Numerical study of the THM effects on the near-field safety of a hypothetical nuclear waste repository—BMT1 of the DECOVALEX III project. Part 2: Effects of THM coupling in continuous and homogeneous rocks

Abstract

An evaluation of the importance of the thermo-hydro-mechanical couplings (THM) on the performance assessment of a deep underground radioactive waste repository has been made as a part of the international DECOVALEX III project. It is a numerical study that simulates a generic repository configuration in the near field in a continuous and homogeneous hard rock. A periodic repository configuration comprises a single vertical borehole, containing a canister surrounded by an over-pack and a bentonite layer, and the backfilled upper portion of the gallery. The thermo-hydro-mechanical evolution of the whole configuration is simulated over a period of 100 years. The importance of the rock mass's intrinsic permeability has been investigated through scoping calculations with three values: 10 -17 , 10 -18 and 10 -19 m 2 . Comparison of the results predicted by fully coupled THM analysis as well as partially coupled TH, TM and HM analyses, in terms of several predefined indicators of importance for performance assessment, enables us to identify the effects of the different combinations of couplings, which play a crucial role with respect to safety issues. The results demonstrate that temperature is hardly affected by the couplings. In contrast, the influence of the couplings on the mechanical stresses is considerable.

A numerical study of THM effects on the near-field safety of a hypothetical nuclear waste repository—BMT1 of the DECOVALEX III project. Part 3: Effects of THM coupling in sparsely fractured rocks

“…7b shows that the time to full resaturation of the buffer varies between 3 to 10 years according to the calculations made by the various teams. The different resaturation times among these models are in line with simulation results obtained for the well-defined axisymmetric problem [2] and for the case of a homogenous (intact) rock [3]. This indicates that the variation of the resaturation times among these models are caused by slightly different input hydraulic properties of the buffer, such as relative permeability and water-retention curves.…”

“…8 presents the KTH results for the evolution of stress in the rock at point R1 at a critical location for the evaluation of rock failure. A shown in the second companion paper [3], the highest potential for rock failure occurs on the floor of the drift. Fig.…”

“…1b) shows that the main effect of the fractures in the near-field rock mass is an accelerated resaturation of the buffer, leading to a much shorter resaturation time in fractured rocks. For the case of a homogeneous, intact, low-permeability rock (k=1×10 -19 m 2 ), the time to full resaturation ranged from 40 to 200 years [3]. By introducing hydraulic conducting fractures, the resaturation time is reduced to 3 to 10 years (Fig.…”

“…The overall aims and definition of the BMT1, as well as the model conceptualization and characterization, were presented in the first companion paper [2]. The second companion paper [3] focused on the impact of THM coupling for a repository located in continuous and homogeneous (intact) rock without fractures. This is the third (and final) paper with focus on the impact of THM couplings in near-field of a repository located in sparsely fractured rock (Fig.…”

“…The THM model conceptualisations and simulation results are then 3 presented, followed by discussions about the impacts of THM couplings and near-field rock fractures on repository performance. The impact of fractures is analysed by a comparison with results in the second accompanying paper on continuous homogeneous (intact) rock [3].…”