Nuclear Decommissioning

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About this resource

This site was created to serve as a generic introduction to training programs in the nuclear decommissioning sector as part of the ELINDER program. The resource aims to give a general overview of the major industrial activities, technology, safety concepts, principles and management approaches in the nuclear sector as preparation for more advanced programs. The site has an e-book format and is split into seven chapters, followed by a multiple choice assessment that covers all content, which can be used to check one's absorption of the information provided.

What is ELINDER?

ELINDER stands for 'European Learning Initiatives for Nuclear Decommissioning and Environmental Remediation' and is a new training and knowledge sharing initiative launched by the European Commission. The program is aimed at students, young professionals and other experienced personnel who may be interested in a career change towards the nuclear decommissioning industry.

The ELINDER program was set up in response to the growing skills shortage facing the decommissioning sector and aims to raise awareness of the appealing career opportunities available in the rapidly growing industry. Nuclear decommissioning is an inevitability for all nuclear reactors currently in operation and poses challenging work, requiring suitably eager, qualified and experienced personnel, which the program aims to attract.

Important nuclear organisations

IAEA - The 'International Atomic Energy Agency', an international forum that aims to promote the peaceful development of nuclear energy and discourage the use of nuclear weapons.

ICRP - The 'International Commission on Radiological Protection', an international non-governmental organisation that provides guidance and recommendations on radiation protection.

ICRU - The 'International Commission on Radiation Units and Measurements', a sister organisation to the ICRP that aims to standardise the units and quantities of ionising radiation.

EAEC/Euratom - The 'European Atomic Energy Community', a Europe-wide organisation created as part of the EURATOM treaty that aims ensure an equal access supply of nuclear fuels to EU nations.

NDA - The 'Nuclear Decommissioning Authority', a public body of the Department of Business, Energy and Industrial Strategy in the UK. The body aims to oversee decommissioning efforts in the UK by offering work contracts to site licence companies.

ONR - 'Office for Nuclear Regulation', the UK regulator of the civil nuclear industry.

Other useful e-learning resources

TU Delft 'Understanding Nuclear Energy' e-module

IAEA e-learning modules - Decommissioning

The World Nuclear Association provides an extensive educational resource, covering much of the nuclear industry.

Khan Academy contains some useful videos if you think you need to brush up on your physics.

About the author

This resource was created by Daniel Lloyd and Dawid M. Hampel under the supervision of Dr. Tzany Kokalova Wheldon at the University of Birmingham, UK.

'Nuclear 101' - Introduction to Nuclear Power

So you're interested in the nuclear industry, but where do you begin?

This chapter aims to give a broad introduction into nuclear energy, focusing on the basic science behind the process. This topic is covered fairly extensively online, so this section is designed to act as a refresher course to ensure key terms and processes are understood in later chapters. Alternate or more detailed resources will be linked at the end of the section.

Nuclear overview

Nuclear power has been around in the form we know since the 1950s, it involves the exploitation of large energy releases that occur when certain large atomic nuclei split into two smaller fragments following the absorption of a neutron; more neutrons are released during the process which allows the 'fission' to be self-sustaining i.e. 'critical'. The heat from this process is transferred to a fluid (gas or water) which drives steam turbines, producing electricity. Simple.

Useful facts about nuclear energy:

  • Nuclear reactors produce large and consistent amounts of energy that is not reliant on external factors such as wind or sunlight, making them very useful for baseload generation.
  • The electricity produced does not involve the release of carbon dioxide, making nuclear much more environmentally friendly than fossil fuels. Wastes are also contained onsite, unlike most other power generating industries.
  • Nuclear fuel is energy dense, meaning far less volume is required compared to fossil fuel thermal plants. One uranium pellet the size of a coin contains the same amount of energy as almost a tonne of coal!
  • Current reserves of uranium are set to last for several hundred years at current consumption, with the advent of new advanced reactors such as fast breeders, reserves of nuclear fuel could extend to thousands of years.
  • 31 countries operate nuclear facilities and most operate without incident.

Philippsburg nuclear power plant

Figure 1 - Philippsburg Nuclear Power Plant, Germany. Houses one PWR and one BWR type reactor. [1]

What about nuclear waste?

Alongside this energy production comes 'fission products', which make up the majority of our more problematic nuclear waste. As the name suggests, these are comprised of the smaller atoms produced via fission and will continue to give off energy through radioactive decay; as they are no longer useful in the reactor, they must be removed and managed safely so that they pose no risk to the public or the environment, which is a big challenge in itself. The 'Radioactive Wastes and Disposal' chapter explores this in more detail.

Safety

When asked about nuclear power, many people voice concerns over the safety of the industry; this stigma likely stems from disasters such as the one at Chernobyl in the USSR, or Three Mile Island in the US. In reality, most nuclear plants operate without any kind of incident and are subject to much harsher regulations and environmental controls than other power generation methods. In fact, a recent paper estimated 1.8 million lives have been saved by nuclear power, due to decreased carbon and particulate emissions [2].

Cost

Another obstacle to nuclear energy is cost; high construction expenses are unavoidable when dealing with nuclear power due to complex designs and the rigorous safety standards that must be adhered to, therefore new reactors cannot be deployed as quickly or cheaply as fossil fuel power stations. Increased standardisation of plants alongside increasing pollution taxation may help to reduce the impact of the steep initial funds required for a nuclear program. Furthermore, nuclear power also offers a nation relative energy independence, avoiding the costly embargoes and threats associated with the volatile oil and gas market.

Comparison to other energy sources

It wouldn't be very fair if we focussed entirely on nuclear energy as all methods of power generation have their advantages and disadvantages, the table below aims to compare the most popular methods across key characteristics.

Fuel Energy Density Costs Environmental Impact
Bio fuel V low-low Low Low
Renewables Low Moderate Low-moderate
Coal Moderate Moderate V high
Oil Moderate Moderate High
Natural Gas Moderate Moderate Moderate-high
Nuclear (Natural U) High V high Low-moderate
Nuclear (Enriched U) V high V high Low-moderate
Breeder Reactor (U/Th) V high V high Low-moderate

Table 1 - Comparison of common energy sources. Bio-fuel refers to fuels derived from plant sources or wastes, such as ethanol or wood. U denotes uranium; natural uranium contains 0.711% fissionable U-235, enriched uranium contains 3-5% U-235. Th denotes thorium. Environmental Impact refers to the pollution generated during power production only.

The science behind nuclear power

Nuclear power is all about atoms. To recap, an atom consists of a nucleus comprised of positively charged protons and neutrons with no charge, this is surrounded by one or more negatively charged electrons that form the 'electron cloud' and balance the overall charge of the atom; the nucleus makes up most of the mass of the atom and the much smaller electrons orbit the nucleus, much like planets around a star. The energy release when an atom with a large number of protons and neutrons splits is what is exploited for nuclear power.

Neutrons and fission

Neutrons are key to the whole fission reaction. When a neutron is fired at a nucleus there's a chance it will be absorbed and incorporated into the host nucleus, which will enter an excited, unstable state. The nuclide cannot exist in this excited state(*) for long and will either convert to another nuclide via the emission of a beta particle (electron or positron), or it will undergo fission and split into two fragments. Most of the energy released by the reaction is in the kinetic energy of the fragments which is immediately converted to heat in the fuel, some energy is also released via gamma rays and neutrons.

A possible fission reaction is as follows:

U-235 + n => U-236* => fission products (e.g. Kr-89 + Ba-144) + 200MeV + 3n

The reaction will usually release 2-3 neutrons which can go on to repeat the process with other fissile nuclides in the fuel, forming a chain reaction.

We want to increase the likelihood of neutron absorption as much as possible and the main way we do this is by using thermal neutrons. A neutron is thermal when it is 'slowed down' enough so that it is in thermal equilibrium with its surroundings, this increases the capture cross section of the neutron and therefore the chance that fission will occur; this slowing down is achieved with the use of a moderator, which we will look at later on.

Sometimes 'fast neutrons' can be used, for instance in fast breeder reactors, where fertile fuels are converted to fissile ones which then undergo fission. See 'Reactors' for more information.

Chain reaction illustration

Figure 2 - Illustration of a chain reaction - neutrons cause fission in the the U-235, forming fission products and several more neutrons which continue the reaction; neutrons absorbed by U-238 do not. [3]

Criticality

To produce continuous, reliable electricity we need to maintain a critical core at all times, this means that the reaction must be self sustaining - where neutrons released from fission go on to cause fissions themselves; the numerical measure of this is the effective neutron multiplication factor, k. The desired state in the core is k=1, where the reaction is just about maintaining itself (critical), this is maintained by carefully balancing the amount of fuel present in the reactor and the deployment of neutron absorbing control rods. If k drops to below 1, the reaction can no longer sustain itself and it will begin to slow and eventually stop (though residual heat will remain for weeks). If k>1, the reaction is known as 'supercritical', where the rate of fission is increasing, this state may seem alarming, but it is essential for the initial build up of energy when the reactor is brought online, though it is imperative that k is brought back down to 1 (critical) as soon as the desired operating level is reached.

Fusion

Fusion is often heralded as the solution to the world's energy crisis, a source of near limitless energy coupled with little to no greenhouse emissions. This may well be the case one day, but currently, nuclear fusion creates as many problems as it solves.

This topic will not be focussed upon in this resource, but in short, fusion can be considered to be the opposite of fission in some ways; it involves the fusing together of light atomic nuclei such as hydrogen and helium under very high temperatures and pressures, releasing energy as they do so. This process is the very same as the one that occurs in our sun, relying on the huge gravitational forces present there to allow the process to occur; on Earth, it is much more difficult to provide conditions suitable to even initiate fusion, let alone sustain and exploit it.

This is all a topic of future research though, technological progress is always accelerating and great strides have already been made into fusion. It is always important to remember that there is never a 'miracle cure'; while fusion could provide huge amounts of clean energy with little risk of out of control reactions, it will still result in medium term radioactive waste due to neutron bombardment of reactor materials, though the long term radiological hazards of this waste will be much lower than those from fission. The potential release of tritium into the environment is another issue to consider, as this can be very difficult to contain and decontaminate due to it easily replacing the hydrogen in water molecules which could pose major threats to our aquifers.

