Water and Gas Movement in Mx80 Bentonite Buffer Clay
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:DWHUDQG*DV0RYHPHQWLQ0[%HQWRQLWH%XIIHU&OD\ Stephen T. Horseman1, Jon F. Harrington1 and P. Sellin2 1 British Geological Survey, Keyworth, NG12 5GG, UK ([email protected]/+44 115 9363261) 2 Svensk Kärbränslehantering AB (SKB), Stockholm, Sweden ([email protected]/+46 8 6624974). $%675$&7 This paper describes a long-term laboratory test designed to examine the sensitivity of gas flow in Mx80 buffer bentonite subject to a constant volume boundary condition. A constant volume and radial flow (CVRF) apparatus was designed to enable gas flow from a centrally located injection filter to be independently monitored at three sink-filter arrays mounted around the circumference of the clay specimen. Axial and radial total stresses and internal porewater pressure were continuously monitored. Gas entry, breakthrough and peak gas pressures were found to be systematically higher under constant volume boundary conditions than under previously reported constant stress and radially-constrained test conditions [6, 9, 10]. The observation that gas pressures are sensitive to test boundary conditions supports the hypothesis that gas entry is accompanied by dilation of the bentonite fabric. Gas penetration of the clay caused a substantial increase in total stress and internal porewater pressure. Abrupt drops in gas pressure, accompanied by similar drops in total stress, were interpreted as fracture propagation events. The outflow of gas was always non-uniformly distributed between the sinks. Furthermore, the distribution of flow between sinks often changed abruptly during the course of an experiment indicating that gas pathways were very unstable. When gas injection stopped, the gas pressure and rate of outflow spontaneously declined with time. Under constant volume conditions, the gas pressure at the asymptote exceeded the internal porewater pressure by an amount equal to the capillary pressure. In constant volume tests on clay with high water saturation, capillary pressure has a value close to the measured swelling pressure of the clay. ,1752'8&7,21 In the KBS-3 concept, copper/steel canisters containing spent nuclear fuel will be placed in large diameter disposal boreholes drilled into the floor of the repository tunnels. The space around each canister will be filled with pre-compacted bentonite blocks. Over time, the bentonite blocks will draw in the surrounding groundwater and swell, closing up any construction gaps. Because of the important buffering effect of the bentonite on the local water chemistry, this barrier is usually referred to as the bentonite buffer. The copper/steel waste canisters are expected to have a very substantial life in the repository environment. However, for purposes of performance assessment, it is important to consider the possible impact of groundwater penetrating one of the canisters. Under certain conditions corrosion of the steel inner of each canister will lead to the formation of hydrogen. Radioactive decay of the waste and the radiolysis of water will produce some additional gas. Dependi
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