Modelling silicitation of a clay buffer subject to opposing temperature and hydration gradients

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Following emplacement and closure of an engineered barrier system (EBS), it is important to understand what kind of phenomena may take place as a consequence of the coupling between various physical and chemical processes: the buffer resaturation profile, temperature gradient, material/mineral alteration (through heating or precipitation), corrosion and gas generation, and aqueous chemistry. These governing processes take place together, and focusing on any one of them in isolation is difficult to justify, and may invoke unrealistic assumptions. Just as for other well-known coupled process models, such as geometrical fingering when flow is fully coupled to flux driven mineral dissolution, it is expected that the resulting phenomena are robust with respect to the precise details of the submodel terms (fingering models are functionally the same for all types of mineral dissolution), and the important point is to identify which parameters can most effectively control or limit various effects. Hence the impact of coupling can be understood qualitatively by coupling relatively simplified submodel terms, before stepping on to a quantitative understanding. Here we take a phenomenological view and consider the problem of a resaturating compacted clay buffer in which there remains a relatively dry zone close to the hot waste canister due to compression of air (and possibly vapour initially until pore pressures exceed 1.5 MPa), displaced by incoming pore water. The ‘dry’, low thermal conductivity, region is ‘baked’ by the prevailing temperature, resulting in the possible breakdown of the clay structure. Throughout the remaining partially and fully saturated clay pore space, aqueous silica complexes are formed (enhanced by temperature) and redistributed from the heated zone by diffusive transport. On cooling these result in the precipitation of amorphous silica, causing strong cementation and the formation of a clay stone layer adjacent to the metal overpack. There is also significant silicitation as far as the outer clay boundary. These phenomena were observed in published experiments, and are shown to arise robustly within a suitable coupled model of thermal, hydration and chemical alteration. Their relevance to barrier performance and potential impact upon performance assessment will be discussed.