Abstract
Post-closure performance assessment (PA) calculations suggest that deep borehole disposal of cesium (Cs)/strontium (Sr) capsules, a U.S. Department of Energy (DOE) waste form (WF), is safe, resulting in no releases to the biosphere over 10,000,000 years when the waste is placed in a 3–5 km deep waste disposal zone. The same is true when a hypothetical breach of a stuck waste package (WP) is assumed to occur at much shallower depths penetrated by through-going fractures. Cs and Sr retardation in the host rock is a key control over movement. Calculated borehole performance would be even stronger if credit was taken for the presence of the WP.
Highlights
Deep borehole disposal (DBD) for the geologic isolation of spent nuclear fuel (SNF) and high-level radioactive waste (HLW) has been considered for many years, beginning with evaluations of nuclear waste disposal options by the National Academy of Sciences in 1957 [1]
Nominal scenario performance assessment (PA) simulations with PFLOTRAN included a baseline deterministic run [5]
A set of probabilistic realizations to examine the sensitivity and importance of long-term radionuclide transport to selected processes and parameters [5]
Summary
Deep borehole disposal (DBD) for the geologic isolation of spent nuclear fuel (SNF) and high-level radioactive waste (HLW) has been considered for many years, beginning with evaluations of nuclear waste disposal options by the National Academy of Sciences in 1957 [1]. The DBD concept, illustrated, consists of drilling a large-diameter borehole into crystalline basement rock to a depth of about 5000 m, placing waste packages (WPs) in the lower, waste emplacement zone portion of the borehole, and sealing and plugging the upper portion of the borehole with a combination of bentonite, cement plugs, and sand/crushed rock backfill. Multi-barrier design: the safety of a DBD system includes contributions from natural barriers (deep, isolated low-permeability host rock) and engineered barriers Overlying sediments are represented as a single vertically and laterally homogeneous region This model simplification does not impact the results, because radionuclides are not transported beyond the crystalline basement rock in any of the simulations.
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