Further reading

TUDelft 'Understanding Nuclear Energy E-learning module contains several lectures on nuclear fundamentals

Scientific American blog post on the lives saved by nuclear energy

World Nuclear Association has a very detailed article on the science behind nuclear power

References

[1] By Lothar Neumann, Gernsbach [1] - Karlsruhe:Bild:Philippsburg2.jpg, CC BY-SA 2.5, https://commons.wikimedia.org/w/index.php?curid=10060036

[2] http://pubs.acs.org/doi/abs/10.1021/es3051197?source=cen

[3] By User:Fastfission - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=522592

History of Nuclear Power

This chapter will provide a brief introduction to the history of nuclear power, focusing on events relevant to electricity production and the development of modern nuclear reactors. More detailed information can be found at the resources linked at the end of the chapter.

Side note: Details on each reactor type mentioned in the text will be covered in later chapters, so don't worry if you don't know the differences between them yet. This section is also quite wordy, a cup of tea is recommended!

Early years - 'Discovery'

Long before the idea for nuclear power was even conceived, scientists were observing the effects of ionising radiation on photographic plates. Wilhelm Roentgen was the first to document these effects in 1895 when investigating electron beams in glass tubes under vacuum; he noticed that photographic plates near his apparatus exhibited fluorescence when the device was powered. Roentgen's studies into this effect lead to the discovery of X-rays, and he took the first ever X-ray image of his wife's hand just weeks later, paving the way for the indispensible medical technology we take for granted today.

Early X-Ray image

Figure 3 - The early X-ray image Rontgen took. [1]

The year after Roentgen's work, Henri Becquerel observed a similar effect on photographic plates when left near uranium salts, the Curie family were able to separate the elements that caused this effect - radium and polonium, and called the effect radioactivity.

In 1902, Ernest Rutherford began experimenting with radioactivity and was the first to prove that alpha and beta radiation accompanies the emission of X-rays. His study into this lead to the modern understanding of the structure of atomic nuclei, theorising the existence of another particle - the neutron.

Scientific study continued, and by 1932, Rutherford discovered that firing accelerated protons at lithium atoms could 'split' them, releasing large amounts of energy. It was around this time that James Chadwick, a doctoral student of Rutherford's discovered the neutron, opening up promising new pathways for research which were snapped up by other scientists including Enrico Fermi, who utilised slow neutrons to produce a wider range of radionuclides than could be achieved with protons.

The Manhattan project - 'Fission'

In 1938, chemists Otto Hahn and Fritz Strassmann made one of the most important (and potentially most deadly) discoveries of the 20th century by firing neutrons at uranium atoms. They realised that the uranium atom had split into two near equal mass elements, contradicting Fermi's belief that he had formed a new transuranic element. Lise Meitner and Otto Frisch later explained this behaviour, suggesting that the neutron was absorbed by the nucleus resulting in strong vibrations and eventual splitting, naming the phenomenon 'fission'.

Hahn and Strassman soon suggested that other neutrons could also be released alongside energy during the reaction. This lead many scientists to theorise that the reaction could be self-sustaining and could therefore have possible military applications; with the threat of Nazi Germany looming over the horizon, this option was pursued by many countries. Rudolf Peierls of the University of Birmingham was the first to calculate the critical mass of uranium needed for a self-sustaining reaction and found that it was sufficiently small to be carried by air - it was previously believed that the amount required would be far too large to even consider an airborne bomb! The information was presented to the UK government in a document known as the Frisch-Peierls Memorandum which detailed the critical mass, delivery method and type of detonator appropriate for use in a bomb.

Nuclear chain reactions were the centre of scientific discussion and in 1942 the world's first man made self sustaining nuclear reaction took place in the Chicago Pile 1 (CP-1) reactor, created by Fermi underneath Chicago University. This development sparked American interest in an atomic bomb program which until now had been very slow compared to other countries; they named it the 'Manhattan Project' and it would result in the two catastrophic nuclear detonations over Japan at the closure of the Second World War.

The first British nuclear explosion

Figure 4 -'Operation Hurricane' - the first British nuclear weapon detonation. [2]

Civilian power - 'Golden age'

After the end of WWII and the deployment of the atomic bomb, nuclear power didn't lose its steam. As well as the continued research and production of nuclear warheads for the Cold War, research into civilian power generation was also starting to gain traction. In Idaho 1951, electricity was generated for the first time via nuclear power from the liquid metal cooled 'EBR-1' reactor with an output of only 100kW; which though small, was an important proof of concept.

Most development in nuclear power went towards powering submarines, but by 1954 the Obninsk power plant in the USSR was the first reactor to provide electricity to a grid, with an output of 5MWe, and was the prototype to the Russian RBMK reactor fleet that included Chernobyl. The Calder Hall-1 reactor opened in 1956 in the UK and was the first commercial nuclear power plant worldwide, producing 50MW for the national grid, the first of the MAGNOX type reactors that dominated UK early build. 1957 saw the launch of both EURATOM and the IAEA, with the aim of greater international cooperation and regulation of nuclear matters.

Focus shifted to improving the safety and performance of existing technology and in 1960 the first pressurised water (PWR) and boiling water (BWR) reactors were built by Westinghouse and the Argonne National Lab respectively, which went on the be the standardised designs for the most common reactor types used today. In 1962 the first CANDU (Heavy water) design reactor was built in Canada.

The 70s saw a large increase in nuclear new build, particularly in the UK and France. The UK began the rollout of its unique AGR type plants, replacing the costly and outdated MAGNOX reactors which relied too heavily on bespoke components. France on the other hand began its nuclear program as a response to the 1973 oil crisis, which saw the price of oil skyrocket due to the Arab embargo of Western states; nuclear power was seen as an effective method to increase the energy security of the nation and prevent such problems in the future.

Nuclear stagnation and revival - 'Trust'

Large construction costs, delays and Cold War politics meant that the popularity of nuclear power saw a dramatic slump from the late 70s onwards. The near catastrophic events at Three Mile Island, 1979 and the Chernobyl disaster, 1986, almost dealt the killing blow to the industry and many new builds were cancelled, despite a global increase in electricity demand.

This slump remained until the mid 90s, where the demand for low carbon electricity production and the impressive safety record of nuclear power (including disasters!) made nuclear a viable part of the energy mix again. At this time, the EPR began to replace older generation reactors in much of Europe. This 'renaissance' gained traction and new reactors were being planned in several countries, particularly Asia; China alone currently plans to increase its nuclear fleet by over 100 units.

This trend did not last forever though. In March 2011, the extraordinary 9.1 magnitude Tohoku Earthquake struck Japan and triggered a large tsunami which overwhelmed the flood defences at the Fukushima-Daiichi power plant and caused a loss of cooling to the reactors. While radiation was released, it fortunately blew offshore and avoided populated areas, the worst effects were political rather than physical. This close call caused many nations to re-think their nuclear strategy; Germany chose to phase out nuclear power entirely and many other European countries put plans on hold.

However, the future still remains bright for the nuclear industry, fast breeder and modular reactors are seeing increased interest, having strong advantages over the current established technology and may hold the solution to many of our energy issues today. We will look into these in later chapters.

Further reading

Naturally, when history is concerned you could go on forever; another more detailed look at the history of nuclear power was produced by the US Department of Energy, find the pdf here.

A more detailed outline of history of nuclear power can be read on the World Nuclear website.

References

[1] By Wilhelm Röntgen; current version created by Old Moonraker. - Flipped version of File:X-ray by Wilhelm Röntgen of Albert von Kölliker's hand - 18960123-03.jpg (now deleted as a duplicate)., Public Domain, https://commons.wikimedia.org/w/index.php?curid=7267529

[2] By (Naval Historical Collection) - This image is available from the Collection Database of the Australian War Memorial under the ID Number: P00444.045 Public Domain, https://commons.wikimedia.org/w/index.php?curid=8751512

Nuclear Reactor Types and Technology

A nuclear reactor is a device that initiates, controls and contains the nuclear chain reaction; over the last half a century there have been many differing designs, though the core components remain generally the same. The role and design of each essential component will be explained in this section, as well as an overview of the major reactor types and how they differ from each other. As this is primarily a decommissioning resource, there will be a focus on past and current designs.

It is worth noting that while this page only discusses reactors used in civil power generation, nuclear reactors are used in a number of industries for various purposes - from medical research to powering submarines; though the basic principles are identical.

Main components

The core

The term 'core' is thrown around a lot, but it isn't strictly a component in itself. The core is essentially where the nuclear reaction takes place in the power plant and so is where all the reactive components of the reactor are located, including the fuel, moderator and control rods.

Nuclear fuel

The fuel is one of the most important aspects of a nuclear plant. Fuels are typically ceramic to ensure that no warping, melting or burning takes place under the high temperature conditions within the core, though some reactors do use metallic fuels along with strict temperature controls. The type of fuel used is often a decision based on what resources are more readily available to a nation rather than what is more efficient, this was especially true early in the industry - though standardisation is increasing in the modern age.

Uranium fuel pellet

Figure 5 - A uranium oxide ceramic fuel pellet. [1]

The fuel is normally contained within pins or rods which are grouped into assemblies and inserted in cycles into the core, this means that when one group needs to be removed, there are always more inside so the reactor does not need to be shut down and power can be produced continually. A typical cluster will begin to lose its efficiency after around 12-24 months, when it is then removed and cooled in specialised ponds, the removed fuel will still retain over 95% of its potential, but a build-up of neutron absorbers in the fuel renders it unusable in its current state.

The choice of fuel often influences much of the design of the reactor, uranium is the most common element used, though plutonium and thorium are seeing increased interest. Some reactors can operate on natural metallic uranium which requires little processing, whereas others require enrichment to increase the proportion of fissile material present in the fuel to allow criticality to persist; more recent developments even allow some reactors to 'breed' more fuel from fertile nuclides which increases reserves of nuclear fuel dramatically.

