HIPed Tailored Ceramic Waste Forms for the Immobilisation of Cs and Sr
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HIPed Tailored Ceramic Waste Forms for the Immobilisation of Cs and Sr Melody L. Carter and E. R. Vance Australian Nuclear Science and Technology Organisation, Institute of Materials Engineering, Menai, NSW 2234, Australia ABSTRACT This paper illustrates the benefits of hot isostatically pressed (HIPed) tailored ceramic waste forms for the immobilisation of Cs and Sr separated from spent nuclear fuel. Experimental data on microstructure and aqueous durability are presented for Cs- and Sr-bearing hollanditerich tailored ceramics prepared with 12-18 wt% waste (on an oxide basis). MCC-1 type leach testing, on the sample containing 12 wt% waste at 90oC for 28 days revealed extremely low normalised 7-28-day Cs and Sr release rates of 0.003 and 0.004 g/m2day respectively.
INTRODUCTION The U. S. Advanced Fuel Cycle Initiative (AFCI) is conducting research on aqueous separations processes for the nuclear fuel cycle. This research includes development of solvent extraction processes for the separation of cesium (Cs) and strontium (Sr) from spent nuclear fuel to reduce the short-term decay heat load. The thermal load on the repository is greatly reduced if the emplacement of Cs/Sr waste is delayed until these relatively short-lived (principal half-lives of ~30 years) materials decay. The baseline process for separation of Cs and Sr from dissolved spent light water reactor fuel, as part of the Uranium Extraction Plus (UREX+) process [1], is a solvent extraction method utilising chlorinated cobalt dicarbollide and polyethylene glycol (CCD/PEG) in a phenyltrifluormethyl sulfone (FS-13) diluent. Cs and Sr are stripped from the CCD/PEG organic stream with an aqueous solution of guanidine carbonate and diethylenetriaminepentaacetic acid (DTPA) yielding a metal carbonate product solution (Table 1). The Cs/Sr strip solution from the UREX+ process will require treatment and solidification for managed storage. Storage will require a waste form that is stable, does not produce radiolysis products (e.g., buildup of potentially explosive gases like hydrogen), has high product density (to minimise storage volume and disposal costs), and has properties that enhance heat management. The selection and integration of appropriate waste form processing technology is an essential component of developing a waste form solution for this separated waste stream. Generically, synroc is an advanced crystalline ceramic comprised of geochemically stable natural titanate mineral analogues, some of which have immobilised uranium and thorium in the natural environment for many millions of years [2]. However synroc can take various forms depending on its specific use and can be tailored to immobilise particular components in the high level waste (HLW) by incorporating them into the crystal structure of the mineral analogue phases. The principal advantage of the synroc-C ceramic, targeted to PUREX HLW, is that the waste ions are incorporated in durable titanate mineral phases (HLW loading can be varied between 0 and 35 wt.%), which are considerably more insol
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