Concrete is used as a barrier on surface or near-surface facilities for the final disposal of low- and intermediate-level radioactive waste, where gas can be generated and affect the hydraulic properties and the processes taking place in concrete. In this framework, gas-transport properties of concrete samples were investigated using two different laboratory test set-ups: a non-steady-state equipment working under low injection pressures; and a newly fine-tuned steady-state set-up working under different pressures.Permeability decreased with water content increase but was also greatly affected by the hydraulic history of concrete (i.e. if it had been previously dried or wetted). The intrinsic permeability determined with gas flow was about two orders of magnitude higher than that determined with liquid water (10−16 v. 10−18 m2), probably due to the chemical reactions taking place during saturation (carbonation). The relative gas permeability of concrete increased sharply for water degrees of saturation smaller than 50%.The boundary conditions also affected the gas permeability, which seemed to be mostly conditioned by the back pressure and the confining pressure, on the whole decreasing as the effective pressure increased. It is considered that the Klinkenberg effect was not relevant in the range of pressures applied.
The backfilling and sealing of deposition galleries or holes and access galleries and shafts is an important part of nuclear waste underground repository design. Any opening created during the construction of the repository is a potential preferential pathway for water, gas and radionuclides migration, and has to be effectively sealed. Bentonite or bentonite-based mixtures have been proposed as backfill and sealing materials for their low permeability, high swelling capacity and high retention capacity. This work reports the results of two laboratory tests in which two different sealing materials were subjected to conditions similar to those in a repository: a high thermal gradient and hydration with host rock water (Figure 1). The materials are contained in cylindrical cells with a plane heater at the bottom and water supply through the top surface. The temperature and relative humidity (RH) of the materials are measured online during the tests by capacitive sensors placed at different positions. The water intake and the heater power are also measured online. In addition, cell B is instrumented with a ring load cell located on the top of the cell to determine the axial pressure generated during the test. In one of them the material used is MX80 bentonite pellets (B) and in the other is a 65/35 sand/bentonite mixture (S/B). The superficial thermal conductivity of these granular materials in their as-received state are 0.33 and 0.12 W/m·K for the mixture and the pellets, respectively. The S/B mixture has a predominant macroporosity with a pore mode about 204 µm, whereas in the B pellets mesopores of pore mode about 0.014 µm predominate (Villar 2013). For a dry density of 1.53 g/cm 3 a swelling pressure of about 4 MPa is expected for MX-80 bentonite and of 0.15 MPa for the sand/bentonite mixture at a nominal dry density of 1.45 g/cm 3 . These values were obtained with deionised water, but they are expected to be lower if the saturation water is saline. Figure 1: Experimental setup for the infiltration testsThe initial water content and the average dry density of the materials inside the cells were 5.9% and 1.46 kg/m 3 for the B pellets and around 4% and 1.5 g/cm 3 for the S/B mixture.The tests were performed in two stages. In the first one, only a thermal gradient was prescribed with no hydration taking place. This stage tries to simulate the early stage of a barrier in a low-permeability argillaceous rock where hydration from the host medium will be minimal.The temperature of the heaters placed at the bottom of the columns was increased to 100°C and then to 140°C. The stabilisation of the temperature was very quick; the low thermal conductivity of the dry materials caused a high thermal gradient near the heater, and low temperatures in the rest of the columns. The movement of water in the vapour phase as a result of the thermal gradient was evinced by the sharp increase of relative humidity recorded by the sensors closest to the heater -followed by a continuous decrease-and the slower increase recorded in the mi...
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