High level waste is very radioactive, mainly because of the presence of fission products, and generates a great deal of heat. This heat generation has to be taken into account when storing HLW and designing facilities for its management in the long term. HLW arises at Sellafield as a highly radioactive nitric acid liquid waste solution following the separation of uranium and plutonium from spent nuclear fuel during reprocessing. These liquid wastes are being converted into a passively safe solid form, as borosilicate glass within 150-litre stainless steel containers, using a process called vitrification. Once vitrified, the solid waste is stored in a naturally cooled, shielded concrete store.

Intermediate level waste is less radioactive than HLW, and does not generate sufficient heat for this to be taken into account in the design of the facilities for management, but can require significant shielding. Some of the waste can have a very long half-life. The form of the waste is much more varied than HLW. Major components include metal items, such as nuclear fuel cladding and reactor components, graphite from reactor cores, and sludges from treating radioactive liquid effluents, as well as some wastes from medical and industrial users. It is stored in tanks, vaults and drums, usually with some shielding to protect operators from radiation. Waste is progressively being immobilised in cement-based materials within 500-litre stainless steel drums or, for larger items, within high capacity steel or concrete boxes.

Low level wastes are less radioactive than ILW and consist largely of redundant equipment, protective clothing and packaging. In future, as nuclear plants are decommissioned, the main components will include soil, concrete and steel items such as ducting, piping and reinforcement. Most of the existing LLW is disposed of at the national Low Level Waste Repository (LLWR) near Drigg in Cumbria. It is compacted, packaged in large metal containers and put in an engineered vault a few metres below the surface. Some LLW that does not meet the LLWR site acceptance criteria because of its radioactivity or physical/chemical properties, is included in the inventory for geological disposal.

Materials that may be declared to be waste

Spent nuclear fuel is a mixture of plutonium, uranium and waste materials. It can be reprocessed to extract the plutonium and uranium; or it can be managed in another way, such as packaging it and placing it in a waste repository, as is planned in a number of countries that have nuclear power stations. The UK stocks of non-reprocessed spent fuel consist of arisings from Magnox and Advanced Gas-cooled Reactors and from the Sizewell B Pressurised Water Reactor, along with submarine reactor spent fuel and smaller quantities of specialist fuel from research and prototype reactors. There are also small quantities of unirradiated fuel that are included in the spent fuel category.

Plutonium is created as a by-product of the use of uranium fuel in nuclear reactors and is used in nuclear weapons. It can represent a significant health hazard if inhaled or ingested. It is contained within spent fuel removed from a reactor, but can be extracted by reprocessing the spent fuel. Plutonium could, in this extracted form, be used to make some reactor fuels. The UK’s stockpile of plutonium is stored at Sellafield and future arisings from reprocessing will be stored at the same place. The NDA is evaluating the options for managing this stockpile, which include consideration of use in reactor fuel and various methods of encapsulating it as a waste.

Uranium is found naturally, and can be processed to give highly enriched, low enriched and depleted uranium (these forms having different concentrations of uranium-235). It is also a product of the reprocessing of spent fuel and can have similar uses to plutonium. Less radioactive (‘depleted’) uranium has a wider range of uses, including artillery shells and aircraft counterweights. Uranium is stored at several nuclear sites and future arisings from reprocessing will occur at Sellafield. As with plutonium, the NDA is evaluating options for managing this stockpile. The vast majority of the uranium inventory is depleted and the potential risks associated with this material are considerably less than for most other materials in the inventory of wastes and materials that may be declared to be waste.

Processes that give rise to wastes

Descriptions of the processes that give rise to radioactive wastes are given in the 2007 UK Radioactive Waste Inventory report 'A Review of the Preocesses Contributing to Radioactive Wastes in the UK', which can be downloaded from www.nda.gov.uk/strategy/waste/waste-information.cfm