Investigating Effect of Clay Composition on Safety Function Performance in a Geological Disposal Facility (GDF)

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Abstract

A legacy of radioactive waste has accumulated since the late 1940s and safe containment of long lived, highly radioactive waste is crucial for the future of nuclear power. A geological disposal facility (GDF) is the preferred method for the safe disposal of radioactive wastes; a multifaceted approach, using both engineered and natural barriers, to maximise the time between the breakdown of barriers and the final interaction with the environment and subsequently people. Clay is likely form an integral part of the engineered barrier system (EBS) surrounding the waste canisters in many proposed GDFs for heat generating radioactive wastes. The clay selected for this purpose would need to have the necessary physical and chemical properties to protect the waste container against corrosion and also to limit the release of radionuclides from the waste after container failure. Clays have a number of advantageous properties, such as high sorption capacity for radionuclides, small pore structure restricting microbial activity, and stability over geological time scales. Substitution of cations (Fe2+/3+, Mg2+, Al3+) into octahedral and tetrahedral (Al3+ and Si4+) sheets give a net negative charge on the clay layers giving interlayer spaces in-between; hydrated cations balance the negative charge within the interlayer space and cause the clay to swell filling surrounding gaps/cracks, avoiding advective flow, stabilizing the canister, and making diffusion the predominant transport mechanism within the barrier. A number of challenges such as heat (from the high level wastes <130 °C), radiation (gamma/alpha), waste package corrosion products (Fe/ Cu release), and ground water infiltration (ion exchange, swelling) would be present in the GDF environment and need to be examined with respect to the clay barrier material. Clay characterisation is fundamental to understanding the limitations of the barrier as each challenge is confronted. The coupled effects of heating (25-500 °C) and gamma irradiation on a number of montmorillonites and pseudo-alteration products (nontronites), heating (120 °C) and alpha irradiation on a model montmorillonite, and the effect of gamma irradiation on U(VI) and Cr(VI) sorption on a model montmorillonite and a nontronite, were studied using a combination of methods including XRD, IR, XRF, XCT, XAS, zeta potential and EPR. The results provided a positive outlook for the use of montmorillonite (bentonite) as an engineered clay barrier material. Montmorillonite was shown to mitigate potential detrimental effects whilst maintaining its bulk advantageous properties. Heating studies showed limited effects to the clay (>160 °C), with small changes being resisted further by divalent interlayer cations. gamma irradiation was shown to generate charge defects within the clay, increasing surface potential and activating redox properties (Fe); alpha irradiation showed localised amorphisation of the clay structure with long range order maintained. Maximising the ability of the clay barrier to withstand the challenges expected in the GDF environment would allow for the strengthening of public opinion and a faster, smaller (footprint), cheaper and safer GDF for high level, heat generating, radioactive wastes to be produced.

Keyword(s)

Clay Minerals; Engineered Clay Barrier ; Geological Disposal ; Montmorillonite; Nuclear; Radioactive Waste

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