Modeling transient thermal conditions for nuclear waste in deep boreholes

Previous deep borehole studies have focused on feasibility, providing proof-of-concept that a deep borehole repository can meet containment standards; but the basic design is not yet optimized. The thermal analysis presented in this paper provides insight into a design-to-performance approach, namely by modeling how changes in disposal depth, disposal zone rock type, disposal zone geometry, and initial heat load affect facility performance. As compared to a 5 km vertical deep borehole, a shallower borehole with multiple branches from a single vertical access shaft could cost less than a very deep vertical bore. This paper also considers heat transport from disposal in deep sedimentary rock in addition to disposal in granite. Borehole geometry changes, especially near-horizontal boreholes in sedimentary rock, can lead to longer disposal zone lengths and decrease the mechanical load on disposal canisters. The combination of shallower wells, rock types which require less effort to drill through, and the savings of a shared vertical access hole could considerably decrease facility construction costs. Cost savings could be redistributed among involved parties, which in turn may increase the likelihood of project success. Thermal transport analysis is also useful because it can provide insight into the physical processes relevant to the transport of radioisotopes in groundwater, including thermally-driven fluid flow and canister corrosion rates. Modeling the link between canister temperature and corrosion rates can inform better estimates of materials costs for canisters.