EXECUTIVE SUMMARY
A crucial element of the long-term postclosure safety strategy for the potential monitored geologic repository at Yucca Mountain is to contain high-level radioactive waste (HLW), and to keep that waste and its container as dry as possible. This report is one of nine Process Model Reports (PMRs) developed to address the technical basis supporting the Total System Performance Assessment (TSPA) model. These PMRs are supported by Analysis and Model Reports (AMRs) that contain more detailed technical information. The waste package (WP) is a major component of the engineered barrier system (EBS) and contributes to isolation of HLW during the preclosure and postclosure periods. It also reduces the uncertainties associated with performance of the repository. As a result, the integrity of the WP and the drip shield (DS) have been designated as "Principal Factors" important to the repository safety strategy. The WP, protected by a DS, will be subjected to degradation processes in the repository that will eventually impact postclosure performance. Some of the important conditions contributing to WP degradation include: humidity and temperature in the emplacement drift; chemistry of the dripping water; and the corrosion susceptibility of the WP materials. Eight process-level models or analyses, four abstraction models, and two engineering calculations were developed and documented in individual AMRs or Calculations. This PMR provides a summary of process-level and abstraction models, as well as a summary of their utilization in the integrated WP degradation model contained in the WAPDEG code. This report also includes discussions on the features, events, and processes (FEPs) relevant to the performance of the WP materials. These FEPs that are deemed important to repository performance are evaluated, either as components for the TSPA, or as separate analyses in the AMRs. In addition to describing the process-level and abstraction models, this PMR addresses two of the Key Technical Issues (KTIs) in the Issue Resolution Status Report (IRSR) prepared by the U.S. Nuclear Regulatory Commission (NRC). These issues are the Container Life and Source Term (CLST) and the TSPA and Integration (TSPAI). Several subissues that relate directly to the WP are discussed in this PMR, along with the approach used by the Yucca Mountain Site Characterization Project (YMP) to meet the acceptance criteria for each of the KTIs. This PMR also addresses relevant issues identified by other agencies and organizations, including the Nuclear Waste Technical Review Board (NWTRB). The integrated WP model incorporated into the WAPDEG code was used to develop the results documented here. The analyses have shown that both the DS and WPs do not experience significant failures within the regulatory time period (10,000 years). The materials selected for the DS and WP outer barrier, Titanium Grade 7 and Alloy 22, respectively are highly corrosion resistant under the repository exposure conditions. Both the DS and WP degrade by general corrosion (GC) at very low rates. However, degradation modes such as localized corrosion (LC) (pitting and crevice corrosion), stress corrosion cracking (SCC), and hydrogen-induced cracking (HIC) could lead to premature failure of the WP or DS, if any degradation is initiated. Fortunately, the selected materials appear to be immune to LC under repository-relevant conditions. With appropriate processing, the initiation of SCC and HIC (in the case of Titanium Grade 7) are delayed to a point where acceptable repository performance is achieved. To preclude SCC, post-weld stress mitigation processes will be implemented on the closure welds of the dual-closure-lid WP. The compressive layer that is needed to preclude SCC is produced by either one of two post-weld stress mitigation processes, laser peening or localized induction annealing. The life of the WP is determined by the time required to remove the mitigated layer of material at each weld by GC.