Transient modeling of host rock for a deep borehole nuclear waste repository

Abstract

This work describes a framework built to simulate the thermal and hydrological processes in a deep borehole repository of spent nuclear fuel. Such simulation requires a fully coupled solver, capable of capturing the processes at the scale of tens of kilometers and millions of years. The MOOSE framework was chosen for this purpose, where the FALCON application, developed at INL, was adopted as baseline. This application had previously been applied for simulation of fluid flow and heat transport in geothermal reservoirs, and therefore provided a valuable reference. Additional features were implemented in FALCON in order to simulate deep borehole repositories. Solver options were adjusted for best performance. Code verification was performed on Rayleigh-Bnard convection in a porous medium. Cross-code validation was performed between the FALCON code and the FEHM code on a single borehole test case, and the thermal results were further compared to analytical and simplified numerical models, confirming the potential existence of a second peak of temperature at the scale of thousands of years. Two configurations for the borehole repository were analyzed. The first one consisted of an infinite array of boreholes, which allows one to significantly simplify the geometry, boundary conditions, and test code features. A parametric study of input parameters such as rock permeability, borehole spacing, and pitch length, was performed to assess thermal behavior of the repository. Analysis of the results led to the conclusion that the water flow in the caprock is driven mostly by thermal expansion of water. The displacement length of the water front was found to be negligible in comparison to the depth of the repository. The second configuration included a semi-infinite array of boreholes. This representation is a more realistic approximation of an actual repository, since it includes the modeling of the undisturbed rock surrounding the emplacement zone. It was shown that in this configuration convection can originate between the emplacement region and the rock outside the repository. At rock permeability higher than 10-16 m2 this mechanism can lead to an escape length of the water front larger than the burial depth. However, it was shown that the salinity gradient in the underground water can suppress convection and effectively eliminate water escape.