A nuclide is fissile if it will undergo fission following the absorption of a thermal neutron, this is the trait most commonly desired in nuclear fuel as it provides a simple fuel-energy reaction without any more complicated stages; U-235 is the isotope of choice in most nuclear reactors and is present at around 0.711% in natural uranium, or 3-5% in enriched fuels.

Even if a nuclide is not immediately fissile, it may at least be 'fertile'. This means that the isotope has the potential to form a fissile nuclide following the absorption of a neutron. For example, U-238 is not fissile, but fertile; U-238 is converted to the fissile Pu-239 isotope inside the reactor which helps extend the life of the fuel, (U-238 makes up over 99% of a typical uranium sample - this is a lot of potential!).

See 'Fuel Cycle' for more information on the enrichment and reprocessing of fuels.

Cladding

The cladding is essentially what isolates the fuel from the external environment, preventing any interaction between the fuel and the coolant; it also serves to contain the highly radioactive fission products that are generated during the reaction. It is essential that the cladding material is a good conductor of heat and not prone to distortion at high temperatures, as well as having a low neutron capture cross section so that it does not interfere with the criticality of the chain reaction.

The most common cladding materials today are stainless steel and zircalloy (zirconium alloy), though a magnesium alloy known as magnox was used in older British reactors (see - Magnox).

Magnox fuel rod

Figure 6 - Magnox fuel cladding - the uranium metal fuel rod was contained in the centre, also note the unusual cooling fins. Length is around a metre. [2]

Moderator

As mentioned in 'Nuclear 101', the chance of fission occurring in the fuel is increased when the neutrons are 'thermal' - in thermal equilibrium with their surroundings , this means they must be 'slowed down' before interacting with fissile nuclides, and this is the job of the moderator.

A moderator should be comprised of light nuclei, so that energy loss per collision is as high as possible. A good way of visualising this is to imagine a snooker table where the cue ball represents a neutron, if the cue ball strikes the cushion (a large nucleus), it is rebounded back without losing much speed; whereas if the cue ball strikes another ball (light nucleus), it is slowed down far more. The moderator should also be dense to increase the chance for a neutron to scatter (i.e. putting more snooker balls on the table), this is why hydrogen gas is not appropriate in a reactor - flammability aside, despite being the lightest element. Again, the material must have a low neutron absorption rate; water and graphite are ideal for this.

Coolant

Sustained nuclear fission generates a lot of heat, the coolant fluid keeps the reactor temperature at a manageable level and transfers the heat from the core to the steam generators. Most reactors have several coolant loops so that the steam used to generate electricity does not come in contact with the reactor itself, though in direct cycle reactors such as BWRs, there is only one direct cycle. Each loop is self contained, with the exception of the final loop which is generally sourced from a large body of water, this loop does not come into contact with radioactive material.

Naturally, the coolant must have a high heat transfer coefficient and have low neutron absorbance, but it is also important that it does not react with materials inside the reactor. Water and Heavy water (D2O) are the most common coolants (and moderators), though many older generation British reactors utilised CO2 gas; liquid metals are used in fast breeder reactors due to their high heat transfer capabilities and low neutron moderation.

Control rods

Control rods are a vital part of the reactor as without them we could not control the fission reaction rate; they are created from materials with high neutron absorption cross sections such as boron and cadmium in a composition that covers the entire spectrum of neutron energies in the reactor. The rods are inserted incrementally to slow the reaction as it builds, and fully lowered to stop it* - the rods are usually held by electromagnets so that in the case of a power failure, the rods are automatically dropped without reliance on any external systems.

*while the critical reaction is stopped in seconds, delayed neutrons and decay heat will keep the reactor hot for days following shutdown, failure to control this heat in an emergency situation is the cause of most major nuclear accidents.

Pressure vessel

The pressure vessel is what holds the core and all its components together and shields them from the outside world; the vessel contains a cold inlet and hot outlet for each coolant loop to allow flow between the heat exchangers. The vessels must be very robust to withstand the high temperature and pressure present in reactors and are typically made of thick steel lined with high quality stainless steel to prevent corrosion, though some early designs used pre-stressed concrete as it was much easier to work with (Magnox).

Heat exchangers

Most reactor types have heat exchangers, which allow the hot primary coolant in the reactor to transfer its heat to an isolated secondary feedwater loop, which is allowed to boil and produce steam; the terms 'boiler' and 'steam generator' are also used. The primary coolant is cooled during the process and returned to the core.

Containment

While the pressure vessel isolates the core from the immediate environment, it doesn't do much to block penetrating radiation such as gamma rays; the containment is a large reinforced concrete structure that houses the entire reactor and steam generators, shielding the external environment from harmful effects. While its use is predominantly for passive safety, the containment is also the final line of defence in the event of a meltdown and should be able to contain any core material that escapes the pressure vessel, confining the danger to a localised area that is much easier to clean up.

Reactor types

Now you know a little bit about the components of a nuclear reactor, it's good to know how these vary between common reactor types. This section will give a brief overview into the main types of nuclear power plant, as well as the advantages and disadvantages of their design.

Broadly speaking there are two categories of reactor, distinguished by what neutron energies they utilise; these are termed fast or thermal reactors - the latter being most widespread.

Gas-cooled - Magnox and AGR

The Magnox stations were part of a relatively unique line of gas-cooled reactors of British design that were commissioned from the late 50s to the early 70s, and were later superseded by the similar, more advanced AGR plants; due to the cold war, these reactors were designed as much for plutonium production as they were for power generation. Magnox-type were amongst the first commercial nuclear power plants to operate and their design was upgraded with each new build - the last Magnox reactor to be built would have been almost unrecognisable from the first. This lead to issues with standardisation and an overreliance on bespoke parts, which made maintenance and life extension very expensive.

The name Magnox comes from the unique magnesium-aluminium alloy used to clad the fuel, the material has a very low neutron absorption rate which allows for the use of un-enriched metallic uranium fuel; a major benefit at the time as the UK had no enrichment facilities. Each fuel pin is a metre long and has fins along its outer edges which allow for better heat transfer to the CO2 gas coolant; these are placed between graphite blocks in the core, which act as the moderator. Control rods were inserted top down into the reactor, between fuel rods.

The core was contained within a thick steel pressure vessel in early models, though the cost and difficulty of making these lead to the use of post-stressed concrete vessels in later designs which could withstand higher pressures, increasing coolant efficiency. The heat from the CO2 was then used to generate steam in heat exchangers, these could be located inside the pressure vessel in concrete designs, reducing the likelihood of leaks.

The issues associated with this type of reactor generally stem from the fuel assemblies. Magnox will ignite and distort at temperatures higher than 520 Celsius and uranium will undergo phase change swelling at 660 Celsius, these factors limited the operating temperature to around 350 Celsius, which impacted the efficiency of the design.

No Magnox reactors remain active in the UK.

Magnox reactor schematic

Figure 7 - A schematic diagram of a Magnox reactor [3]

Advanced Gas-Cooled Reactors (AGRs) were designed to address the shortcomings of the magnox program, they retained many of the same features as the late magnox designs but were more efficient due to a higher operating temperature; the coolant, moderator and pressure vessel were near identical.

The biggest changes were made to the fuel assemblies, stainless steel was used to clad the fuel instead of magnox, and while this allowed for much higher gas temperatures to be reached, it has a much higher absorption cross section for neutrons; this necessitated the use of a different fuel type - Uranium oxide ceramic pellets, enriched to around 3% U-235. The ceramic fuel is able to withstand temperatures of over 2500 Celsius, far higher than would be expected in a reactor core, this makes the fuel much safer and resistant to warping in comparison with metallic fuels, these pellets are packed inside thin steel pins and inserted into a graphite sleeve to form a fuel element, ready to be placed into the reactor.

While the AGR design was very efficient even by today's standards, it still had some drawbacks. The output temperature is restricted by the cladding to 650 Celsius, though this is still much higher than most thermal reactors; the biggest issue stems from the graphite moderator, which is slowly degraded by high temperatures and oxidation by the CO2 coolant, which restricts the life span of the fuel elements.

Seven AGR reactors remain active in the UK at this moment in time, though closure and decommissioning is scheduled to begin within the next decade.

AGR schematic

Figure 8 - A schematic diagram of an Advanced Gas Cooled Reactor. Numbers on diagram: 1. Charge tubes 2. Control rods 3. Graphite moderator 4. Fuel assemblies 5. Concrete pressure vessel and radiation shielding 6. Gas circulator 7. Water 8. Water circulator 9. Heat exchanger 10. Steam [4]

Light water cooled - PWR and BWR

Pressurised Water Reactors (PWR) are easily the most widespread nuclear stations; originally an American design, they became very popular due to a focus on standardisation and mass production, which meant that running costs were low compared to other reactors.

Regular or 'light' water is used as both the moderator and the coolant, reaching over 320 Celsius in the core; because of this, the water is kept at very high pressure (160 bar) to prevent boiling, which necessitates the use of thick steel pressure vessels to withstand such conditions. A unique component of PWRs is the pressuriser, which is used to maintain pressure in the system and is able to heat or condense steam contained within it to keep hydrostatic pressure at a desired level.

The fuel is uranium oxide pellets enriched up to 4%, contained within thin zirconium alloy (zircaloy) pins over 3m long, which are grouped to form very large fuel arrays; control rods are contained within around a third of these arrays at the expense of some fuel pins. Heat from the fuel is transferred via the coolant to the heat exchangers, which use water in a separate loop to create the steam that drives the turbines as usual. The whole array is placed within a concrete containment structure.

The sheer popularity of this type of reactor is a benefit in itself, the design is well known and has been significantly mass produced. The moderator/coolant choice also provides a safety benefit: should a loss of control occur, the water will eventually form steam - the low density results in poor moderation and causes the reaction to slow on its own.

PWR design is not faultless though, they have lower efficiency than their gas-cooled counterparts due to a lower operating temperatures, and the use of high pressure systems increases the likelihood and severity of loss of cooling accidents.

