A Risk-Informed Approach to Performing the Pre-Closure Safety Analysis for the Deep Borehole Disposal of Radioactive Waste

Deep borehole disposal is an emerging concept which involves disposing of radioactive waste in boreholes several kilometers deep then backfilling to minimize radionuclide transport and distribute weight. Unfortunately, little documentation exists on borehole safety case development, especially for pre-closure operations. This study aims to close this gap by proposing a graded approach matrix based on consequence, maturity, and complexity. Two case studies are used to demonstrate this application: Hanford CsCl capsules and spent PWR assemblies. Simulations are used to predict potential for dose exposure, revealing CsCl capsules have a much higher potential for being above the 5 rem public dose exposure regulation, with exceedance for a waste package containing 18 CsCl capsules between .13 and 3.5 km. Sensitivity analyses reveal weather related input parameters dominate uncertainty, and therefore a site-specific study is recommended for future work. This study may be helpful to inform the regulatory framework for borehole disposal.