Investigation on gas migration. Technical barriers concrete and bentonite as well as granite of the surrounding host rock

The Japanese Radioactive Waste Management Funding and Research Centre (RWMC) perform R and D programmes regarding disposal of radioactive waste. Intermediate level waste is foreseen to be disposed in concrete silos with additional sealing built in stable geological formations. Considerable amounts of gases (hydrogen, hydro carbons, and carbon dioxide) will be produced by anaerobic corrosion and microbial degradiation of the waste components. If the gas migrates at very low rates through the Engineered Barrier Systems (EBS) or the surrounding host rock, gas pressure will increase in the disposal cavern or in the EBS. This gas pressure may affect the stability of the repository system and may lead to unacceptable transport of radionuclides into the geosphere. For long-term safety aspects the gas and water permeability of each component of the EBS and of the surrounding host rock are of importance. Furthermore, the evaluation of the gas behaviour in the complete repository is essential for the construction of the disposal system. Therefore RWMC proposed the Gas Migration Test (GMT) in the frame of the Grimsel phase V project (1997 - 2002). In 1997, RWMC finalized the test programme, started the excavation of the silo cavern, and the field experiments under the experimental lead of NAGRA with the support of Obayashi (Japan). On behalf of the German Ministry for Education and Research (BMBF), the Repository Safety Research Division of GRS worked out a proposal for investigating the gas migration in the EBS and the host rock within this project, which was accepted by RWMC and NAGRA. In 1998 GRS joined the GMT projected and started the installation and the investigation in the test field in 1999. (orig.)