Disposal Concepts/Thermal Load Management (FY11/12 Summary Report)

This report is part of study to identify reference geologic disposal concepts for generic studies in the Used Fuel Disposition R&D Campaign. This report summarizes the work on both enclosed and open modes, which has been expanded to include thermal analysis of open modes, a range of spent nuclear fuel (SNF) burnup, additional disposal system description, and cost estimation.
Thermal management for geologic disposal is used to limit various temperatures inside and outside the waste package. The chief constraint is controlling degradation of clay-based buffer or backfill materials in close proximity to the waste package. Further away, or for concepts that do not have buffer or backfill, the next constraint is limiting thermal damage to the host rock caused by processes such as desiccation and thermal expansion of solids or pore fluids. No limitation on repository thermal loading has been identified that derives from thermal degradation of the waste package or its contents (SNF or high-level waste, HLW); these components can resist temperatures at least 100 C° hotter than can the materials surrounding waste packages. Other thermal management constraints were identified but not evaluated by this study, including large-scale effects from thermal expansion, and brine migration in salt. Measures available to limit these effects include selecting host rock with superior heat dissipation properties, decay storage or preclosure repository ventilation, smaller waste packages, larger waste package and drift spacings, waste segregation in different parts of a repository, and reliance on far-field barriers. Deep borehole disposal has been evaluated in parallel studies, and all thermal limits considered in this study would be met because of small package size, large spacings, and reliance on far-field barriers.
This study identified two major categories for waste package emplacement modes: “open” where extended ventilation can remove heat for many years following waste emplacement underground; and “enclosed” modes for clay/shale and salt media. For the enclosed modes, waste packages are emplaced in direct contact with natural or engineered materials which may have temperature limits that constrain thermal loading. All disposal concepts developed internationally and in this report fit into this categorization scheme. Enclosed modes include vertical and horizontal borehole emplacement in borings constructed from underground, backfilled alcoves, and deep boreholes. In-drift emplacement can be open or enclosed depending on whether buffer and/or backfill is installed around waste packages at emplacement. Emplacement drifts may be kept open for ventilation, then backfilled or isolated by seals prior to closure.