Abstract
Repository designs frequently favour geological disposal of radioactive waste with a backfill material occupying void space around the waste. The backfill material must tolerate the high temperatures produced by decaying radioactive waste to prevent its failure or degradation, leading to increased hydraulic conductivity and reduced sealing performance. The results of four experiments investigating the effect of temperature on the permeability of a bentonite backfill are presented. Bentonite is a clay commonly proposed as the backfill in repository designs because of its high swelling capacity and very low permeability. The experiments were conducted in two sets of purpose-built, temperature controlled apparatus, designed to simulate isotropic pressure and constant volume conditions within the testing range of 4–6 MPa average effective stress. The response of bentonite during thermal loading at temperatures up to 200 °C was investigated, extending the previously considered temperature range. The results provide details of bentonite’s intrinsic permeability, total stress, swelling pressure and porewater pressure during thermal cycles. We find that bentonite’s hydraulic properties are sensitive to thermal loading and the type of imposed boundary condition. However, the permeability change is not large and can mostly be accounted for by water viscosity changes. Thus, under 150 °C, temperature has a minimal impact on bentonite’s hydraulic permeability.
Highlights
The production of energy from nuclear fuels inherently generates radioactive waste as a by-product, and the requirement for long-term disposal of this waste is an inevitable consequence.Geological disposal of radioactive waste is universally favoured with many nations choosing either clay-based host formations [1,2] with an additional clay backfill material occupying the void space around the waste, salt formations or crystalline host rocks, for example the Forsmark Spent FuelRepository which has a granodiorite host rock with a clay backfill material [3,4]
The first phase in CVRF-1, CVRF-2 and ISO-1 test was the hydration phase with equal up and downstream pressures applied by the injection and backpressure pumps
The intrinsic permeability of the bentonite measured in the four tests showed a general trend for decreasing permeability with increasing bentonite measured in the four tests showed a general trend for decreasing permeability with temperature (Figure 11); the ISO-1 data showed this trend most clearly
Summary
The production of energy from nuclear fuels inherently generates radioactive waste as a by-product, and the requirement for long-term disposal of this waste is an inevitable consequence.Geological disposal of radioactive waste is universally favoured with many nations choosing either clay-based host formations [1,2] with an additional clay backfill material occupying the void space around the waste, salt formations or crystalline host rocks, for example the Forsmark Spent FuelRepository which has a granodiorite host rock with a clay backfill material [3,4]. Geological disposal of radioactive waste is universally favoured with many nations choosing either clay-based host formations [1,2] with an additional clay backfill material occupying the void space around the waste, salt formations or crystalline host rocks, for example the Forsmark Spent Fuel. The only operational geological disposal facility for intermediate- to high-level waste is the Waste Isolation Pilot Plant (WIPP) in a salt formation in Carlsbad, New Mexico; many other proposed sites globally are in the conceptual, planning or approval stage [1,3,4,13]. The waste is either placed in large boreholes in the floor of deposition boreholes (“in-floor”) or the waste is stacked in the tunnels themselves (“in-tunnel”), with a clay backfill material occupying the void space [14]. Bentonite is commonly proposed for use in the engineered barrier system (EBS) and as a backfill material in many proposed facilities [15,16,17] because
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