Deep Borehole Disposal Safety Case

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.

Deep Borehole Disposal Safety Case

“…Incidents leading to failure included events with minimal effects (e.g., minor worker exposure, low probability of radionuclide release), moderate effects (e.g., moderate worker exposure), and potentially significant effects (e.g., high probability for radionuclide release). For the PCSA base case assumptions, the probability of a successful borehole completion was 95.4%, the probability of a minimal or moderate incident was 4.6%, and the probability of a potentially significant event was only 0.0005% [9]. With achievable minimization of human error probabilities and increased component reliability, the probability of a successful borehole completion rises to 99.4%, and the probability of a potentially significant event drops to 0.0001% [9].…”

“…The pre-closure assessment basis associated with the reference design [15] includes a description of the surface and subsurface facilities (i.e., the borehole) and their operation for use in a quantitative pre-closure safety analysis. Specific pre-closure basis information for the reference case and reference design includes [8,9,16]…”

“…In general, a pre-closure operational safety analysis provides a quantitative estimate of the occupational dose from on-site radiation levels and radiological exposures, and the dose to the public from off-site releases of radioactive materials. For this preliminary DBD safety case, a simplified pre-closure safety analysis (PCSA) was performed that calculated the probability of success (which is equal to 1 -probability of failure) of emplacing 108 waste packages (containing all the Cs and Sr capsules) in a single borehole [9,15,16]. Incidents leading to failure included events with minimal effects (e.g., minor worker exposure, low probability of radionuclide release), moderate effects (e.g., moderate worker exposure), and potentially significant effects (e.g., high probability for radionuclide release).…”

“…The formal concept of a safety case for the long-term disposal of high-activity waste in an engineered facility located in a deep geologic formation was first introduced by the Nuclear Energy Agency (NEA) [3][4][5] More recently, the U.S. Department of Energy Office of Nuclear Energy has published safety case overviews relevant to geologic disposal in the U.S. [6] and specific to the DBD concept [2,[7][8][9].…”

“…The post-closure assessment bases associated with the reference design [14] include a description of the natural and engineered barriers for use in the quantitative post-closure PA. Specific post-closure basis information includes [8,9]:…”