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Nuclear Fuel Cycle

There are several clearly identifiable stages in the life of nuclear fuel, known as the fuel cycle.

The majority of nuclear fuel is made from enriched uranium (although UK Magnox stations use natural un-enriched uranium). Uranium in the form of uranium ore concentrate is readily available on the world market where commercial demand for uranium is principally determined by the requirements of nuclear electricity generation.

In 1999 the main five sources of production were Canada, Australia, Niger, Namibia and Uzbekistan. Current world requirements for uranium are forecast to rise from about 61,000 tonnes in 2000 to about 72,000 tonnes in 2020 primarily as a result of reactor construction programmes in China, Japan and Korea.

The next step in producing fuel for AGRs and PWRs is to convert uranium into uranium hexafluoride which, for AGR and PWR fuel (Magnox stations use natural (un-enriched) uranium) is then enriched to increase the proportion of the Uranium 235 (U235) from approximately 0.7 per cent to typically between 2.5 and 5.0 per cent. The enriched material is then converted into either AGR or PWR ceramic fuel pellets which are then packed into stainless steel tubes for AGRs to form fuel pins or zirconium alloy tubes for PWRs to form fuel rods. A number of these pins or rods are then assembled into a fuel element.

Following the delivery of AGR fuel elements to a power station, the elements are inspected and stored. Each station typically stores up to six months' supply of fuel. To refuel the reactor, several elements are stacked to form a fuel assembly, which is loaded into the reactor using a fuelling machine. AGRs typically contain 300 to 330 fuel assemblies per reactor.

A fuel assembly remains within an AGR for a period of typically four to eight years, after which it is removed from the reactor using the fuelling machine and moved to a store for up to a month to allow some of its radioactivity to decay. It is then moved by the same fuelling machine to an irradiated fuel dismantling cell where the assembly is dismantled. The fuel elements are then passed to water tanks called cooling ponds, where they remain for a minimum period of 60 days and typically for between three to six months, before being loaded into specially designed flasks and transported off site for reprocessing or long term storage.

In the UK, PWR fuel for the next refuelling is delivered approximately six weeks before refuelling takes place and will then be inspected and stored. PWR fuel elements remain within the reactor for typically three to five years. The PWR must be shut down for refuelling at intervals of 12 to 18 months, depending on the stations' output and fuel enrichment. PWR refuelling takes place with the primary circuit cooled and depressurised. The top of the pressure vessel is removed, the internal structures above the fuel are disconnected and the fuel elements are lifted from the pressure vessel and passed to the cooling ponds. Approximately two thirds of the discharged fuel and one third of new fuel is then returned to the reactor vessel and the structure is reassembled.

Spent PWR fuel is stored on site in cooling ponds pending transport off site or long term storage.

Reprocessing

Reprocessing of spent fuel involves the separation of uranium and plutonium from nuclear waste products by dissolving the fuel in nitric acid. The resulting materials are then stored. Commercial scale reprocessing facilities currently exist in France, at BNFL's Sellafield site in the UK and in Russia, India and Japan.

Spent Magnox, AGR and PWR fuel contains approximately 96 per cent uranium, 3 per cent nuclear waste products and 1 per cent plutonium. Some of the nuclear waste products are highly radioactive. Reprocessed uranium can be recycled once it has been converted, enriched and fabricated into new AGR or PWR fuel elements.

Plutonium from reprocessing can be stored in accordance with international safety and security requirements or combined with fresh uranium to form mixed oxide fuel ("MOX"). MOX can be used as fuel for PWRs but is not currently used in the UK.

Handling of Spent (Irradiated) Fuel

Spent fuel is fuel which is finally removed from a reactor. Spent AGR and PWR fuel elements are stored for long periods prior to final disposal, or, after a period of at least three years for AGR spent fuel or five years for PWR spent fuel, can be reprocessed. The Government’s policy on reprocessing, remains as set out in the White Paper "Review of Radioactive Waste Management Policy" published in July 1995 (Cm 2919), - ie that the question of whether to reprocess or hold the spent fuel in long term storage is a matter for the commercial judgement of the owner of the spent fuel subject to meeting the necessary regulatory requirements.