Abstract
The safety case for deep borehole disposal of nuclear wastes contains a safety strategy, an assessment basis, and a safety assessment. The safety strategy includes strategies for management, siting and design, and assessment. The assessment basis considers site selection, pre-closure, and post-closure, which includes waste and engineered barriers, the geosphere/natural barriers, and the biosphere and surface environment. The safety assessment entails a pre-closure safety analysis, a post-closure performance assessment, and confidence enhancement analyses. This paper outlines the assessment basis and safety assessment aspects of a deep borehole disposal safety case. The safety case presented here is specific to deep borehole disposal of Cs and Sr capsules, but is generally applicable to other waste forms, such as spent nuclear fuel. The safety assessments for pre-closure and post-closure are briefly summarized from other sources; key issues for confidence enhancement are described in greater detail. These confidence enhancement analyses require building the technical basis for geologically old, reducing, highly saline brines at the depth of waste emplacement, and using reactive-transport codes to predict their movement in post-closure. The development and emplacement of borehole seals above the waste emplacement zone is also important to confidence enhancement.
Highlights
This paper develops a preliminary safety case for the deep borehole disposal (DBD) concept
There are some differences in the use of the term safety assessment across national programs and over time; the definition used in Freeze et al [8] and in this report is: Safety Assessment—An iterative set of assessments for evaluating the performance of a repository system and its potential impact that aims to provide reasonable assurance that the repository system
The pre-closure and post-closure safety assessments, augmented by confidence enhancement analyses, collectively suggest that the DBD concept is a viable approach for safe disposal of radioactive wastes
Summary
This paper develops a preliminary safety case for the deep borehole disposal (DBD) concept. Existing general U.S regulations for disposal of high-activity wastes in geologic repositories (10 CFR 60 (NRC)) and 40 CFR 191 (EPA), first promulgated in 1983 and 1985, respectively, remain in effect, and could be applied to disposal of nuclear waste in deep boreholes, as written [13]. These existing regulations would likely be superseded, since they were developed more than 30 years ago and are not consistent with the more recent thinking on regulating geologic disposal concepts that embraces a risk-informed, performance-based approach, such as that represented in the site-specific regulations for Yucca Mountain (10 CFR 63 (NRC) and 40. Key considerations of the aforementioned regulations that might provide insight to future DBD regulations are summarized in Freeze et al [8]
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