PWR schematic

Figure 9 - A schematic diagram of a Pressurised Water Reactor. [5]

Boiling Water Reactors (BWRs) are similar to PWRs in many ways, though the water is allowed to boil inside the reactor meaning only one coolant loop is used to generate electricity; the steam passes through driers to remove excess moisture and enters the turbines to generate electricity. All of this results in a larger core, though as the pressure is around half that of a PWR (80 bar) the pressure vessel doesn't need to be quite as thick, operational temperature is around 285 Celsius.

Another unique feature of BWRs is that the control rods are inserted from the bottom of the pressure vessel as the top half of the reactor is less dense due to steam, so inserting control rods from above would be ineffective at slowing the reaction rate.

The simpler design of a BWR has the benefit of reducing construction and maintenance costs, though boiling water in the pressure vessel leads to complications in predicting the conditions in the core and a direct cycle runs the risk of radioactive material entering the turbine hall, which now needs to be shielded, this also increases decommissioning costs further down the line.

BWR schematic

Figure 10 - A schematic diagram of a Boiling Water Reactor. [6]

Heavy-water moderated - CANDU

Heavy water (sometimes stylised as D2O) is simply water that has a higher concentration of deuterium (a hydrogen isotope) than normal; heavy water is less absorbing of neutrons than light water, and so enrichment of fuel is not required. This substance is used as the moderator and coolant in Canadian Deuterium Uranium (CANDU) reactors, another example of a pressurised system, though unlike PWRs, there is no pressure vessel. Instead, the moderator is placed in an unpressurised container known as a calandria, which hosts several horizontal pressurised tubes that contain the un-enriched uranium oxide fuel (clad in zircaloy) that is cooled by more heavy water in a cooling loop, which then moves to the steam generators. Control rods are inserted vertically into the calandria, and as usual, all radioactive components are located within a concrete containment structure. Operational temperature is just over 300 Celsius.

One major advantage of the CANDU design is that it can accept a wide variety of fuels, including thorium, newer models have been altered to take advantage of this.

The lack of a pressure vessel means that CANDU reactors can be made much smaller than other designs, the relatively simple construction also makes installation much easier. Furthermore, CANDU plants boast a very high energy output to fuel ratio - making them very cost effective, though this is somewhat balanced by higher moderator/coolant costs and a less robust overall structure.

CANDU reactor schematic

Figure 11 - A schematic diagram of a CANDU Reactor. [7]

Light water, graphite moderated - RBMK

RBMK is a reactor type built in the Soviet Union in the 70s, the design was essentially converted from older plutonium producing reactors with the aim of developing a model that could also generate power; it was these designs that were used to create the reactors in Chernobyl.

RBMK translates to 'High Power Channel-type Reactor' and has a fairly unusual design; it is a pressurised light-water cooled, graphite moderated system that uses enriched uranium oxide fuel, clad in zircaloy. Unlike western reactors, the RBMK had no containment aside from the concrete lined radiation shield the reactor was placed inside.

The coolant is direct cycle and allowed to boil, which can cause instability due to a 'positive void co-efficient' caused by the use of the graphite moderator with water coolant. Essentially, when water boils in a BWR, it produces steam, which moderates neutrons less and the reaction slows; in RBMK reactors, water is relied on more as a neutron absorber than moderator, this means that when steam is produced, there is less cooling and less absorption of neutrons, but the graphite moderator is still maintaining the reaction as normal. This effect is controllable at high power levels, but a mistake during routine testing meant that the reactor was run at a low power level, allowing this void coefficient to increase. When the power was returned, the effect spiralled out of control - the reactor got hotter, more steam was produced, less neutrons were absorbed by water and the reaction rate increased exponentially in the space of several seconds, causing a large explosion and the release of large amounts of radioactive material.

Several RBMK reactors still operate in Russia today, though with heavily modified designs to address the shortfalls of the original.

RBMK reactor schematic

Figure 12 - A schematic diagram of a RBMK Reactor. [8]

Future designs - next-gen and fast reactors

New generation reactors are mostly focussed on upgrading and standardising existing plants. New PWR light water designs such as the European Pressurised Reactor (EPR) and the Westinghouse AP1000 are generally focussed on increased thermal efficiency and better fuel burn up, as well as lower energy production costs and compatibility with mixed oxide (MOX) fuels - which contain uranium and plutonium oxides. The Advanced Boiling Water Reactor is a joint effort by Hitachi and Toshiba to modernise the older BWR design that is popular in Japan, again focussing on higher efficiency and power output.

CANDU reactors have also seen continuing development, these tend to focus on increasing fuel flexibility and recycling, as well as extending overall plant life. India is also weighing in on heavy water reactors with the Advanced Heavy Water Reactor (AHWR) that will use thorium as its primary fuel.

Fast Breeder Reactors (FBRs) are seeing increased interest due to their ability to create more fissile nuclides than they use during power production, though they are comparably expensive to operate while uranium costs are low. FBRs are a type of fast reactor - i.e. they utilise fast neutrons, not thermal; this means that a moderator is not required and high energy neutrons are used to split fissile isotopes in fuel, though at a slower rate than thermal reactors. To counteract this lack of efficiency, FBRs typically use either highly enriched (>15% U-235) uranium fuels, or more commonly, un-enriched plutonium; these fuels allow the amount of neutrons produced remain high enough to sustain the reaction, as well as allow conversion of non fissile isotopes to fissile ones.

Naturally, producing more fuel than they use up is a very desirable trait of FBRs, but another benefit is that they can actually fission actinides (the radioactive products of fission), this reduces the amount of long lived radioisotopes in nuclear wastes and also allows for the recycling of 'burned' fuels from other reactor types. FBRs fuels include uranium, plutonium and thorium - making them very fuel diverse.

The coolant for FBRs tends to be liquid metals such as sodium, this is because breeder cores require a coolant with a very high heat capacity and a high boiling point, something that can't be achieved with traditional coolants without significant pressure; low neutron moderation is also an essential trait of the coolant, as any slowing down inside of the reactor is undesirable. While sodium acts as an excellent coolant, the cooling loops must be well engineered to prevent any leaks as any contact with water will cause fires.

There are already several active fast reactors commissioned today, though most are designated as research reactors. Widespread adoption did not occur during the 'nuclear golden age' due to proliferation concerns because of the ability to generate weapons grade plutonium, as well as the relative high cost of the technology. Despite this, the benefits of FBRs are gradually becoming more and more desirable and will likely see increased research and implementation in the coming years, especially when uranium stocks begin to fall.

Further reading

Much of the major content has been covered in this chapter, but the following resources offer some extra detail if desired:

TUDelft 'Understanding Nuclear Energy - E-learning module offers several lectures on reactors, including next gen.

An explanation of reactor decay heat

For more information on advanced reactors, the following pages by whatisnuclear and the World Nuclear Association are both useful.

References

[1] By NRC - http://www.nrc.gov/images/reading-rm/photo-gallery/20071114-022.jpg, Public Domain, https://commons.wikimedia.org/w/index.php?curid=5227313

[2] http://collection.sciencemuseum.org.uk/objects/co8233887/magnox-fuel-cans-from-sizewell-a-nuclear-power-station-1968-nuclear-fuel-cans Creative Commons Attribution-NonCommercial-ShareAlike 4.0 Licence

[3] By Emoscopes, CC BY 2.5, https://commons.wikimedia.org/w/index.php?curid=2455075

[4] By MesserWoland - own work based on Image:AGR reactor schematic.png by Emoscopes, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=1902036

[5] By U.S.NRC. - http://www.nrc.gov/reading-rm/basic-ref/students/animated-pwr.html, Public Domain, https://commons.wikimedia.org/w/index.php?curid=2320214

[6] By U.S.NRC. - http://www.nrc.gov/reading-rm/basic-ref/students/animated-bwr.html, Public Domain,

[7] CC BY-SA 2.5, https://commons.wikimedia.org/w/index.php?curid=564652

[8] By Fireice - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=3579057

Nuclear Fuel Cycle

The nuclear fuel cycle describes all stages that the fissile material we use in reactors goes through, from taking it out of the ground, to eventually returning it there - albeit in a much altered state. These steps become a cycle when recycling/reprocessing gets involved; used fuels still retain most of their potential, but a build up in neutron absorbing fission products makes the fuel inefficient to keep using. Reprocessing separates the fissile material remaining in the old fuel to create new fuel, this is known as a 'closed' or reprocessing cycle; whereas once-through or 'open' fuel cycles skip any recycling, and used fuel is sent to a specialised facility for long-term disposal, alongside other unusable nuclear wastes.

This page will give you an overview of all the steps involved in the uranium fuel cycle. Further reading and information on other fuel cycles will be included at the end of the section.

Mining

Uranium is a fairly widespread element in the Earth's crust, present in most rock and seawater in minute quantities far too small to feasibly extract. An area that contains a high enough concentration of uranium to be economically viable is known as an orebody, these are typically regions with uranium rich minerals such as uraninite (pitchblende), though lower grade ores are also commonly mined; typical uranium concentrations can be as low as 0.1%. Deposits that are currently economically unfeasible to obtain are known as reserves, and may become ores when the financial climate alters; often when traditional resources start to dwindle or become too difficult to extract.

The 3 largest producers of uranium are Kazakhstan, Canada and Australia.

There are a number of methods available to extract uranium from the ground, the traditional route is to simply to mine it, though a method known as in-situ leaching is becoming increasingly popular. Mining uranium is much the same as mining any deposit; underground mineshafts are used to extract concentrated or deep deposits, causing little disturbance on the surface and producing much less waste, though ventilation and radiation dose are a concern when dealing with uranium-bearing ores. Open-pit mining is utilised for more disperse, low quality deposits, where large volumes of material must be excavated to extract a high enough concentration of ore; this method has significant impact on the surface environment and produces a lot of spoil, but has far lower financial and engineering requirements.

In-situ leach is a less mechanical approach to mineral extraction, where acidic (or in carbonate rich regions, alkaline) groundwater is circulated through an ore deposit to dissolve the uranium, allowing the solution to be recovered at the surface with little disturbance of the underlying geology; oxidants may be used to reduce the level of acidity/alkalinity required in the groundwater. This method may only be used for deposits within porous hot rock, confined above and below by impermeable strata, to allow the solution to permeate through to the deposit and avoid any possible contamination of aquifers.

