Sensitivity Studies of the Effect of Cladding Degradation on TSPA Results

Abstract

ABSTRACTThe Commercial Spent Nuclear Fuel (CSNF) cladding directly influences the dose by reducing the release rate of radionuclides compared to bare fuel. A cladding degradation model was introduced in the Total System Performance Assessment – Viability Assessment (TSPA-VA) and has evolved into the TSPA-SR (Sight Recommendation), Rev 00 and most recently the Supplemental Science and Performance Analyses (SSPA) models. TSPAs are large computer models that predict how the radionuclides might escape through the various barriers and migrate through the different geological regions and predict the dose to the critical population. The major components in the cladding degradation model are initial cladding failure, creep, stress corrosion cracking (SCC), localized corrosion, mechanical failures (due to seismic events and rock overburden) and cladding unzipping. In the bare fuel case, the dose is controlled by the failure rate of the waste packages (WPs), the solubility limits for certain radionuclides, and the rate of diffusion of the radionuclides through the cracks in the WPs. The cladding degradation model involves two steps, perforation followed by unzipping. Sensitivity studies have shown that cladding limits dose primarily by preventing dissolution of the UO2 fuel (when the cladding is not perforated). If all the cladding is assumed to be perforated and cladding unzipping occurs, the peak dose would be within 4% of that of bare fuel. The UO2 dissolution rate (the driving mechanism for cladding unzipping) is sufficiently rapid that assuming instant dissolution increases the dose by only 12% from the base case. In the current cladding degradation model, cladding accounts for a reduction in dose for the first 100,000 years of a factor of 16 when compared with bare fuel. This is because of the low initial cladding failure rate (2.1% including SCC and creep) and few failures from localized corrosion (late WP breach and limited water entering the WPs). Failure of the cladding by rock overburden accounts for an increasing trend in dose at times greater than 100,500 years. This mechanism increases the peak dose and delays the peak from approximately 200,000 to 300,000 years. In conclusion, the cladding degradation model, mostly through preventing water from contacting the UO2, reduces the dose predicted in the TSPA analysis.