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Radioactive Wastes
(March 2001)
Radioactive waste is produced through the generation of electricity using nuclear fission. It also arises through coal fired electricity generation and is released into the environment through oil exploration.
What makes the nuclear power industry virtually unique though when it comes to dealing with its waste products is that the costs of waste disposal are incorporated into the electricity sold. The costs of managing and disposing of wastes from nuclear power plants represent about 5% of the total costs of electricity generated. Nuclear power is the only bulk energy producing technology that takes full responsibility for all its wastes even though the amount of these wastes is very small relative to those produced by using fossil fuels for electricity generation.
Every part of the nuclear fuel cycle produces some radioactive waste and the cost of managing and disposing of this 'radwaste' is built into the process. Uranium mining for example generates fine sandy tailings, which contain virtually all the naturally occurring radioactive elements found in the uranium ore.
What is radiation?
Everyone is constantly exposed to radiation. When considering radioactive materials and especially wastes the objective is to avoid increasing that exposure significantly. At high levels radiation is dangerous and hence it is important to shield such radiation from people.
Radiation occurs naturally from the decay of particular forms of some elements (radioisotopes) and there are three main types of radiation alpha, beta and gamma. Radioactive emission takes place as an atom disintegrates. As the number of radioactive atoms is reduced by each disintegration, the rate of radioactive emission is said to decay or lessen with time. The half-life of a radioisotope is the time taken for half of its atoms to decay. So something with a long half-life such as uranium 238 (a half life of 4.5 billion years) gives out very low levels of radiation albeit over a geological time scale. Something with a short half life such as radon 220 (half life 56 seconds) emits very much more radiation over a shorter time.
Categories of radioactive waste:
A large portion of radioactive waste produced from the nuclear fuel cycle has radiation levels similar to, or not much higher than, the natural background level. This waste is relatively easy to deal with. Only a small proportion is highly radioactive and requires isolation from people. The general considerations for classifying radioactive wastes are; how long the waste will remain at a hazardous level, what the concentration of the radioactive material in the waste and whether the waste is heat generating.
The persistence of the radioactivity determines how long the waste requires management. The concentration and heat generation dictate how the waste should be handled. These considerations also inform suitable disposal methods. The classification varies slightly from country to country. However in general the internationally accepted categories are.
Classification of radioactive wastes
There are several systems of nomenclature in use, but the following is generally accepted:
Very low level waste or exempt waste. These categories contain negligible amount amounts of radioactivity and may be disposed of with domestic refuse.
Low-level Waste comprises the bulk of waste from the nuclear fuel cycle. It comprises paper, rags, tools, clothing, filters etc which contain small amounts of mostly short-lived radioactivity. It does not require shielding during handling and transport and is suitable for shallow land burial. To reduce its volume, these wastes are often compacted or incinerated before disposal. Disposal sites for low-level waste are in operation in many countries. Worldwide they make up 90% of the volume but have only 1% of the total radioactivity of all radioactive wastes.
Intermediate-level Waste contains higher amounts of radioactivity and normally requires shielding. Shielding can be barriers of lead, concrete or water to give protection from penetrating radiation such as gamma rays. Intermediate-level wastes typically comprise resins, chemical sludges and metal fuel cladding, as well as contaminated materials from reactor decommissioning. It may be solidified in concrete or bitumen for disposal. Generally short-lived waste (mainly from reactors) is buried, but long-lived waste (from fuel reprocessing) will be disposed of underground.
High-level Waste (HLW) contains the fission products and transuranic elements generated in the reactor core which are highly radioactive and hot. High-level waste accounts for over 95% of the total radioactivity produced though the actual amount of material is low, 25-30 tonnes of spent fuel. or three cubic metres per year of vitrified waste for a typical large nuclear reactor (1000 MWe, light water type).
Spent fuel is generally removed from the reactor core under water, and transferred to large water filled pools where the fuel is held on racks underwater. The water both shields the radiation and cools the spent fuel which may be destined either long term storage or reprocessing.
Management of higher level waste:
There are two types of high level waste, fission products and transuranics separated from the spent fuel and the spent fuel elements themselves from the reactor core when they are not reprocessed. Both types of HLW must be treated prior to disposal. HLW from reprocessing is incorporated into solid blocks of borosilicate glass. This process is known as vitrification. For direct disposal, spent fuel requires encapsulation in containers made, for example, of stainless steel or copper.
For reprocessing when the fission products are first extracted from the spent fuel they are in liquid form, having been dissolved in acid (usually nitric acid). This liquid can be safely retained in stainless steel tanks that are equipped with cooling systems until it is converted into a solid, which is a more convenient material for management, storage, transport and disposal. After drying it is incorporated into molten borosilicate glass which is allowed to solidify inside corrosion resistant canister. Vitrification produces a stable solid that has the high-level waste incorporated its structure.