Open pit uranium mine

Figure 13 - An open pit uranium mine. [1]

Milling

Milling of the ore is done to separate the uranium from the ore or leachate solution, this is usually done in close proximity to the mine to reduce the volume of material to be shipped. The ore is ground down to a suitable size and heated if necessary to remove any organic material, the substance is then leached in sulphuric acid to separate the uranium from the gangue (waste rock); the solution is then purified and separated using solvents, and precipitated to form yellowcake (uranium ore concentrate), which is around 80% uranium.

The gangue left over from this process cannot simply be discarded in spoil heaps due to the high concentrations of heavy metals and radioactive decay products associated with uranium deposits; wastes must be disposed of in specially engineered facilities near the mill, isolated from the environment.

Conversion

Conversion is simply the process of converting yellowcake to the form required to make the fuel - this depends heavily on the type being made. To begin making the fuel, the uranium concentrate needs to be purified again to make it nuclear grade by removing volatiles and neutron absorbers that may still be present, this is done by dissolving in nitric acid and separating uranium with the aid of a solvent. The purified uranium is then converted to uranium dioxide (UO2) - which can be used in reactors that do not require enrichment, but otherwise it is reacted with fluoride to form uranium hexafluoride (UF6), which is the compound preferred for the enrichment process as it forms a gas at relatively low temperatures.

Enrichment

In most modern reactors, the natural proportion of U-235 is 0.72% - too low to sustain criticality; the aim of enrichment is to increase the proportion of this fissile isotope in the fuel, usually to around 3-5%. Enrichment takes advantage of the tiny mass difference between U-235 and U-238; UF6 gas is fed into a series of centrifuges and the spinning motion results in the heavier isotope (U-238) moving preferentially to the edge of the cylinder, whereas the lighter U-235 will concentrate slightly in the centre, this can then be extracted and moved onto the next stage and the depleted mix will return to the previous stage to repeat the process again.

The UF6 vapour will continue to move through these centrifuge stages until the required level of enrichment is reached, the is UF6 is reconverted to (enriched) uranium oxide, ready to be formed into true fuel.

Centrifuge array

Figure 14 - A centrifuge array - note the huge amount of stages! [2]

Fuel fabrication

While the uranium oxide is a fuel in the simplest sense, it must be formed and processed to be suitable for use in a nuclear reactor. The UO2 is pressed into pellets and baked at very high temperature to form a heat resistant ceramic which can then be placed within a suitable metal cladding, usually stainless steel or zircaloy; each fuel pellet must be a consistent size and shape to ensure that the fuel burns predictably in the reactor.

Burn-up and used fuel

Nuclear fuel will typically spend between 12 and 24 months inside the core, after which it becomes inefficient to continue using due to a build up of neutron absorbing fission products and heavy metals known as 'reactor poisons'. Upon their removal from the core, the fuel elements are placed in specialised cooling ponds near the reactor to allow excess heat and short lived radioactive nuclides to subside to a level that allows them to be safely stored in dry facilities if present.

The spent fuel is held in this interim storage until its final fate is decided; as the fuel still contains over 95% of its energy potential, reprocessing may be considered to isolate the useful components of the fuel - otherwise, geological disposal may be used as a more permanent solution.

Reprocessing and recycling

While a spent fuel rod is no longer useful in a reactor, the majority of its uranium content remains unused. Reprocessing aims to recover the useful elements from the fuel; uranium and small quantities of plutonium are separated from the undesirable elements to be re-used earlier in the fuel cycle. Firstly, the fuel element, cladding and all is chopped up and dissolved in nitric acid to separate the various substances inside the fuel, the solution then goes through a series of solvent extraction processes to gradually isolate the uranium and plutonium from the fission products. The uranium and plutonium are then converted to oxides and the fission products remain as a radioactive liquor, which is later solidified and disposed of.

The reclaimed uranium will then undergo enrichment again, before being remade into fuel; the plutonium will either be stored, or used to create mixed-oxide (MOX) fuel - a mixture of uranium and plutonium oxides that can be used as a replacement for low-enriched fuels. Depleted uranium (low in U-235), also has the potential to be used in fast reactors, which can breed fissile isotopes from U-238.

Disposal

Note: This is covered in more depth in the 'Waste Management' chapter.

While there is a drive in the nuclear industry to reduce the volume of waste produced, not everything can be reprocessed or recycled; reprocessing liquors, irradiated material, and even spent fuel in once-through cycles must be treated and safely disposed of in specialised facilities. The way a waste is treated depends heavily on its classification; high, intermediate and low level wastes all differ in their radiological properties and ease of handling, though in all cases the waste must be immobilised in concrete or glass and sealed in steel containers so that it may be transported to a secure underground facility for permanent disposal.

Currently there are no facilities for the permanent disposal of high and intermediate level wastes, instead these are kept in extended storage above ground. This approach is adequate for the time being, as current volumes of these wastes are low, though a more permanent solution is something that must be considered for the near future to reduce unnecessary 'janitorial' costs and security concerns associated with surface storage of nuclear material. Studies into various methods of disposal are underway in several countries, though deep geological disposal has seen the most attention. While the construction of an underground geological facility is not too technically demanding, the social response of siting such a structure can be a much more daunting challenge.

Further reading

TUDelft Understanding Nuclear Energy - E-learning module contains a series of lectures on the fuel cycle

The World Nuclear Association have an article on in-situ leach here.

NDA summary on wastes

References

[1] By Ikiwaner - Own work, GFDL 1.2, https://commons.wikimedia.org/w/index.php?curid=8132756

[2] By Nuclear Regulatory Commission - https://www.flickr.com/photos/nrcgov/16042443515/, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=707601

Nuclear Safety and Radiation Protection

Despite the negative connotations of the word, radiation is simply the transmission of energy. There are many forms of radiation and much of it is very familiar to us, including radio waves, microwaves and visible light; these harmless forms of radiation are known as non-ionising and we are exposed to them daily.

The type of radiation that must be shielded against is known as ionising radiation, this includes electromagnetic rays such as X and gamma, and charged particles such as alpha and beta; these forms have high enough energies to damage cells in the body and so great care must be made to limit exposure to radiation workers and the public. Despite ionising radiation being commonly associated with the nuclear power, most of our yearly dose actually comes from natural sources, particularly in areas with granite rich geology.

Radiation protection principals

Radiation protection is all about shielding people from the negative effects of ionising radiation associated with nuclear activity. Fundamentally, protection is about balancing the benefits and risks associated with the use of technology involving radiation, this is done with reference to three major principles, courtesy of the International Commission on Radiological Protection (ICRP):

Justification - any activity that involves exposure to an individual or population should be justified - the benefit must outweigh any potential for harm.

Optimisation - in any exposures deemed necessary, the dose should be kept as low as reasonably achievable (ALARA), taking into account economic and social factors. The number of people receiving a dose should also be minimised.

Limitation - exposure must be constrained by legal dose limits that ensure an individual is not subject to unacceptable risk.

The ICRP describe two major categories of exposure: practices covers human activities that deliberately result in the emission of radiation that leads to exposure, such as power generation or medical imagery; interventions describes activities that are intended to reduce overall exposure to the broader population at the expense of some individuals, for instance the cleanup of a radioactive spill, or the processing of contaminated soil. In both categories, the three protection principles must apply, even with differing severity or time constraints.

Standards for radiation protection are always conservative, i.e. they are created with the worst case scenario in mind. This means that existing dose limits are set well below the thresholds for radiation damage; despite this, doses must always be justified and kept ALARA so that any exposure to individuals is well below these limits.

Types of ionising radiation

Ionising radiation is a type of particle or electromagnetic wave that is of sufficient energy to ionise ('knock') an electron from the shell of an atom. Before we can begin shielding from this, we must first understand the principal forms of ionising radiation we may encounter; each has differing properties that make engineering a suitable shielding for all forms of ionising radiation a difficult task.

The main types of ionising radiation we deal with are:

  1. Alpha particles - short range, heavy particles consisting of two protons and two neutrons that are emitted from large nuclei such as uranium and thorium during alpha decay. Alpha particles are strongly ionising but cannot penetrate very far into matter due to their charge, and are therefore only truly dangerous if ingested, inhaled or enter wounds in the skin.
  2. Beta particles - ejected electrons moving at high speed; short range but more penetrating than alpha particles and so prolonged exposure can lead to injury. Beta particles are emitted during beta decay and can be easily shielded against with very thin metal plates, low energy particles can even be blocked by clothing.
  3. Gamma and X-rays - different from the previous two in that they are high energy electromagnetic waves consisting of high energy photons. While less ionising than alphas or betas, they are far more penetrating and require much more robust shielding such as lead plates or thick concrete walls. Gammas are typically higher energy and are produced from the nucleus of an atom during decay; X-rays are lower energy and instead originate from the rearrangement of electrons within an atom.
  4. Neutrons - uncharged particles that trigger and are released by nuclear fission. While often not the first things considered when dealing with radiation protection, fast neutrons penetrate far and can be very damaging to tissue; they also have the unique ability to irradiate non-radioactive shielding materials, increasing the volume of waste to deal with.

Radiation penetration illustration

Figure 15 - An illustration showing the different types of ionising radiation and their penetration depths [1]

Dosimetric units

In order to effectively protect people, we must be able to quantify and measure radiation induced health issues. We can begin with the most basic unit used in radiation protection, the absorbed dose; this represents the energy deposited by ionising radiation in a mass of tissue and is given the unit gray (Gy), where one gray is the deposition of one joule per kilogram of tissue. A dose of 4-5Gy is lethal to around 50% of the population.

While this unit is simple to calculate, it does not give us very much useful biological information. The density of ionisation varies with each radiation type and affects how much damage it can cause to DNA; gamma rays and beta particles cause relatively low levels of damage that can usually be repaired easily by the body, alpha particles and neutrons on the other hand lead to significantly more harm. We can account for this by assigning each radiation type a dimensionless radiation weighting factor, this quantity is derived by comparing the relative harmfulness of a radiation type in comparison with high energy photons (gamma rays) for the same absorbed dose. These factors allow us to calculate the equivalent dose, which compensates for the difference in damage caused by each radiation type, this is done by multiplying the absorbed dose by the weighting factor for the specific radiation type, its unit is the Sievert (1 Sv = 1 J/kg). In a mixed field exposure, i.e. there are multiple types of radiation present, each equivalent dose must be calculated separately for each type, the sum of these is the total equivalent dose.

