The management of spent fuel from materials testing reactors in the United Kingdom

Four MTR Reactors, DIDO, PLUTO, DMTR and HERALD have been constructed and operated in the UK. Of these DMTR and HERALD have now been shut down. The spent fuel elements from DIDO and PLUTO are treated by chemical reprocessing at Dounreay to recover the unused U235 and concentrate the fission product activity. All of the fuel from the DMTR reactor has already been reprocessed at Dounreay, whilst the final core from HERALD will be reprocessed soon. After reprocessing the first cycle raffinate, which contains the bulk of the fission products from the spent fuel, is currently stored in high integrity, stainless steel storage tanks which are housed below ground in heavily shielded vaults. A new plant has been constructed at Dounreay which will first neutralize with caustic soda and then immobilize this liquor in cement. The resulting products will then be kept in an engineered store until an intermediate level waste repository is available. The uranyl nitrate product from reprocessing is evaporated and then blended up by the addition of U235. The uranium is then processed at Dounreay to make more MTR fuel, which is recycled to the remaining reactors. In the longer term uncertainty exists over the continued availability and economic viability of reprocessing, particularly if alternative, cheaper sources of fuel are used. The alternative of long term storage of the spent fuel, followed by direct disposal, is therefore being examined as a contingency. A review of the options has concluded that the preferred route would involve the interim storage of the elements in special cans in a cross-flow, natural convection, air-cooled vault. The use of free standing, concrete storage casks to store the fuel would be significantly more expensive and is not favoured. (author). 2 refs, 7 figs