Accounting for EBR-II Metallic Waste Form Degradation in TSPA
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Accounting for EBR-II Metallic Waste Form Degradation in TSPA W. L. Ebert,1 M. A. Lewis,1 T. L. Barber,2 and S. G. Johnson2 1 Argonne National Laboratory, 9700 S. Cass Ave., Argonne, IL 60439 2 Argonne National Laboratory-West, P.O. Box 2528, Idaho Falls, ID 83403
ABSTRACT A metallic waste form (MWF) will be used to immobilize cladding hulls from spent sodium-bonded nuclear fuel rods. Laboratory tests were conducted to determine if the HLW glass degradation rate model used in Total System Performance Assessment (TSPA) calculations can be used as an upper bound for the degradation rate of the MWF. Static tests were conducted with monolithic MWF samples in pH buffer solutions to measure the degradation rates at 50, 70, and 90°C. The measured degradation rates of the MWF were compared to the rates calculated with the HLW glass degradation model. The degradation rates were calculated as the product of the specific dissolution rates and the anticipated exposed surface areas of glass and MWF in a breeched canister. This comparison showed that the rate calculated using the HLW glass degradation model bounds the rates measured for the MWF at all pH values and temperatures. These results support the potential use of HLW glass as a surrogate for MWF degradation for the purpose of performance assessment. INTRODUCTION A metallic waste form (MWF) has been developed to immobilize irradiated cladding hulls recovered after electrometallurgical conditioning of spent sodium-bonded nuclear fuel from the Experimental Breeder Reactor-II (EBR-II). The MWF is a multiphase material composed of nearly equal amounts of austenitic stainless steel and Fe2Zr Laves intermetallic phases.1 Most radionuclides are contained in the intermetallic phase. The MWF must be qualified for disposal in the federal high-level radioactive waste disposal system as a non-standard high-level waste (HLW) form. This paper describes an approach to account for the impact of the MWF on released dose in Total System Performance Assessment (TSPA) calculations conducted for the proposed Yucca Mountain site. In TSPA calculations, the release of radionuclides from a waste form is calculated as the product of the specific dissolution rate of the matrix (expressed as the mass per unit surface area per unit time), the exposed surface area, and the radionuclide inventory in the waste form. To eliminate the need to model the degradation of the MWF separately in the TSPA, we are evaluating the use of the HLW glass degradation model that is already included in the TSPA calculations2 to represent the degradation rate of the MWF. In the HLW glass degradation model, the glass degradation rate is calculated as a function of temperature and pH. Model parameter values for the temperature and pH dependencies and a coefficient that accounts for the dependence on the glass and solution compositions were selected to represent the wide range of HLW glass compositions to be disposed of. It was expected that the rate calculated for HLW glass at a particular temperature and pH could b
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