Formulation and Processing of Polyphase Ceramics for High Level Nuclear Waste
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FORMULATION AND PROCESSING OF POLYPHASE CERAMICS FOR HIGH LEVEL NUCLEAR WASTE ALAN B. HARKER, PETER E. D. MORGAN, DAVID R. CLARKE AND JOHN F. FLINTOFF Rockwell International Science Center, 1049 Camino Dos Rios, Thousand Oaks, California 91360, USA ABSTRACT Two basic crystalline phase assemblages have been developed for incorporating the full range of Savannah River Plant waste compositions into polyphase ceramic forms. Both phase assemblages provide crystalline host phases, with stable mineral analogues, for all radionuclides in the waste. The first, an alumina based assemblage, immobilizes the radioactive elements in solid solutions of magnetoplumbite and uraninite with the bulk non-radioactive waste elements being present in spinel and nepheline. The second assemblage uses the titanate based "zirconolite" type fluorite structure and the alumina/iron based magnetoplumbite phases to host the radioactive nuclei with spinel and nepheline, again providing crystalline hosts for the non-radioactive elements. Both phase assemblages can be consolidated to a fine grain ceramic by hot isostatic pressing at 10400C pressures from 20,000 to 30,000 psi. Redox control during processing, just sufficient to reduce uranium to the tetravalent state, is used. INTRODUCTION It is implicit in the concepts of immobilizing radioactive waste in a polyphase ceramic form that chemical additives be used to adjust the waste composition; such additives serve to chemically "tailor" the waste-producing crystalline phases after consolidation which approximate natural mineral assemblages with proven geochemical stabilityl,2,3. In this laboratory, two basic phase assemblages for Savannah River Plant waste compositions 4 have been selected which require minimal tailoring and allow waste loadings of 50-70wt%. The first formulation uses alumina, silica and rare earth oxide tailoring to form an assemblage of magnetoplumbite, uraninite, spinel and nepheline. The second formulation uses alumina, silica, titania, zirconia, calcium oxide and rare earth oxides to form an assemblage of a "zirconolite" type fluorite structure, magnetoplumbite, spinel, and nepheline. The latter assemblage takes advantage of phases proven most leach resistant in the alumina-based assemblage and the titanate-based Synroc-D ceramic form4 . Both of these forms show the presence of additional compatible phases when individual tailoring additives are present in excess. Excess A12 03, in the alumina based form, appears as corundum (a-A1 2 03), while excess TiO2 in the second assemblage produces pseudobrookite, (Fe,Al)TiO 5 , under the mild redox condition and not an ulvospinel type, Fe2TiO 4 SS, which has been seen before in more highly reduced titanate based forms. FORMULATION Both of these ceramic forms take advantage of the bulk composition of the SRP-waste, using desirable phases which naturally form when the untailored
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waste is consolidated under the expected processing conditions. Figure 1 shows an X-ray diffraction pattern of consolidated simulated SRP waste
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