Ceramic Wasteforms for the Conditioning of Spent MO x Fuel Wastes
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&HUDPLF:DVWHIRUPVIRUWKH&RQGLWLRQLQJRI6SHQW02[)XHO:DVWHV E.R. Maddrell, P.K. Abraitis British Nuclear Fuels plc, Sellafield, Seascale, Cumbria, United Kingdom, CA20 1PG $%675$&7 Crystalline ceramic wasteforms have been fabricated to immobilise a combined fission product and actinide waste stream arising from spent MOx fuel. The fuel is conditioned by a UREX process, as contrasted to PUREX, to produce a waste stream containing fission products and transuranics. Zirconia rich Synroc derivatives have been formulated to minimise formation of perovskite. This ensures that the transuranics are predominantly immobilised in zirconolite. For comparison, a wasteform has also been produced in which transuranics and rare earth element fission products are immobilised in a radiation resistant cubic zirconia solid solution, whilst caesium, strontium and barium are partitioned to an alumina rich magnetoplumbite phase. ,1752'8&7,21 Indefinite recycling of plutonium as MOx fuel through light water reactors is not possible because of the progressive increase in the non-fissile isotopes such as 240Pu with each irradiation cycle. One option for disposal of irradiated MOx is for the spent fuel to be conditioned by a UREX process [1] in which the uranium is extracted to produce a waste stream containing the non-volatile fission products and transuranic elements. The high actinide content of this stream requires that it be immobilised in a crystalline ceramic wasteform. Although Synroc C has adequate crystallochemical capacity, it is potentially unsuitable as a wasteform because under the reducing conditions prevailing during fabrication, trivalent actinides, particularly plutonium, will partition partially to the perovskite phase.[2] The perovskite will become metamict and the concomitant swelling, approximately 17 %,[3] will increase the risk of wasteform cracking and accelerated nuclide release. Given that perovskite is only necessary for strontium immobilisation, and that this is both a short lived and relatively minor fission product in spent fuel wastes, it seems imprudent to allow this element to dictate wasteform composition, particularly if this leads to a significant volume fraction of a non-optimal phase. Zirconia rich modifications of Synroc have therefore been designed in which perovskite is minimised, and perovskite ingrowth under progressively reducing conditions is nominally buffered by the presence of the ZrTiO4 phase.[4] In addition to the titanate wasteforms, a wasteform for the same waste stream has been designed based on a cubic zirconia solid solution and an aluminous magnetoplumbite phase in an attempt to harness the superior radiation stability of fluorite structured solid solutions.[5] 'HILQLWLRQRI:DVWHIRUPV Zirconium rich, calcium poor, derivatives of Synroc were formulated for the coimmobilisation of this combined fission product and actinide stream. The waste free Synrocs have been designated Synroc B7 and Synroc B8 and their compositions were determined by replacing CaO with equimolar quantities
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