It may also be useful to take into account the sensitivity of a particular tissue (or organ) to permanent radiation damage. We do this by assigning another dimensionless number known as a tissue weighting factor, which assesses the relative probability of a unit equivalent whole body dose producing cancer in a particular organ. The effective dose can then be calculated by multiplying the equivalent dose by the relevant tissue weighting factor, the total effective dose can then be found by summing the contribution from each affected tissue, this is also expressed in Sieverts.

ICRP recommended effective dose limits for artificial exposure are 20mSv/yr for workers and 1mSv/yr for the general public.

Effects of ionising radiation

Not all of the effects of ionising radiation are bad. Controlled use of radiation can be very beneficial to society: radiotherapy, X-ray imagery, smoke detectors and food sterilisation all utilise ionising radiation and have become essential tools in the modern world. But what if something goes wrong?

The detrimental health effects of ionising radiation are split into two categories dependant on how they arise. Deterministic effects are almost always instant and occur once a threshold dose has been reached; these include multicellular tissue responses such as radiation burns, cataracts and more severe symptoms such as organ failure and death. Deterministic effects cannot occur below the threshold dose, and their severity increases with dose; we use equivalent doses to measure these effects.

Problems that arise on longer time scales are termed stochastic effects and includes slowly developing issues of a single cell origin, such as cancers and inheritable genetic defects. Stochastic effects have no threshold to overcome, meaning they have a chance to occur at very low doses; their probability, not severity increases with dose. Studies on stochastic effects struggle to agree on the likelihood of radiation induced cancer at low doses due to the long timescales involved and the prevalence of cancers from other sources, but for protection purposes a 'better safe than sorry' approach is taken. The effective dose is used to measure stochastic effects.

Protection and prevention measures

Practical radiological protection revolves around the careful monitoring of planned doses, as well as the prevention of unnecessary exposure to workers or the general public. Monitoring is done with the aid of personal dosimeters that are attached to clothing and give an estimate of the dose an individual has been subject to; film badges that contain photographic film that darkens upon exposure were widespread in the past, but electronic dosimeters are becoming increasingly popular in modern times due to their increased accuracy.

Gamma rays and neutrons are the main focus of attention, as alpha and beta particles are far less penetrating. Protection methods are varied, but all are based on a few key principles:

  1. Limit exposure time - in many cases, simply reducing the amount of time people are exposed to radiation is the most effective solution.
  2. Limit distance - due to the inverse-square law, the dose rate from a radioactive source decreases exponentially with distance, this means that where possible, workers should be located as far away from sources as possible.
  3. Shielding - where previous principals cannot be applied, the area must be adequately shielded to reduce the intensity of ionising radiation emitted from the source; common shielding materials include lead, concrete or water. Materials with a high atomic number are preferred, though this can be compensated for with increased thickness; the exception to this rule are neutrons, which must be specifically shielded against with highly absorbing materials such as cadmium. Most shielding is layered in this way to protect against several forms of ionising radiation as it is unlikely that only one type will be present, some materials will even produce secondary radiation from the absorption of gamma rays, which must then be subsequently shielded.
  4. Containment - materials or areas considered highly dangerous must be isolated from the environment; reactor cores for example must have adequate containment to ensure that any radioactive material is confined to the containment vessel in the unlikely event of an accident.

Personal protective equipment (PPE) is essential when undertaking any work in a radioactive area, this ranges from simple paper filter masks, to self contained air fed suits, depending on the dangers present. PPE is described in more detail in the 'Decommissioning' chapter.

Further reading

TUDelft Understanding Nuclear Energy E-learning module contains several lectures on radiation safety.

International Commission on Radiological Protection 'ICRPaedia'.

The 2007 Recommendations of the International Commission on Radiological Protection. An ICRP publication on current radiation exposure protection standards can be found here.

References

[1] By OpenStax - https://cnx.org/contents/havxkyvSZIP Download:https://cnx.org/exports/85abf193-2bd2-4908-8563-90b8a7ac8df6@9.524.zip/chemistry-9.524.zip, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=64301360

Radioactive Wastes and Disposal

One of the consequences of utilising nuclear power is the generation of a radioactive by-product; these wastes are created throughout all stages of the nuclear fuel cycle, though the more hazardous material is formed during the fission of the fuel and its subsequent reprocessing. Despite this, the current volume of waste generated is easily manageable and issues of disposal are often more political than technological; the energy release versus waste volume ratio is also very favourable.

There has been extensive research into disposal but relatively little real world implementation; this chapter aims to give an overview of some of the main disposal options undergoing appraisal, as well as an insight into the classification and management of radioactive wastes.

Management Principles

Wastes are classified as either low, intermediate or high level by their radioactivity and ability to generate heat - each category has different properties that affect the way they are managed. Spent fuel is often designated its own category, where instead of waste, the fuel is considered a potential asset that may be reprocessed to extract value.

The principles of nuclear waste management are similar in many ways to those in other industries - waste production should always be kept as low as possible, and this is achieved by applying a concept known as the waste hierarchy:

  • Prevention - stop the waste being formed in the first place. This is normally achieved with good practice, such as removing the packaging from equipment or materials before entering radiation zones to reduce the volume of irradiated material.
  • Minimisation - where waste production is unavoidable, we should reduce the amount where possible. For example, better sorting and classification of wastes may allow more highly contaminated material to be separated from the bulk of the waste, allowing the rest to be recycled or sent to conventional landfill.
  • Reuse - material or equipment should be reused to extend its useful life and reduce the volume of waste produced. There are many examples of this in the nuclear industry - transport containers for nuclear material are decontaminated and inspected after each trip, allowing them to be reused many times; it would not be feasible to create a new container for each trip.
  • Recycling - converting waste into a new, useful form. Much of the metal and concrete used in nuclear reactors can be melted down or blasted with abrasive particles to remove the contaminated material, this can then be sold as raw product to be used elsewhere. This can vastly reduce the volume of waste produced, and the money raised from this can go back into aiding the decommissioning effort.
  • Energy recovery - while not always possible with radioactive substances, very low activity waste is often sent to incinerators where energy can be recovered via combustion; however, radionuclide emissions must be closely monitored and controlled to remain within yearly limits set by the governing body.
  • Disposal - the final, permanent solution; this should only be considered where all other options have been exhausted. Disposal methods will be featured later in this chapter.

Waste pyramid illustration

Figure 13 - An illustration of the waste hierarchy pyramid [1]

Higher activity waste is treated, immobilised and placed in steel containers, which are stored in specially engineered facilities for periods up to several years in a practice known as 'delay and decay'. This is done to allow any short lived, potentially heat generating radionuclides to decay to levels that make the waste much easier to manage, and may reduce the need for shielding and cooling during transport to a disposal facility.

Once the waste has been allowed to decay for a time, approaches to permanent disposal generally follow one of two principles, similar to those used for municipal waste. 'Dilute and disperse' was an approach common in the past for both nuclear and non-nuclear wastes, where material was simply dumped in the ground or on the sea bed and allowed to naturally dilute over time; this approach often caused problems, as hazardous material could easily leach and form dangerous contaminant plumes that migrated though the environment. This approach caused many problems in old domestic landfills, where methane gas would travel several miles laterally in the lithosphere and accumulate under people's homes, leading to large explosions when eventually sparked.

The more modern approach to waste management is known as 'concentrate and contain', where material is stored underground and lined with several engineered layers that isolate the waste from the environment and provide several barriers that must be breached before any leaching occurs. In nuclear installations, this multi-barrier approach is used, and every aspect of the wasteform acts as a separate containment for radioactive material.

Waste types and management

As mentioned previously, nuclear waste is divided into three main categories:

Low-level waste (LLW) is the lowest major classification of radioactive waste, describing material with a radioactive content that does not exceed 4GBq/t alpha or 12GBq/t beta-gamma activity; LLW does not provide much of a concern from a radiological protection point of view as it does not require shielding during transport or handling. Lightly contaminated gloves, tools, protective clothing, paper and other operational wastes comprise the bulk of the material and are produced throughout the nuclear fuel cycle; hospitals also contribute a moderate amount to the national LLW output.

Waste on the lower end of the LLW category may be reclassified as 'very-low level' or 'exempt' waste, allowing it to be disposed of alongside conventional waste in landfill, or sent to an incineration facility.

Non-exempt waste must be sorted and treated prior to disposal; metals are recycled, and the rest is compacted and stored in large steel containers , similar to those used in shipping, which are filled with cement to immobilise the waste and increase resistance to leaching - it is very important that the waste is evenly distributed in the cement to prevent 'hotspots' of activity. These containers are then sent to specialised near-surface facilities for disposal, such as the Low Level Waste Repository in Cumbria, UK.

LLW comprises 90% of the total volume of radioactive waste, but less than 1% of total activity.

Intermediate-level waste (ILW) exceeds the radiation limits for LLW, but is not significantly heat generating; this means that cooling is not required, but the increased activity may necessitate shielding. ILW consists of fuel cladding, neutron activated reactor materials, chemical sludges, and resins which may be cut up, dried, and compacted if necessary prior to immobilisation in cement. The waste is contained within 500 litre stainless steel drums, which are then held in interim storage until a disposal route is identified. Cast iron boxes are also being considered for larger items, though these must be much thicker due to iron's increased reactivity with water, though this does provide extra shielding. ILW is placed in larger shielded containers for transport.

It should be noted that ILW is not a recognised category in the USA.

ILW makes up 6% of total waste volume and 4% total radioactivity.

High-level waste (HLW) is the final major category of waste and provides the biggest challenges for disposal, as well as the biggest controversy. Strict definitions of HLW vary, but it is broadly any waste material which is sufficiently radioactive to be significantly heat generating, meaning shielding and cooling is always required. The main sources of HLW are fission products and transuranic elements produced during fission, contained within spent fuel elements or reprocessing liquors; due to their long-lived and highly radioactive nature, significant care must be taken during processing and storage to ensure that none of the material leaks into the environment.

