The Potential Role of Diffusion-Induced Grain-Boundary Migration in Extended Life Prediction
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THE POTENTIAL ROLE OF DIFFUSION-INDUCED GRAIN-BOUNDARY MIGRATION IN EXTENDED LIFE PREDICTION C.A. HANDWERKER', J.E. BLENDELL', C.G. INTERRANTE", T.M. AHN"
"National Institute
of Standards and Technology, Gaithersburg, MD 20899. **U.S. Nuclear Regulatory Commission, Washington, DC 20555. ABSTRACT The selection of materials that are suitable for various high-level waste-packaging designs must reflect the need to meet requirements for long-term performance in repository environments that change with time. With this in mind, we examine how grain boundaries in materials are induced to migrate as a result of solute diffusion even at low temperatures, how the composition of the matrix material is changed significantly by this diffusion-induced grain boundary migration (DIGM), and how the changing microstructures and compositions during DIGM lead to major changes in materials performance, such as corrosion or embrittlement. Methods are discussed for prediction of the long-term behavior of materials affected by DIGM. INTRODUCTION Under repository regulations in the United States, a waste package is expected to provide substantially complete containment for a minimum period of 300 to 1000 years. This requirement, given in the Code of Federal Regulations, as 10 CFR 60.113, "Performance of particular barriers after permanent closure," also indicates that the release of any radionuclide from the engineered barrier system that occurs after the containment period shall not exceed a specified rate [1]. If a container is designed to be more durable than the specified containment requirement, it could be used for satisfying some part, or all, of the requirement for a minimum controlled release of radionuclides. If a container is used to satisfy these combined requirements, it could be expected to have a functional utility on the order of 10,000 years. The prediction of performance over such extended periods presents a significant technical challenge. If one assumes that a container, as part of a waste package, provides a significant part of the containment function, then, in the development of performance arguments, one must consider a variety of technical points as well as the uncertainties associated with them [2]. In general, the unique materials problems posed by a high-level waste (HLW) repository are due, in part, to the long minimum containment period and the moderately elevated temperatures of repositories. Containers are likely to be made of metal alloys, ceramics, or composites, the properties and behavior of which may be highly dependent on the stability of their microstructures under these conditions and over extended periods of thousands of years. Historically, uncertainties in materials behavior have led to unexpected failures of components in "well-engineered" structures. Nevertheless, various avenues are open to the engineering community for ensuring that the performance of components of a waste package will be at or above the levels needed for repository systems [3]. An important component of an adequate performan
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