Synroc for Immobilising Excess Weapons Plutonium
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SYNROC FOR IMMOBILISING EXCESS WEAPONS PLUTONIUM
A. JOSTSONS*, E.R. VANCE*, D.J. MERCER* and V.M. OVERSBY**
*Advanced Materials Program, ANSTO, Private Mail Bag 1, Menai, N.S.W., 2234, Australia "**LawrenceLivermore National Laboratory, Livermore, CA 94450, U.S.A. ABSTRACT The immobilisation of excess weapons plutonium into Synroc can meet all of the important criteria discussed by the U.S. National Academy of Sciences (NAS)(1) for disposal, if the disposal option is pursued rather than options that exploit the energy value of plutonium. This paper summarises the relevant background of Pu incorporation into Synroc, the durability of Pu-containing Synroc and outlines a process flowsheet based on the experience with the 10 kg/hr Synroc Demonstration Plant at ANSTO. The extensive solid solubility of Pu in Synroc, coupled with a very high degree of chemical durability under hydrothermal conditions, makes Synroc ideal as a waste matrix for Pu disposal in deep boreholes to minimise diversion and proliferation risks. INTRODUCTION The status of Synroc development has been reviewed recently(2) showing extensive evidence for the chemical durability of Synroc from tests with simulated HLW, real radioactive waste and from accelerated a-decay damage studies. Fabrication of Synroc has been demonstrated on a commercial (10 kg/hr) scale with non-radioactive waste simulants. Synroc-C, a formulation developed for the immobilisation of HLW from the reprocessing of commercial LWR spent fuel, consists mainly of zirconolite (30 wt%), barium hollandite (30 wt%), perovskite (20 wt%) and excess TiO 2 (15 wt%). Zirconolite is a major host for actinide elements that also partition to the perovskite phase. Both of these phases accept rare earth elements. The use of zirconolite-rich formulations of Synroc have been explored(3) to increase the capability for high-actinide waste loadings. Another aim of these studies has been to increase the amount of zirconolite, which is the most durable phase in
Synroc, without compromising the demonstrated process flexibility (of Synroc-C) to cope with unexpected waste stream fluctuations without change to the chemical durability of the wasteform. WASTE LOADING The solubility limit of PuO 2 in Synroc has not been determined because it is dependent on the content of the zirconolite and perovskite phases and competing radwaste ions. However, the solubility is extensive and 11.8 wt% PuO 2 (Pu-238) has been successfully immobilised in solid solution in the host phases in Synroc-C(4). There was no evidence from X-ray diffraction and metallography of insoluble PuO 2 inclusions or segregation of PuO2 in grain boundary phases. Unfortunately, the sub-micron grain size of the specimens precluded measurement of the partitioning of Pu between the co-existing zirconolite and perovskite phases in Synroc-C. The solid solution limits in zirconolite, the major host phase for plutonium in Synroc, 5 have been investigated by Kesson et al.( ) who showed that zirconolite can accept in solid solution 27 wt% U0 2, 20 wt% ThO2 a
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