HLW makes up 3% of the total waste volume and 95% total radioactivity

Nuclear waste container

Figure 14 - Nucelar waste transport flask [2]

HLW undergoes a process known as vitrification, where material is mixed with glass forming compounds and heated to form a molten product, which can then be poured into 150 litre stainless steel drums to form a solid, stable wasteform that is very resistant to leaching and will effectively contain the waste for thousands of years. These containers are then kept in storage to decay for up to 50 years, before being sent to a permanent disposal facility if available. As with ILW, shielded transport containers are used to transfer HLW between sites, as it is not contact handleable.

SYNROC (synthetic rock) is an alternative to vitrification that is currently undergoing consideration, where radionuclides are incorporated within the crystal lattice of the material, providing effective isolation from the environment. SYNROC is a ceramic material composed of various minerals that are very absorbing of problematic radionuclides, the composition of SYNROC can be altered to suit the particular waste being immobilised, which is a major advantage over vitrification. As SYNROC is made up of natural materials, its behaviour in nature over long time periods is well known, which is useful for geologic disposal considerations. SYNROC is combined with the waste, which is then pressed at high temperature to produce a hard, dense rock like material; despite the advantages of SYNROC, the widespread adoption of vitrification means that the latter is often preferred for waste management.

Spent fuel (SF) may or may not be considered waste depending on current government policy. In reprocessing nations, SF is considered an asset based on potential future value and is stored in cooling ponds to await processing. Where spent fuel is not considered an asset, it is treated in much the same way as HLW, and will be disposed of in a similar manner, following an initial decay period. As SF is already contained within its cladding and the fuel pellet itself, processing is much simpler.

Disposal

Final disposal marks the end of the nuclear fuel cycle and aims to isolate waste from people and the environment indefinitely. The challenge of disposal increases with the level of activity in the waste, and whether the waste is heat generating. LLW for example is relatively easily disposed of, either in specialised surface facilities or regular landfills for VLLW; ILW and HLW are much more difficult to manage, and are currently sent to interim storage to decay and await a permanent disposal solution.

For these more problematic wastes, deep geological disposal is the preferred option for most nations, though little progress has been made in actually siting and constructing these facilities; the exceptions to this are Finland and Sweden, which have both successfully identified and licensed sites for their disposal facilities, with initial wastes expected to be accepted within the next ten years.

Any disposal method must contain the waste for thousands of years, and doses inside any facility must be kept as low as reasonably achievable, within prescribed dose limits to account for the possibility that the structure may be purposefully (or accidentally) accessed in the future.

Multi-barrier system

The multi-barrier system is a concept that is present in most planned long term disposal options. As mentioned previously, each aspect of the wasteform and disposal facility acts as a separate barrier that must be overcome before radionuclides are able to penetrate into the environment. The multi-barrier system is as follows:

  • Wasteform - the wasteform should be robust and resistant to leaching; usually immobilised in borosilicate glass, cement or resin.
  • Container - the container should act as a physical barrier that resists corrosion for a long period of time - up to several thousands of years; also allows easier handling and transport of wastes. Usually stainless steel, thick cast iron or copper.
  • Buffer - the buffer is the backfill that surrounds the containers, this is usually a form of swelling clay (bentonite) or cement that acts to slow the release of radioactive material if leaching occurs.
  • Mass backfill - access tunnels and shafts must also be filled to inhibit the flow of groundwater and also provide strength to the whole structure. The design of the facility itself should also help to divert the flow of water away from key areas.
  • Geology - the local geology acts as the final barrier to the leaching of radionuclides into the environment. Many lithologies are suitable for a disposal facility, and this shall be explored in more detail in the next section.

Radwaste container

Figure 15 - A mockup of a stainless steel waste container [3]

Deep geological disposal

Geological disposal is the most popular and consequentially most studied option for intermediate and high level wastes, and involves the burial of material hundreds of metres deep within stable geological strata in engineered storage vaults or boreholes. As wastes remain highly active for long timescales, any facility must provide containment for potentially hundreds of thousands of years to allow the activity to reach acceptable levels for exposure to the environment; use of geological systems helps prolong this containment, as contaminant flow through bedrock can take hundreds of years in low groundwater flow systems, which provides more than adequate time when combined with the multi-barrier approach of the wasteform and facility.

Systems suitable for disposal must be geologically stable and accessible by land, they may constitute:

  • High strength rocks - igneous rocks such as granites are tougher to excavate, but provide good structural integrity for the construction of an underground facility. While porosity is low in these rocks, fractures can allow fairly fast fluid flow which can be an issue for waste containment.
  • Low strength rocks - soft sedimentary rocks such as clays or marls act as 'aquitards', meaning groundwater flow through them is negligible; this is excellent for waste disposal as the release of radionuclides into the environment can be heavily slowed. The low structural integrity of these formations makes construction difficult, though they are much easier to excavate, and clay rich systems are self healing, meaning they will seal any fractures that may occur.
  • Evaporites - large salt deposits provide excellent containment for radioactive wastes. As with clay rich systems, groundwater flow in evaporites is extremely slow; the ductile nature of these deposits also means that over time, access tunnels and vaults will completely self seal to a larger extent than in clays, reducing the need for backfill. However, large enough salt deposits are not common, and there is some uncertainty over their structural properties over long time periods.

A geological disposal facility (GDF) is essentially a large underground cavern dug into bedrock that is used to store highly active wastes; they are the disposal option favoured by most nuclear states. HLW and SF is stored in a separate section of the facility to ILW due to its heat generating nature, this also means that HLW canisters cannot be stacked, leading to a much larger footprint for this end of the facility, despite the smaller actual volume of waste. Access tunnels and processing buildings are also hollowed out of the rock to allow workers to receive and organise wastes, though handling of canisters inside the vaults must be done robotically, as the waste storage facility will not be man-access. GDFs utilise the multi-barrier approach as outlined previously, and all shafts, tunnels and vaults will be backfilled upon closure, sealing the waste inside the facility for (hopefully) thousands of years.

An alternative to a large underground facility is the use of deep boreholes to contain wastes. These are similar in principal to a GDF, though the overall footprint is much smaller; a line of boreholes is dug deep into the underlying rock and waste containers are stacked on top of one another, surrounded by a clay or concrete backfill. An immediate limitation occurs for HLW, as there will be a limit on the number of containers that can be safely stored in a borehole due to heat generation, the overall capacity of the technique is also much lower than for a GDF. An advantage of boreholes is that they may be sited offshore, as drilling technology is well established due to its use in the oil industry, the wastes are also totally irretrievable - which while good for security and proliferation concerns, it may be an issue if the waste becomes an asset in the future. The low capacity of boreholes makes them off-putting for large scale disposal of radioactive waste from the nuclear industry, though they may prove more attractive for small scale wastes, such as those from medicine or research; boreholes may also be utilised within a GDF, as a kind of hybrid design.

Generic design concept for a GDF

Figure 16 - Generic design concept for a geological waste facility - note the much larger HLW/SF section - NDA [4]

Near-surface disposal

Not all radioactive waste disposal concepts take place deep underground, near-surface disposal involves the storage of nuclear waste on or near the surface, within a 100m or so. Canisters are stored in vaults, which are backfilled with clay, buried, and lined with engineered membranes to contain leach fluids and prevent rainwater from penetrating the facility, similar to modern landfills. This type of facility is more common for LLW material, though some nations such as Scotland have been considering this technique for their higher activity wastes. One thing to consider with near surface facilities is their susceptibility to surface processes such as erosion and glaciation, as the depth of the structure is not sufficient to adequately protect from these events, this means that nuclear material has a chance of re-emerging to the surface following a major climatic or geological event.

Other methods of disposal

While the options discussed earlier in this section are the most common concepts for radioactive waste disposal, several other ideas have been researched - with varying feasibility:

  • Long term interim storage - one option is to simply leave wastes in their 'temporary' storage locations over large timescales. While this approach is simple and low cost, the security concerns are very large; the facilities will also require constant upkeep and this burden will fall onto future generations.
  • Disposal in subduction zones - a subduction zone occurs at convergent boundaries of the Earth's crust, where a denser section of crust (usually oceanic) connects with a more buoyant section (e.g. continental), forcing the denser plate to sink under gravity into the Earth's mantle; such areas are accompanied by large scale volcanism such as the ring of fire surrounding the Pacific Ocean, and the formation of deep trenches. Such trenches could be used to dispose of radioactive wastes, which would be subducted and melted, though the actual behaviour of the wastes in this kind of environment is not well known; disposal of nuclear material in international waters is also prohibited by several agreements.
  • Sea bed disposal - waste containers could be simply placed on or within the sea bed, allowing the wastes to dilute over time into the ocean. While this approach was actually taken in the past by many nations for LLW and ILW, international agreements since then have prohibited this kind of disposal.
  • Outer space - a more outlandish option is to attach waste containers onto rockets to eject into space, removing the waste from the Earth completely. While totally removing the waste is an attractive option, the dangers and expense involved with rocketry are far too great to ever consider with current technology, but who knows what the future holds.
  • Transmutation - an interesting concept is to partition and transmute long lived radionuclides into more stable or short lived forms by forcing the capture of a neutron or causing fission. While this approach may lead to some nasty short lived radionuclides, the timescales of these wastes will be greatly reduced, reducing the need for long term containment.

Informational videos

The two videos describe modern radioactive waste disposal methods.

Further reading

TUDelft Understanding Nuclear Energy - E-learning module contains lectures on radioactive wastes.

The IAEA 'Spent Fuel and Radioactive Waste Management, Decommissioning and Environmental Remediation' - E-learning module contains several in depth lectures on all aspects of radioactive wastes, management and disposal.

The World Nuclear Association has a good article on waste treatment here.

NDA management of nuclear waste page.

