Integrated Analysis for Long-Term Degradation of Waste Package at the Potential Yucca Mountain Repository for High-Level Nuclear Waste Disposal

Abstract

The technical basis for Site Recommendation (SR) of the potential repository for high-level nuclear waste at Yucca Mountain, Nevada has been completed. Long-term containment of the waste and subsequent slow release of radionuclides from the engineered barrier system (EBS) into the geosphere will rely on a robust waste package (WP) design, among other EBS components as well as the natural barrier system. The WP and drip shield (DS) degradation analyses for the total system performance assessment (TSPA) baseline model for the SR have shown that, based on the current corrosion models and assumptions, both the DSs and WPs do not fail within the regulatory compliance time period (10,000 years). From the perspective of initial WP failure time, the analysis results are encouraging because the upper bounds of the baseline case are likely to represent the worst case combination of key corrosion model parameters that significantly affect long-term performance of WPs in the potential repository. The estimated long life-time of the WPs in the current analysis is attributed mostly to the following two factors that delay the onset of stress corrosion cracking (SCC): (1) the stress mitigation to substantial depths from the outer surface in the dual closure-lid weld regions; and (2) the very low general-corrosion rate applied to the closure-lid weld regions to corrode the compressive stress zones. Uncertainties are associated with the current WP SCC analysis. These are stress mitigation on the closure-lid welds, characterization of manufacturing flaws applied to SCC, and general corrosion rate applied to the closurelid weld regions. These uncertainties are expected to be reduced as additional data and analyses are developed.

Review of the Modelling of Corrosion Processes and Lifetime Prediction for HLW/SF Containers—Part 2: Performance Assessment Models

“…Although it is commonly assumed that the container no longer serves any function after first penetration of the corrosion allowance, some PA models do account for the effect of the remaining barrier. For example, in the Waste Package Degradation (WAPDEG) PA model for the corrosion of Alloy 22 waste packages in the Yucca Mountain repository, the container surface was divided into ~1400 "patches", each approximately the size of a corrosion coupon used in the complementary long-term corrosion tests used to provide input data for the model [25][26][27]. Each patch was assigned a corrosion rate from a Weibull distribution of the experimentally observed rates, so that some patches corroded faster than others.…”

“…Therefore, for models that included a DS, failure of the DS was a prerequisite factor for localized corrosion. In the CNWRA [81] and DOE [12,82,83] models, the initiation of crevice corrosion was assessed based on the electrochemical criterion E CORR ≥ E RCREV , where E RCREV is the crevice repassivation potential (also see the discussion in Section 3.2.4 of [1]). DOE screened out localized corrosion based on these various criteria for the initiation of crevice corrosion and concluded that localized attack was not possible at any time in the evolution of the repository environment (Figure 5).…”

“…Stress corrosion cracking of the closure lid welds was also included in the DOE [12,82,84,85] and EPRI [76,78] PA models (The WP was designed with two welded closure lids, inner and outer, with only the outer surface of the outer closure lid weld stress relieved by a non-thermal process. The non-closure lid weld and the body of the WP would be heat treated to relieve residual stresses).…”

“…The inner closure lid weld was only considered susceptible to SCC once the outer lid had failed. SCC was predicted to lead to some WP failures, with the earliest failure in the DOE model after 11,000 a [82].…”

Radioactive Wastes

“…To evaluate the long-term safety of Yucca Mountain, Wilson et al (2002) conducted the total-system performance-assessment simulations and found that risk was dominated by igneous activity at early times, because the robust waste-package design prevented significant nominal (nondisruptive) releases for tens of thousands of years. The technical basis for Site Recommendation (SR) of the potential repository for high-level nuclear waste at Yucca Mountain was reviewed (Lee et al, 2002a), showing that longterm containment of the waste and subsequent slow release of radionuclides from the engineered barrier system (EBS) into the geosphere relied on a robust waste package (WP) design, among other EBS components as well as the natural barrier system. The WP and drip shield (DS) degradation analyses for the total system performance assessment (TSPA) baseline model for the SR showed that, based on the current corrosion models and assumptions, both the DSs and WPs did not fall within the regulatory compliance time period (10,000 yr).…”