Synroc-D Type Ceramics Produced by Hot Isostatic Pressing and Cold Crucible Melting for Immobilisation of (Al, U) Rich N
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Synroc-D Type Ceramics Produced by Hot Isostatic Pressing and Cold Crucible Melting for Immobilisation of (Al, U) Rich Nuclear Waste Eric R Vance, Michael La Robina, Huijun Li, and Joel Davis Institute of Materials and Engineering Science, Australian Nuclear Science and Technology Organisation, Menai, NSW 2234, Australia ABSTRACT A synroc-D ceramic consisting mostly of spinel, hollandite, pyrochlore-structured CaUTi2O7, UO2, and Ti-rich regions shows promise for immobilisation of a HLW containing mainly Al and U, together with fission products. Ceramics with virtually zero porosities and waste loadings of 50-60 wt% on an oxide basis were prepared by cold crucible melting (CCM) at ~1500oC, and also by subsolidus hot isostatic pressing (HIP) at 1100oC to prevent volatile losses. PCT leaching test values for Cs were < 13 g/L, with all other normalised elemental extractions being well below 1 g/L. INTRODUCTION The synroc family of titanate ceramics [1] targeted to high-level waste from nuclear fuel reprocessing or weapons production are still diversifying as high-level wastes vary in composition. The original synrocs targeted Purex reprocessing waste (Synroc-C) [1] or spent fuel (synroc-F) [2]. The principal phases were zirconolite (CaZrTi2O7) which can include rare earth fission products and waste actinides into its crystal structure, perovskite (CaTiO3) which can include Sr as well as rare earths and actinides, and barium hollandite (empirical formula BaAl2Ti5O14) which can incorporate Cs and Rb as well as small amounts of Sr. Rutile which housed no fission products or actinides was also included in the waste form formulation to allow chemical flexibility in terms of waste/precursor ratios, insofar as such variations would only produce changes in phase abundances and not new phases. With U as the main cation in spent fuel, the principal phase in synroc-F was the pyrochlore-structured CaUTi2O7, with small amounts of hollandite, perovskite and rutile also being present[2]. In the early 1980s, work was also done on synroc-D, a synroc variant targeted to Al- and Ferich Savannah River HLW. In this formulation, spinel was present to include the majority of the Al and Fe, with nepheline (NaAlSiO4) also introduced to deal with the SiO2 in the waste as well as Cs (subsequent investigations have shown that Cs tended to form pollucite (CsAlSi2O6) rather than inhabit the Na site in nepheline). More recently, synrocs rich in pyrochlore-structured Ca(U,Hf,Pu,Gd)Ti2O7 have been proposed for immobilising surplus impure Pu together with U[4]. Also, synrocs based on CaUTi2O7 has been proposed for intermediate level wastes arising from the production of 99Mo [5].
Here we have studied the incorporation in a synroc-type formulation aimed at ENEA-based HLW which is rich in U and Al if the different tanks are combined-see Table I. While hot isostatic pressing in collapsible stainless steel cans at ~ 1150oC/100MPa is our favoured consolidation method, it was of interest to carry out parallel studies by cold-crucible melting.
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