References

[1] By Drstuey at the English language Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=3518269

[2] Bill Ebbesen - Transferred from en.wikipedia to Commons., CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=11432531

[3] By Tiia Monto, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=34944259

[4] http://www.westcumbriamrws.org.uk/images/0810-02-NDA%20-%20UK%20Geological%20Disposal%20Facility%20Concept%20for%20higher%20strength%20rock.jpg - Courtesy of the NDA

Decommissioning and Site Restoration

Decommissioning of a nuclear power plant involves the safe dismantling and demolition of all buildings and infrastructure onsite, alongside the progressive removal of all radioactive material; it occurs when the facility reaches the end of its useful life, i.e. maintaining operation of the plant is no longer economically viable. Decommissioning of a plant is inevitable, though many operators aim to delay the process as much as possible by upgrading and maintaining areas of the facility to extend the lifetime of the reactor past its original specifications, this is often done to avoid the formation of an 'energy gap' once a reactor is switched off if no replacement is available, or to take advantage of new technology being developed.

Much of a nuclear site remains uncontaminated throughout its life, making the majority of decommissioning work fairly simple, only areas that are subject to high levels of radiation pose a problem, such as reactor components and shielding. Older sites can often be more challenging than newer ones; early reactor buildings were not designed with decommissioning in mind, this combined with poor practices in the past can lead to very complex clean-up projects that infamously run well over budget and feature prominently in sensationalist newspaper headlines. Many of the mistakes of the past have been rectified, and new-build reactors are designed specifically to make the decommissioning phase simpler and cheaper; despite this, decommissioning is still a long and expensive process, and should always be planned for well in advance.

Nuclear plant undergoing deconstruction

Figure 17 - A nuclear plant undergoing dismantling, the containment building can be seen. [1]

Why decommission?

At the most basic level, we decommission because it is required by law; the UK government policy Cm 2919 for example calls for decommissioning to begin as soon as reasonable practicable (ALARP) to deliver a systematic reduction of radioactive hazards.

Decommissioning also takes place in response to safety concerns; old reactor buildings and containment will be less stable and harder to manage the longer they are left unused. Leaving radioactive material on site for extended periods of time is also a security risk, meaning constant surveillance will be required while the site is still under nuclear licence.

Economic factors also contribute to the decision. The operating efficiency of a plant will decline over time until it becomes unfeasible to continue operation, at which point the decommissioning process will begin. It is often prudent to begin work as soon as possible to avoid excess security and maintenance costs while the site is generating no profit; the eventual site restoration will allow the land to be used for other purposes, rather than acting as a 'money drain'.

Approaches to decommissioning

There are many strategies for implementing decommissioning work; the IAEA outline three major options that are taken into account when deciding on national policy:

  • Immediate dismantling aims to begin decommissioning work shortly after the facility ceases to operate, with the intention of releasing the site from regulatory control as soon as possible. Following an initial transition period, decommissioning efforts will continue in phases until the site has reached a state deemed acceptable to be released from its nuclear site licence by the regulatory body; this is usually when the site is demonstrably free from any radioactive hazard. This method allows the land to become economically productive again sooner; it also reduces the amount of job cuts that need to be made in the short term as the expertise from existing workers can be utilised throughout the decommissioning process. While this option reduces the burden on future generations, it does not allow radioactive material to decay for an extended period, increasing the difficulty of the work.
  • Deferred dismantling is an alternate approach that aims to place the site in a state of passive safety in preparation for an extended period of monitoring to allow radionuclides to decay or technology to improve; the deferral period may be upwards of half a century, after which the main reactor buildings will be decontaminated and decommissioned. During preparations for this phase, the reactors will be defueled to remove the majority of the radioactive hazard on the site. While this method allows short-lived radionuclides to decay, it increases the length of time that the land is effectively unused, and thus increases the burden on future generations.
  • Entombment describes a strategy in which radioactive material is permanently stored on site following defueling of the reactors. The overall area containing radioactivity will be reduced in scope and remaining material will be encased in a concrete structure - effectively forming a repository, which should provide long lasting protection against exposure to people and the environment. While this method is simple and relatively quick, it does not allow the land to be reused in any way; the permanent (near) surface storage of ILW may also prove to be an unacceptable hazard to many nations.

All of the above options will be considered by governing bodies and a combination of approaches may be taken; it should be noted that a policy of 'no action' is never considered acceptable.

Cost

Financial expenditure must never be a barrier to decommissioning or any kind of radioactive hazard reduction/removal. Budgeting for this phase must be completed and suitable insurances taken out before the reactor is even built, and this is often required by the regulatory body, though some operators dedicate a percentage of yearly operational revenue towards a decommissioning fund instead. The operator of a nuclear site is responsible for all costs of delicensing and the approach taken will dictate the extent of these, it is often a balancing act between maintenance and monitoring costs versus the benefits of deferred dismantling.

End-life phases of a nuclear plant

The following gives a walkthrough of the life cycle of a nuclear power plant undergoing a variant of the deferred dismantling strategy. The procedure equally applies to immediate dismantling strategies, excluding extended care and maintenance periods; entombment options more closely resemble waste disposal principals (see: Waste Management chapter).

Phase 1: Generation

Most nuclear reactors have a commercial life of around 30-40 years, though this is often extended by several years depending on demand; over its lifetime, a reactor will supply several tens of terawatt hours of electricity to the national grid [2]. Once it is deemed uneconomical to continue running the reactor, the decommissioning phase will begin.

Phase 2: Defueling

As the name implies, most of the work in this phase involves the removal of nuclear fuel from the site, either to a reprocessing facility or to permanent disposal. Defueling removes 99% of the radioactive hazard from the site and allows other decommissioning work to proceed, though this phase can take several years due to monthly activity limits that gauge how much radioactive material a reprocessing/disposal facility is allowed to accept. Loose surface contamination is also cleaned up during this phase, alongside the removal of hazardous substances or chemicals from the facility, such as asbestos.

Phase 3: Preparation for safe enclosure

The main aim of this stage is to prepare the facility for a period of 'care and maintenance', and involves the demolition of most of the structures on site - with the exception of reactor buildings, which are sealed and placed into a state of passive safety. A waste processing and storage facility is built during this phase, allowing most of the radioactive material on site to be containerised and stored. Workforce restructuring will also take place in preparation for the extended maintenance period, which will operate on minimal staff levels.

Phase 4: Safe enclosure

Once preparations are complete, the site will enter an inactive phase of care and maintenance, where no major decommissioning work will take place. The period may last several decades to allow short lived radionuclides to decay, and it is imperative that the site is in a passively safe condition throughout this time; meaning no active safety measures such as coolant pumps or ventilation systems are necessary. The remaining buildings will be periodically inspected and maintained to ensure that no significant deterioration of the containment occurs; the surrounding land will also be tested and monitored to ensure that no leaks occur to the environment. The workforce requirements are minimal for this stage, mostly involving security and repair work.

Note: this phase does not feature in immediate dismantling strategies.

Phase 5: Final site clearance

The final stage in the life plan of a nuclear facility is done with the aim of terminating the nuclear site licence, meaning the operator no longer has responsibility over the site and the land can be sold or redeveloped. To achieve this goal, the site must be in a condition considered suitable for the next land use and demonstrate that no radioactive hazard remains on site, i.e. radiation levels are at or below background; the local community should be heavily involved in the process of deciding the fate of the land once remedial work has been completed.

Remaining buildings and infrastructure on site should be decontaminated in preparation for demolition, with particular care taken with reactor buildings - sections of which will have to be processed and stored as waste; an additional facility will likely be built for this purpose. Any remaining containerised wastes on site will be sent to a disposal facility and the storage building itself will be demolished; once all this work is completed, the land will be remediated and sculpted if necessary. Regulators will then inspect the site, and if all conditions for delicensing have been met, the land will be cleared for further use, and the operator will be free of responsibility.

Decontamination

The term decontamination has been thrown around a lot in this resource, but what actually is it? and how is it achieved?

Decontamination is simply the process of removing contaminants from the surface, pores and fissures of an object or material; contamination may be anything from harmful microbes to radioactive substances, though naturally we focus on the latter. Decontamination methods belong to one of three main groups:

  • Non-attritive: non abrasive techniques such as vacuuming, sweeping, washing and scrubbing.
  • Chemical: methods involving chemical solutions, such as the use of airborne or spray reagents and foams.
  • Physical: abrasive solutions, such as high pressure water jets, sand blasting, shaving, milling and drilling.

Personal protective equipment (PPE)

The use of PPE is essential in many reactor operation or decommissioning projects and the level of protection necessary depends on the risks and consequences associated with the work.

Low consequence work, or areas with large particulate contamination require the lowest levels of protection; a respirator and a set of overalls are used to prevent the inhalation of dangerous particles and prevent them from entering breaks in the skin.

Medium consequence work requires more robust protection, a rubber air hood that covers the head and shoulders with its own filtered air supply is used alongside overalls to protect against finer particles.

High consequence projects necessitate the use of a full air-fed suit, which covers the entire body and airways to provide total protection against particulate contamination. The suit is under slight positive pressure, which means air is blown out of the suit in the event of a puncture, reducing the extent of contamination inside the suit. Work duration should be limited in high risk areas, as PPE cannot provide much protection against penetrating radiation.

Workers in personal protective equipment

Figure 18 - An example of an inspector wearing overalls and a respirator. [3]

Further reading

TUDelft Understanding Nuclear Energy - E-learning module contains a lecture on decommissioning.

The IAEA 'Spent Fuel and Radioactive Waste Management, Decommissioning and Environmental Remediation' - E-learning module contains several in depth lectures on decommissioning and site restoration.

IAEA - Decommissioning strategies document

Interesting Scientific American article on reactor lifetimes

References

[1] By Nuclear Regulatory Commission - http://www.nrc.gov/reading-rm/photo-gallery/index.cfm?&cat=Nuclear_Reactors, Public Domain, https://commons.wikimedia.org/w/index.php?curid=3429592

[2] https://magnoxsites.com/wp-content/uploads/2014/03/Bradwell-Lifetime-Plan.pdf

[3] By IAEA Imagebank - https://www.flickr.com/photos/iaea_imagebank/8656859137/, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=56218966