GLASS-CERAMICS IN A COLD-CRUCIBLE MELTER :THE OPTIMUM COMBINATION FOR GREATER WASTE PROCESSING EFFICIENCY

摘要

Improving the efficiency of nuclear waste immobilization is constantly desired by all nuclear waste managementprograms world-wide.For high-level and other waste to be vitrified in traditional ceramic Joule-heated meltersoperated at temperatures up to 1150°C,process flexibilities including waste loadings are often restricted by thistemperature limit as well as the need to consider wasteform corrosion of refractory linings and electrodes.Newmelter technologies,such as the cold-crucible melter (CCM),enable processing up to significantly highertemperatures free of many of the limitations of conventional melters.Higher processing temperatures open up theway for wider composition and processing envelopes to be considered for the vitrification process,including thepossibility for higher waste loadings.In many instances the presence of crystals in the final cooled wasteform is not considered desirable within presentlyexisting glass specifications.For some feed compositions increased waste loadings can lead to the formation of largeamounts of crystals,and thus to a significant departure from the 'glass' state..Nevertheless it is recognized that,ingeneral,increasing the acceptable volume fractions of crystals in the glass offers the best opportunity to increasewaste loading,all other factors being equal.In addition,the deliberate promotion of specific crystalline phases bydesign may enhance the quality of the wasteform,for example by partitioning a long-lived radionuclide into a verystable crystalline phase,or by depleting the glass in detrimental elements.In order to explore the potentialimprovements by harnessing the higher achievable processing temperatures and immunity to refractory corrosionavailable with the cold-crucible melter,and after promising indications for synroc-based matrices,it was decided toinvestigate the feasibility of designing and producing via melting new high temperature 'glass-ceramic' wasteformsfor high level waste immobilization.The INEEL calcines were selected as example feed compositions.These calcines have a wide range of problematiccompositions.They either have high amounts of crystal-forming components,and/or components that lead tocorrosive melts,and for good measure,the components in some waste types are quite refractory for vitrification aswell.The recent DOE High-Level Waste Melter Review Report concluded that,for the INEEL calcine wastes inparticular,the CCM could have sufficient advantages over the Joule-heated ceramic melter to justify its evaluationfor direct vitrification of these wastes.Based on the extensive ceramic design experience of ANSTO,in collaborationwith the CEA and COGEMA for a CCM implementation,a preliminary set of waste forms has been developed thatimmobilize long-lived waste actinides into highly chemically durable crystalline phases by design,using refractorycrystal-forming components already in the wastes to advantage,while at the same time maintaining a very goodoverall leach resistance for the glass-ceramics even after "canister centerline cooling" ((CCC)heat treatments.Thispaper presents the results of a 50 kg technological scale test in the CCM of a glass-ceramic formulation for theaverage Bin Set 2 formulation,at a conservative waste loading of 50 %.