Durability of Pu-doped Titanate and Zirconate Ceramics Designed for Pu Immobilisation
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Durability of Pu-doped Titanate and Zirconate Ceramics Designed for Pu Immobilisation Y. Zhang, K.P. Hart, B.D. Begg, E.A. Keegan, A.R. Day, A. Brownscombe and M.W.A. Stewart Australian Nuclear Science and Technology Organisation, PMB 1, Menai, NSW 2234, Australia ABSTRACT Ceramics rich in pyrochlore-structured titanate and fluorite-structured zirconate phases designed for surplus Pu immobilisation, with and without process impurities, have been leach tested at 90oC in deionised water. The zirconates consist mainly of a defect-fluorite with secondary impurity-containing phase-powellite/scheelite (when sintered in Ar but not when sintered in air), a spinel or magnetoplumbite type phase, a glass forming silicate and a secondary U-rich phase (when sintered in air with added impurities). The pyrochlore-rich baseline titanate ceramic consists of pyrochlore, brannerite and Hf-rutile. When impurities are added zirconolite and a silicate glass are also present. The pyrochlore-rich titanate with 5 wt% of impurities sintered at 1300oC is highly durable. A well-densified zirconate version without impurities has comparable elemental releases to those of the titanate ceramic but a zirconate with 5 wt% of impurities sintered at 1400oC in air or Ar shows much higher U and Ca releases than the titanate ceramic. Sintering atmospheres, changing from Ar to air, can influence Pu and U release rates up to an order of magnitude. High Ga releases from zirconates with impurities show that the secondary phase containing Ga is not durable. The higher processing temperature and the apparent inability to incorporate many impurity elements suggest that zirconates are not as flexible as titanates in respect of processing conditions and aqueous durability. INTRODUCTION Defect-fluorite-structured zirconates have recently been proposed as hosts for surplus Pu and other actinides [1,2]. However, the current proposal for Pu-immobilisation utilises a baselinetitanate nominally composed of 95 wt.% pyrochlore (Ca0.89Gd0.22Hf0.23Pu0.22U0.44Ti2O7), 1 wt.% HfO2 and 4 wt.% TiO2. The Gd and Hf are present as neutron absorbers for criticality control and 238 U is added to eliminate the long-term criticality risks associated with the formation of the 239 Pu progeny, 235U, and to allow for the presence of U in real Pu waste streams. Stewart et al. [3] reported on the preparation and characterisation of Ti- and Zr- versions of the baseline pyrochlore-rich ceramic with Zr replacing Ti in the above formulation. These samples were made with and without minor constituents that represent impurities in actual Pu wastes. This paper reports some preliminary results on aqueous durabilities of the Pu- and U-doped Ti- and Zr versions of these baseline-ceramics. EXPERIMENTAL Samples were made via the alkoxide-route, with and without ~ 5 wt.% of process impurities (Table 1) as described elsewhere [3]. Zirconate and titanate samples were prepared by sintering in air or argon for 4 hours at temperatures between 1300 and 1500oC. Backscattered electron micrographs for zirconates
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