Sensitivity Studies of Waste Package Performance in High-Level Waste Management

Abstract

An important attribute of the high-level waste (HLW) disposal program in the United States (U.S.) is the estimated long life of waste packages (WPs). The outer container of the WP is proposed to be constructed of a nickel-based alloy, Alloy 22 (58Ni-21Cr-14Mo-3W-4Fe), which is highly corrosion resistant. Under nominal conditions of the proposed repository at Yucca Mountain (YM), Nevada, WPs are anticipated to have a long life because of their extremely slow uniform corrosion rate. The technical basis for this expected long life of geological scale is still subject of discussion. In this paper, the NRC Total-system Performance Assessment (TPA) Code is used to evaluate uncertainties associated with the WP performance within a 10,000-year period. Early WP failure may occur due to localized corrosion or high frequency of manufacturing defects. Localized corrosion may occur under mixed salt deposits on the WP surface at temperatures above 100°C. High frequencies of WP juvenile failure are considered to account for the limited industrial experience with Alloy 22 and uncertainty in the long-term performance of Alloy 22 as container material. The localized corrosion and the high frequency of manufacturing defects were simulated with the TPA code by lowering the critical relative humidity for the onset of aqueous corrosion, RH_c, and increasing the fraction of initially defective WPs. By sampling RH_c from various normal distributions, the mean annual total effective dose equivalent (TEDE) approached 1 mrem/year at 10,000 years. Assuming a fixed fraction of initially defective WPs (1 percent), the mean annual TEDE approached 0.07 mrem/year at 10,000 years.