Gas Migration through Bentonite/Sand: Lessons Learned from the GMT in situ Test

A multibarrier system consisting of cement-based backfill, support structures, and bentonite-based buffer is one of the foreseen Engineered Barrier System (EBS) concepts for the Japanese Transuranic (TRU) waste repository. A certain amount of gas will be produced by anaerobic corrosion of metals and microbial degradation of organic materials disposed in such a repository. The gas may accumulate to a certain pressure level before migrating through the EBS, which consequently may inadvertently affect the EBS performance. How will gas migrate through the engineered barriers and how do models predict this migration are two of the basic questions that have motivated performance of the large-scale Gas Migration Test (GMT), which was performed at Nagra's Grimsel Test Site (GTS) in Switzerland. The GMT project was initiated in 1997 under the leadership of the Radioactive Waste Management Funding and Research Center (RWMC) in Japan and was completed in March 2007 as planned. Within the final year of the project the main emphasis was on the summary and discussion of lessons learned from GMT and how the results can be applied within other work, in particular, Japanese TRU programme. This paper provides an executive summary of the GMT project from the view point of identified key lessons learnt from GMT in situ test.