In either case however there is a cooling period of 20 to 50 years between removal from the reactor and disposal, with the conditioned spent fuel or conditioned HLW being retained in interim storage. This is because the level of radioactivity and heat from the used fuel fall rapidly in these years down to about one thousandth of the level at discharge in 40 years. Such long term storage facilities may be at one central place as in Sweden or at the reactor site, as in the US. They may again be underwater or dry storage, where circulating air removes the heat generated by the spent fuel. The structure and design of both the building and containers protects the outside world from radiation exposure and the fuel from potential outside hazards. Many countries are developing plans for disposal of HLW in geological repositories buried in stable rock formations hundreds of metres beneath the surface.
Management of operational waste: low and intermediate waste
The intermediate-level waste (ILW) along with the low-level waste represent some 90% of the total volume of radioactive waste generated during the lifetime of a nuclear power plant. This relatively large volume of long-lived and short-lived ILW contains only about 1% of the total radioactivity. Only a small proportion of the intermediate-level waste remains significantly radioactive for years but all ILW requires shielding when it is handled. Low-level waste (LLW) and short-lived intermediate-level waste is of three kinds:
Process wastes result from the treatment, purification and filtration systems of fluids in direct contact with the parts of the reactor that may be contaminated by radioactivity. These wastes include:
Technological wastes arise from the necessary maintenance carried out on a nuclear power plant. Technological waste represents half the volume of LLW and short-lived ILW, but contains little radioactivity.
Solid technological wastes might contain rags, cardboard, plastic sheets, bags, tools and protective clothing. Liquid technological wastes comprise mainly oils, small amounts of lubricants and organic solvents used for decontamination.
Decommissioning wastes occur at the end of a nuclear reactor's life. After the spent fuel is removed the plant is decommissioned and eventually demolished. During this process, large amounts of wastes are generated, though most is not radioactive. About a tenth of it contains some radioactivity up to the intermediate level.
Plant operators make constant efforts to reduce the quantities of waste that are generated. Waste is collected, sorted and then conditioned. The management strategy chosen depends upon the origin and radioactivity level of the waste. LLW, with the lowest concentrations of radioactivity, is usually retained in metal drums, which are often compacted after filling to reduce the volume. Other techniques may also be used to effect volume reduction. These include: melting of metallic waste, incinerating of the combustible parts of waste (whilst retaining the radioactive ash) and super-compacting waste to reduce the total volume further.
Low level wastes that contain slightly higher radioactivity levels are stabilised by cement or an organic solid (bitumen or resin) and then placed in concrete containers for shielding. Disposal sites for such wastes are in operation in many countries. Typically, these are shallow earth burial sites, which provide a suitable facility to contain the wastes safely. A 1000 MWe nuclear power reactor can be expected to produce around 100m 3 of low level waste every year.
Long-lived intermediate level waste
Typically, these wastes arise from dismantled internal structures of the reactor core, which become radioactive after prolonged operation. They also include: the control rods, which regulate the nuclear reaction, the source assemblies, which are used to initiate a nuclear reaction after new fuel has been loaded, and other rods that limit the reactivity of fresh fuel. ILW is treated and conditioned by incorporating it into cement and then placing it in concrete containers. In some instances, the conditioned waste might subsequently be placed into an additional container, made of metal. Special packages are used for transporting long-lived intermediate level waste. These packages meet internationally approved standards that ensure that the waste is safely contained.
Ultimately long-lived ILW will go to deep geological disposal as with high-level waste.
Sweden has already done this but in most countries, long-lived waste is being safely stored and contained at interim storage facilities. The maintenance of a 1000 MWe nuclear power reactor produces less than 0.5m 3 of long-lived ILW each year. If the spent fuel goes for reprocessing, then the cladding from the spent fuel adds an additional 3m 3 of ILW.
Spent Fuel: Reprocessing and Recycling
Fresh uranium oxide fuel contains up to 5% U-235. When the fuel reaches the end of its useful life, it is removed from the reactor. At this point it typically contains about 95% U-238, 3% fission products (the residues of the fission reactions) and transuranic isotopes, 1% plutonium and 1% U-235. The plutonium is produced by the neutron irradiation of U-238.
Spent fuel still contains about a quarter of the original fissile U-235 as well as much of the plutonium which has been formed in the reactor. Reprocessing separates out this uranium and plutonium. Several reprocessing facilities, Sellafield in the UK, La Hague in France, and Chelybinsk in Russia are in operation. The wastes left after reprocessing can then be disposed of, while the uranium and plutonium may be recycled for use in a nuclear reactor as mixed oxide (MOX) fuel. This is called the 'closed fuel cycle' because the useful ingredients of spent fuel are recycled.
With the recycling option the energy potential can be realised in new nuclear fuel since Pu-239 and U-235 contained in the spent fuel are fissile.
Waste from reprocessing:
The reprocessing of spent fuel gives rise to low, intermediate and high level wastes:
High level waste comprises the non-reusable part of the spent nuclear fuel itself both fission products and transuranic elements other than plutonium. The fission product leftovers are vitrified, i.e. incorporated into glass. Hulls and end fittings from the fuel assemblies are compacted, to reduce the total volume of the waste, and are frequently incorporated into cement before being placed into containers for disposal as ILW.
