Tables Figllres 1 2
IntroductionOne of the most signrficant factors impacting the performance of waste package container materials under repository relevant conditions is the thermal environment. This environment will be affected by the areal power density of the repository, which is dictated by facility design, and the dominant heat transfer mechanism at the site. The near-field environment w i l l evolve as radioactive decay decreases the thermal output of each waste package. Recent calculations (Buscheck and Nitao, 1994) have addressed the importance of thermal loading conditions on waste package performance at the Yucca Mountain site. If a relatively low repository thermal loading design is employed, the temperature and relative humidity near the waste package may significantly affect the degradation of corrosion allowance barriers due to moist air oxidation and radiolytically enhanced corrosion.An evolution of the waste package design effort has resulted in an increased emphasis on the performance of the engineered banier system (EBS). The EBS includes the waste package and the near-field engineered repository environment. This evolution has prompted research and analysis related to the development of a multiple barrier Advanced Conceptual Design (ACD) for the waste package disposal container. The ACD systems currently under consideration consist of a multiple barrier design. The outer barrier w i l l be fabricated from a corrosion allowance material, such as carbon steel. The inner barrier will employ a corrosion resistant material, such as Alloy 825. The thickness of each barrier is currently a variable in the container design process. The predicted long-term performance of each banier material under repository relevant conditions will dictate the ultimate thicknesses of each barrier. Degradation modes surveys for Alloy 825 (Fanner, et al., 1988 and Gdowski, 1991) and carbon steel (Vinson, et ai ., 1995) have been completed previously.A significant factor related to repository thermal performance that impacts the degradation of the container is the time period over which each container is predicted to remain dry. During this time period, a majority of the degradation of the container will occur due to simple moist air oxidation. However, current repository thermal designs include a lower thermal loading option which may result in container surface temperatures dropping below the boiling point of water in relatively short times following emplacement. This could result in a sigmficantly more ag,sressive corrosion environment for the corrosion allowance material employed in the waste container. Various options addressing this potentially more aggressive environment are currently 1 being considered. These options may employ an additional outer barrier that is moderately corrosion resistant under expected low thermal loading repository conditions or replacement of the carbon steel corrosion allowance barrier with a more corrosion resistant material, such as a nickel-copper alloy (Monel400).The est...