Thermal Conversion of Cs-exchanged IONSIV IE-911 into a Novel Caesium Ceramic Wasteform by Hot Isostatic Pressing
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Thermal Conversion of Cs-exchanged IONSIV IE-911 into a Novel Caesium Ceramic Wasteform by Hot Isostatic Pressing Tzu-Yu Chen1, Joseph A. Hriljac1, Amy S. Gandy2, Martin C. Stennett2, Neil C. Hyatt2 and Ewan. R. Maddrell3 1 School of Chemistry, University of Birmingham, Edgbaston, Birmingham, B15 2TT, U.K., 2 Department of Materials Science and Engineering, University of Sheffield, Sheffield, S1 3JD, U.K., 3 National Nuclear Laboratory, Sellafield, Seascale, Cumbria, CA20 1PG, U.K. ABSTRACT Hot Isostatic Pressing of Cs-exchanged IONSIV IE-911 samples is shown to produce a mixture of ceramic phases, the nature and mass fractions of these have been determined by Rietveld analysis of powder X-ray diffraction data. The main Cs phase that forms is Cs2TiNb6O18, after this reaches approximately 30% of the total crystalline content the remaining Cs is partitioned into Cs2ZrSi6O15. Durability tests using the PCT-B method for 7 days at 90 qC with deionised water lead to Cs leach rates of 0.032 and 0.038 g∙m-2∙day-1 for samples exchanged to 6 and 12 wt% Cs, respectively, indicating a durable wasteform is produced. INTRODUCTION IONSIV IE-911 is a commercial mixture of a crystalline silicotitanate (CST) with a formula of (H3O)xNay(Nb0.3Ti0.7)4Si2O14·zH2O, where x~2, y~1 and z~4, and an amorphous Zr(OH)4 binder in a 4:1 ratio. It has been widely used in the nuclear industry as an inorganic ionexchanger to separate 137Cs[1-3] and shows excellent selectivity even in the presence of large amounts of Na+, K+, Mg2+, Ca2+ or Ba2+ and over a broad pH range.[4] After use, however, the options for long term storage or disposal are limited and warrant further investigation. Particular points to consider are large volumes of ILW due to the very low bulk density of the material as used in pellet form, 1.1 kg m–3, potential back-exchange or leaching especially at higher temperatures, and large fraction of Ti which limits the amount that can be incorporated into most glasses. The thermal behaviour of IONSIV has been previously reported,[5] CST starts to lose crystallinity at around 500 qC and with further heating new ceramic phases begin to form until the onset of melting at around 1000 qC. The reported crystalline phases that form from Csexchanged IONSIV are Cs2ZrSi3O9, Na(Ti,Nb)O3 and Na2Ti6O13. Hot isostatic pressing (HIP) is a more advanced ceramic processing technique that involves heating a powder sample in a sealed and evacuated metal container whilst simultaneously applying pressure. It has been widely studied for nuclear waste treatment for more than fifteen years[6-10] and offers several advantages over heating powders or pressed pellets. After HIPing, internal pores and defects within a solid body collapse, as a consequence a homogeneous material with a uniform grain size and a nearly 100% density is achieved.[11, 12] The waste is enclosed in a sealed can and processed at relatively low temperature, therefore there are no high temperature volatility losses and no expensive off-gas emission processing system is required. Here we repo
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