Safety Evaluation Report Related to Disposal of High-Level Radioactive Wastes in a Geologic Repository at Yucca Mountain, Nevada

ion and Integration For the Nominal Modeling Case in the TSPA analysis, DOE implemented the model abstractionfor general corrosion of the drip shield in the Waste Package and Drip Shield DegradationSubmodel, as described in SNL (2008ag, Section 6.3.5.1), in which the drip shields weredistributed in the five percolation subregions. The Waste Package and Drip Shield DegradationSubmodel considers only general corrosion breach of the Titanium Grade 7 plates. DOEconcluded that the drip shield would protect the waste package against seepage if the dripshield plates are intact, even if the drip shield supports collapsed and the sidewall buckled(SAR Section 2.3.4.5.3.1). For each realization, DOE sampled one general corrosion rate forthe outside of the drip shield plates (under the aggressive condition) and one for the inside ofthe drip shield plates (under the benign condition) from the respective distributions given inSNL (2008ag, Table 6.3.5-3). The corrosion rates were applied to all drip shields, regardlessof the percolation subregion, such that all drip shields in a given realization failed at the sametime. The output of the Waste Package and Drip Shield Degradation Submodel was the fractionof that for drip shields in each percolation subregion breached by general corrosion as afunction of time. This output was provided to the Waste Form Degradation and MobilizationModel Component and the Engineered Barrier System Flow and Engineered Barrier SystemTransport Submodels. SAR Figures 2.1-8 and 2.4-24 showed the distribution of calculated failure times for theTitanium Grade 7 drip shield plates in the Nominal Modeling Case, on the basis of the modeldescribed in SAR Section 2.3.6.8.1. DOE’s analyses calculated that most drip shield failuresoccur between 260,000 and 340,000 years after repository closure. DOE (2009cn, Enclosure 5,Figure 1) showed a modified distribution of failure times considering both a higher corrosion rate(based on additional uncertainties associated with specimen cleaning) and lower corrosion rate(based on potential decrease in corrosion rate over time). The modified distribution shows thatmost drip shield plate failures occur between 80,000 and 500,000 years after repository closure.In either case, DOE concluded that there is negligible probability of drip shield plate breach bynominal processes within 12,000 years after repository closure, the time period during whichDOE calculates that the waste package is susceptible to localized corrosion if contacted byseepage water. For the Seismic Ground Motion Modeling Case in the TSPA analysis, DOE alsoimplemented the Waste Package and Drip Shield Degradation Submodel to calculate thetiming and magnitude of drip shield plate breach by general corrosion, as outlined inSNL (2008ag, Section 6.6.1). Both the Titanium Grades 7 and 29 corrosion rates are sampled