The Underground as a Storage Facility. Modelling of Nuclear Waste Repositories and Aquifer Thermal Energy Stores

Sammanfattning: The underground is a vast region, which is, too a large extent, unused by man. It has a large potential as a storage facility due to its vastness and availability. This thesis, which consists of eleven papers and reports, deals with nuclear waste repositories in solid rock and with aquifer thermal energy storage systems. All these storage systems induce multidimensional, time-variable thermo-hydro-elastic processes in the ground in and around the storage region. The partial differential equations that govern the physical processes are solved analytically in some cases, and in other cases numerical models are developed. Many methods of classical mathematical physics are employed for the solution. The analytical approach provides a deeper physical understanding of the processes and their interactions. At large depths, the salinity of groundwater, and hence its density, often increases downwards. In the first study, the upward buoyancy flow of groundwater in fracture planes due to heat release from the nuclear waste is studied considering the added effect of a salt gradient. The aim of the study is to determine the natural barrier effect caused by the salt. A simple formula for the largest upward displacement from the repository is derived. There may be a strong natural barrier, which is independent of fracture permeabilities. In two papers, the temperature field in rock due to a large rectangular grid of heat-releasing canisters containing nuclear waste is studied. The solution is by superposition divided into different parts. There is a global temperature field due to the large rectangular canister area, while a local field accounts for the remaining heat source problem. A complete analytical solution is presented In the next set of papers, the thermoelastic response from the rectangular field of nuclear waste is analysed. An analytical solution for any rectangular heat source in a semi-infinite medium is derived. The thermoelastic problem for the rectangular heat source in an infinite surrounding is solved exactly. An important step is the introduction of so-called quadrantal heat sources. The solution for the rectangle is obtained from four quadrantal solutions. The condition of zero normal and shear stresses at the ground surface is fulfilled by using a mirror heat source and a boundary solution. The boundary solution accounts for the residual normal stress at the ground surface. Using a Hertzian potential, a surprisingly simple solution is obtained. The analytical solution is applied for the Swedish KBS-3 concept to determine the complete three-dimensional stress and strain fields at any time during the first thousand years. Regions of largest tensile stress are identified. Another study concerns the use of heat as a tracer to investigate flow in a fracture plane. One result is that a commonly used two-dimensional, analytical solution cannot describe the complete process in the fracture plane and surrounding rock. Two papers deal with the thermohydraulic evaluations of two aquifer heat storage projects in Southern Sweden. Both plants have been successfully simulated using models based on conformal flow and entropy conservation techniques. In the last paper, the conformal flow technique is presented together with the very useful thermal front tracking model.