Survey on the state of the arts of development of gas migration models in barrier system

Under the environmental condition of deep geological repository, Gas production resulting from mechanisms of metal corrosion and microbial degradation is expected to occur, These phenomena may lead to overpressurization in the repository, degradation of barriers and enhanced release of radionuclides. Preliminary evaluation of the 'gas impact' on repository performance, using a molecular diffusion model and a conventional continuum two-phase flow model 'TOUGH2' code independently, was carried out. The analytical result was described in H12 report by JNC. In H12 report, subjects of the assessment of gas impact was described as 'further gas permeability data of the buffer and rock mass must be acquired by experiments, and verification of predicted performance regarding computer model of gas migration will be essential'. In the context of above mentioned subjects, the newly developed flow model based on Kozeny-Carman model was applied for the simulation of the gas injection experiments on pure bentonite sample. The simulation results of gas injection test were compared with measured test's data. The results of the simulation were in reasonably agreement with obtained experimental data around gas breakthrough phenomenon, but it was difficult to simulate 'burst flow' which was result from preference pathway created at the interface between specimen and vessel occurred immediately after the gas breakthrough. A surveillance of the development states of gas flow models and assessments of gas impact was carried out. The results of the surveillance clarified the progress of flow model development and subjects to application of gas impact assessment to performance assessment. Finally, a conceptual plan of in-situ experiment at Horonobe deep geological laboratory was investigated. Required experiments in the geological laboratory and equipments were extracted and a stepwise schedule of each experiment was considered. (author)