A Preliminary assessment of gas diffusion and migration

Tanai, Kenji ; Sato, Haruo; not registered; not registered

In the anaerobic environment in the deep underground water, carbon-steel overpack corrodes and generates molecular hydrogen. It is conceivable that this hydrogen either dissolves into the porewater of the buffer and migrates through the buffer. If the rate of aqueous diffusion of hydlogen is too low compared to the rate of hydrogen generation, the concentration of hydrogen at the overpack surface will increase until a solubility limit is attained and a free hydrogen gas phase forms. It is possible that the pressure in this accumulating gas phase will increase, affecting the stability of the buffer or the surrounding rock mass. There is also a concern of possible effects on nuclide migration, as it is also conceivable that the flow of gas could push out radionuclide-bearing porewater in the buffer when it floes through the buffer. As such, experimental and analytical study must be carried out on such phenomenon to evaluate such potential phenomena. (1)Diffusion experiment of dissolved hydrogen. According to the test result concerning the effective diffusion coefficient of the dissolved hydrogen in buffer material, the effective diffusion coefficient of reference buffer material (70wt% bentonite + 30wt% sand mixture, dry density 1.6Mg m) ranges from 10 m s to 10 ms. The value of the effective diffusion coefficient measured for a dry density of 1.8 Mg m is slightly smaller than the value in that for a dry density 1.6 Mg m. And the effective diffusion coefficient at 60C tends to have slightly larger value than that at 25C. Test results from the foreign countries show the diffusion coefficient in the range between 10 ms to 10ms. Basically, these test results reported here are in the same range as these other results. (2)Gas permeability. Studies of the gas permeabinty of buffer material have been carried out by Pusch et al., Volckaert ...