The U.S. Nuclear Regulatory Commission (NRC) staff has developed a performance assessment capability to address three programmatic areas in nuclear waste management: high-level waste, low-level waste, and decommissioning of licensed facilities (license termination). The NRC capability consists of: (1) methodologies for performance assessment; (2) models and computer codes for estimating system performance; (3) regulatory guidance in various forms, such as regulations, Branch Technical Positions, and Standard Review Plans; and (4) a technical staff experienced in executing and evaluating performance assessments for a variety of waste systems. Although the tools and techniques are refined for each programmatic area, general approaches and similar issues are encountered in all areas.

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Since the Yucca Mountain project in the U.S. was defunded in 2010, the notion of disposing of spent nuclear fuel (SNF) in deep boreholes has been reinvigorated, most recently by private companies proposing to utilize lateral drilling technology to excavate boreholes for SNF disposal in sedimentary rock. It is claimed that this approach will alleviate site characterization efforts and expand the availability of potential disposal sites. However, long-term safety will hinge upon the prevalence of geochemically reducing, highly saline, and slow-flowing fluids around the waste emplacement zone, and to quantify these parameters in fluids sampled from depths >1 km will present a challenge. Regional data indicate only a narrow geographical extent of such conditions in the conterminous United States. Furthermore, models of radionuclide transport from disposal boreholes must take into account processes that may accelerate degradation of the canisters, plug, and SNF itself, such as radiolysis and attack by constituents of hydrothermal brines, coupled with hydrogeologic features that promote advective groundwater flow. This review summarizes some geologic considerations, most notably those related to geochemistry, that challenge the long-term safety case for deep borehole disposal of SNF.

The U.S. Nuclear Regulatory Commission (NRC) staff has developed a performance assessment capability to address three programmatic areas in nuclear waste management: high-level waste, low-level waste, and decommissioning of licensed facilities (license termination). The NRC capability consists of: (1) methodologies for performance assessment; (2) models and computer codes for estimating system performance; (3) regulatory guidance in various forms, such as regulations, Branch Technical Positions, and Standard Review Plans; and (4) a technical staff experienced in executing and evaluating performance assessments for a variety of waste systems. Although the tools and techniques are refined for each programmatic area, general approaches and similar issues are encountered in all areas.