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

Rutqvist, J.(a) , Chijimatsu, M.(b ), Jing, L.(c) , Millard, A.(d) , Nguyen, T.S.(e) , Rejeb, A.(f ), Sugita, Y.(g) , Tsang, C.F.(a)

International Journal of Rock Mechanics and Mining Sciences
Volume 42, Issue 5-6 SPEC. ISS., July 2005, Pages 745-755

As a part of the international DECOVALEX III project, and the European BENCHPAR project, the impact of thermal-hydrological-mechanical (THM) couplings on the performance of a bentonite-back-filled nuclear waste repository in near-field crystalline rocks is evaluated in a Bench-Mark Test problem (BMT1) and the results are presented in a series of three companion papers in this issue. This is the third paper with focuses on the effects of THM processes at a repository located in a sparsely fractured rock. Several independent coupled THM analyses presented in this paper show that THM couplings have the most significant impact on the mechanical stress evolution, which is important for repository design, construction and post-closure monitoring considerations. The results show that the stress evolution in the bentonite-back-filled excavations and the surrounding rock depends on the post-closure evolution of both fields of temperature and fluid pressure. It is further shown that the time required to full resaturation may play an important role for the mechanical integrity of the repository drifts. In this sense, the presence of hydraulically conducting fractures in the near-field rock might actually improve the mechanical performance of the repository. Hydraulically conducting fractures in the near-field rocks enhances the water supply to the buffers/back-fills, which promotes a more timely process of resaturation and development of swelling pressures in the back-fill, thus provides timely confining stress and support to the rock walls. In one particular case simulated in this study, it was shown that failure in the drift walls could be prevented if the compressive stresses in back-fill were fully developed within 50 yr, which is when thermally induced rock strain begins to create high differential (failure-prone) stresses in the near-field rocks.

a Lawrence Berkeley National Laboratory, Berkeley, CA, United States
b Hazama Corporation, Tokyo, Japan
c Royal Institute of Technology, KTH, Stockholm, Sweden
d Commissariat a l'Energie Atomique, Paris, France
e Canadian Nuclear Safety Commission, Ottawa, Ont., Canada
f Institut de Radioprotection et de Sûreté Nucléaire, Paris, France
g Japan Nuclear Cycle Development Institute, Ibaraki, Japan