The major commercial reprocessing plants operating in France and UK also undertake reprocessing for utilities in other countries, notably Japan. Most Japanese spent fuel is reprocessed in Europe, with the vitrified waste and the recovered uranium and plutonium (as MOX) being returned to Japan to be recycled.
Recycling:
Among the benefits of recycling identified by those countries that are utilising MOX fuel are conservation of uranium, minimising the amount of high-level radioactive, reducing reliance on new uranium supply, reducing the fissile plutonium inventory and reduction of spent fuel storage requirements.
Plutonium recycling
Plutonium is recycled through a special fuel fabrication plant to produce mixed oxide (MOX) fuel. MOX fuel is a mixture of plutonium and uranium oxides (formed from natural, depleted or reprocessed uranium). MOX fuel containing 5 to 7% plutonium has characteristics that are similar to uranium oxide based fuel and used as part of a reactor's fuel loading. There are 34 reactors licensed to use MOX fuel across Europe with seventy-five others in the licensing process. Japan for example has plans to introduce MOX fuel into twenty of its reactors by the year 2010. It should be noted that plutonium arising from the civil nuclear fuel cycle is not suitable for bombs because it contains far too much of the Pu-240 isotope, due to the length of time the fuel has been in the reactor.
Uranium recycling
Uranium from reprocessing, sometimes referred to as Rep-U, must usually be enriched, and to facilitate this it must first be converted to UF6.
International organisations- and safety standards
In 1997, 'The Joint Convention on the Safety of Spent Fuel Management and the Safety of Radioactive Waste Management' was adopted by a diplomatic conference of the International Atomic Energy Agency (IAEA). It is the first international agreement on the safety of spent fuel and the management of radioactive waste. It is due to come into force in June 2001. A full copy of this document can be found on the IAEA website.
The international nuclear industry has an exceptional safety record in the management of its wastes. The industry has managed these wastes since they were first generated over forty years ago. The rules governing the management of radioactive wastes are more stringent than those for any other type of hazardous material. Current practice provides effective mechanisms for the management of highly radioactive waste in interim storage facilities. However, several countries are carrying out research to develop and engineer new facilities that will provide a permanent repository for these wastes.
Developing of safety standards is based on the wealth of experience provided through several national and international organisations.
The IAEA: The International Atomic Energy Agency (IAEA) is the international organisation that oversees the peaceful uses of atomic energy. It is an agency of the United Nations, that is based in Vienna, Austria and was founded in 1957. The IAEA has about 115 member states, including both countries with nuclear energy programs and countries without. The IAEA develops safety standards, guidelines and recommendations and provides technical guidance to member states regarding radiological practices and protection. Member states use the standards and guidelines in developing their own legislation, regulatory documents and guidelines. The IAEA'S Waste Safety Section works to co-ordinate the development of internationally agreed standards on the safety of radioactive waste. In addition, the IAEA helps member states by providing technical assistance with services, equipment and training and by conducting radiological assessments.
The OECD/NEA (Nuclear Energy Agency of the Organisation for Economic Co-operation and Development) is based in Paris, France. Many of its activities are similar to those of the IAEA. It has a variety of waste management programs involving its 27 member states. It works closely with the IAEA on nuclear safety standards and other technical activities
Commission of the European Communities: The European Commission supports research and development projects, sponsors international symposia and provides training opportunities in a number of areas including radio- active waste management. It co-operates with the IAEA on many of its programs.
Disposing of high level wastes
Final disposal of high level wastes is required in due course but there is no technical or logistical reasons why this is urgent. Rather the contrary, the longer HLW waste is in storage, the easier it is to handle safely. HLW is accumulating at about 12,000 tonnes a year worldwide. High-level wastes are highly radioactive for a long time so must be isolated from people for thousands of years while their radiation levels drop.
Geological repositories are planned in stable rock formations in the main countries utilising nuclear energy. It is the responsibility of each country to dispose of its wastes. Typically a repository will be 500 metres down in rock, clay or salt. The idea is a multiple barrier concept:
Other means of stabilising high-level waste are at the research stage. One of the more advanced is a substance called Synroc. This is an advanced ceramic principally comprising three natural titanate minerals which are geo-chemically stable and which together have the capacity to incorporate into their crystal structures nearly all of the elements present in high-level radioactive waste, thereby immobilising them.
There is an interesting example from nature of long term geologically storage over millions, of years. Several nuclear reactors were discovered in 1972 at the Oklo uranium mine in the West African republic of Gabon. The deposit of ore, which contained about 3% U-235, began a self-sustaining chain reaction millions of years ago. Like all reactors, this one created its own high-level waste, up to 5000 kg of fission products and transuranic elements which today are found only in used fuel. The Oklo chain reaction occurred intermittently for more than 500,000 years. Despite its location in a wet, tropical climate, Oklo's uranium deposit and high-level waste has remained securely locked in this natural repository for the past 2000 million years. Many of the waste products stayed where they were created or moved only a few centimetres before decaying into harmless products.
Such natural analogues provides practical confirmation that disposal of today's nuclear wastes in the manner outlined will be both secure and safe indefinitely.
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