High Level Waste (HLW) Vitrification Experience in the US: Application of Glass Product/Process Control to Other HLW and
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High Level Waste (HLW) Vitrification Experience in the US: Application of Glass Product/Process Control to Other HLW and Hazardous Wastes Carol M. Jantzen and James C. Marra Savannah River National Laboratory Aiken, SC 29808 ABSTRACT Vitrification is currently the most widely used technology for the treatment of high level radioactive wastes (HLW) throughout the world. At the Savannah River Site (SRS) actual HLW tank waste has successfully been processed to stringent product and process constraints without any rework into a stable borosilicate glass waste since 1996. A unique “feed forward” statistical process control (SPC) has been used rather than statistical quality control (SQC). In SPC, the feed composition to the melter is controlled prior to vitrification. In SQC, the glass product is sampled after it is vitrified. Individual glass property models form the basis for the “feed forward” SPC. The property models transform constraints on the melt and glass properties into constraints on the feed composition. The property models are mechanistic and depend on glass bonding/structure, thermodynamics, quasicrystalline melt species, and/or electron transfers. The mechanistic models have been validated over composition regions well outside of the regions for which they were developed because they are mechanistic. Mechanistic models allow accurate extension to radioactive and hazardous waste melts well outside the composition boundaries for which they were developed. INTRODUCTION Borosilicate glasses have been used in the US and in Europe to immobilize radioactive HLW for ultimate geologic disposal. Vitrification has also been developed as a technology to immobilize low activity waste, low-level wastes, mixed (radioactive and hazardous) wastes, and TRU wastes in durable glass formulations for permanent disposal and/or long-term storage. Waste glass formulations must maximize the amount of waste to be vitrified so that waste glass volumes and the associated storage and disposal costs are reduced. Moreover, glass formulation optimization for HLW [1,2,3] or other wastes must simultaneously balance multiple product/ process (P/P) constraints (Table I). Table I. Waste Glass Product and Process Constraints Product Constraints Process Constraints melt viscosity chemical durability glass homogeneity liquidus waste solubility thermal stability regulatory compliance melt temperature/corrosivity mechanical stability radionuclide volatility REDOX* * controls foaming and melt rate
Most P/P properties, other than melt temperature, cannot be measured directly. The waste streams are often highly variable and difficult to characterize. In addition, in the US, the P/P constraints must be satisfied to a very high degree of certainty (>95%) as the canister geometry makes rework (remelting) of the product impossible. This requires a “systems approach” so that the P/P constraints can be optimized simultaneously [1]. The “systems approach” ensures that the final product safeguards the public, and that the production process used is safe to
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