Sealing of a Deep Horizontal Borehole Repository for Nuclear Waste

Abstract

The depth and layout of a horizontal borehole repository has the potential to offer strong isolation of nuclear waste from the surface. However, the isolation may be compromised by the borehole used to access the repository, as it could provide a direct fast-flow path transporting radionuclides from the disposal section to the accessible environment. Thus, backfilling the disposal section and sealing the access hole are considered essential engineered safety components. To analyze the importance of plugging the open space between canisters and sealing the access hole, we numerically calculate non-isothermal fluid flow and radionuclide transport through the borehole and the surrounding geosphere for a variety of scenarios, which include backfill materials with different sealing properties and configurations that potentially induce strong driving forces along both the horizontal and vertical sections of the borehole. The simulations indicate that the dose contribution of radionuclides released through the access hole is small, even if the backfill material is of poor quality or has deteriorated, and even if considerable horizontal and vertical pressure gradients are imposed by assuming the underlying formation is overpressured and that the disposal section is intersected by faults activated during a seismic event. The modeling also reveals that the low influence of backfill integrity on repository performance partly arises from the very high length-to-diameter ratio of the borehole, which favors the radial diffusion of radionuclides—as well as pressure dissipation and associated advective transport—into the surrounding formation rather than axial transport along the borehole. The integrated modeling approach also exposes the importance of accounting for the connections and feedback mechanisms among the various subcomponents of the repository system.

Post-Closure Safety Analysis of Nuclear Waste Disposal in Deep Vertical Boreholes

“…This reduces risks and costs related to long-distance transportation. Due to the relatively low sensitivity of performance with respect to the details of host-rock properties and the ability to tolerate conservative assumptions [19,20,42], site requirements, specifically regarding properties and the extent of a formation suitable to host a borehole repository, would be easier to meet [15]. Moreover, while the probability of being affected by certain disruptive events (e.g., seismicity) may increase, the consequences of such events would be compartmentalized and thus diminished [19,42].…”

“…Some disruptive events may lead to an early release of radionuclides. Once dissolved in the pore-water, radionuclides become mobile and can be transported by diffusion or advection along the borehole [11,20,29], into the near field, and through the geosphere to the biosphere.…”

“…The corrosion and eventual perforation of the canisters and casing are approximated by a time-dependent increase in permeability [19], with the assumption that the canister is breached after 10,000 years. The role of borehole seals has been examined for a large, centralized, vertical borehole repository in [7,11,69,70], and for a smaller, modular, horizontal borehole repository in [20]. While backfilling and plugging the borehole is an integral part of repository closure activities, it is difficult to assess or predict the long-term performance of such seals.…”

“…As a variant of such a vertical borehole repository, directional drilling technology can be used to gradually deviate from the vertical direction of the access hole at a kick-off point above the target depth and to create a horizontal disposal section within a suitable formation, which may be a sedimentary or crystalline host rock. The deep horizontal borehole concept has been described in [17], with generic safety calculations for a repository in shale discussed in [18][19][20].…”

“…Deep borehole repositories are at a relatively early stage within this process, specifically because no disposal sites have been identified and because research and development by national waste organizations focus on mined repositories. Nevertheless, a series of preliminary safety analyses for borehole repositories have addressed a subset of features, events, and processes (FEPs) for generic sites and reference repository designs [6,7,9,11,[18][19][20]28,29]. While these studies discuss many aspects (including operational safety during the pre-closure phase [30] and economics [11,31]), the post-closure analyses focus on thermal effects, borehole sealing issues, as well as driving forces and stabilizing effects, such as density stratification.…”