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

摘要

Following emplacement and closure of anengineered barrier system (EBS), it si important to understandwhat kind of phenomena may take place as a consequence of thecoupling between various physical and chemical processes: thebuffer restoration profile, temperature gradient, material/mineralalteration (Through heating or precipitation), corrosion and gasgeneration, and aqueous chemistry. These governing processestake place together, and focusing on any one of them in isolationis difficult to justify, and may invoke unrealistic assumptions.Just as for other well-known coupled process models, such asgeometrical fingering when flow is fully coupled to flux drivenmineral dissolution, it is expected that the resulting phenomenaare robust with respect to the precise details of the submodelterms (fingering models are functionally the same for all types ofmineral dissolution), and the important point is to identify whichparameters can most effectively control or limit various effects.Hence the impact of coupling can be understood qualitatively bycoupling relatively simplified submodel terms, before steppingon the a quantitative understanding.Here we take a phenomenological view and consider theproblem of a resaturating compacted clay buffer in which thereremains a relatively dry zone close to the hot waste canister dueto compression of air (and possibly vapour initially until porepressures exceed 1.5 MPa), displaced by incoming pore water.The 'dry', low thermal conductivity, region is 'baked' by theprevailing temperature, resulting in the possible breakdown of theclay structure. Throughout the remaining partially and fullysaturated clay pore space, aqueous silica complexes are formed(enhanced by temperature) and redistributed from the heated zoneby diffusive transport. On cooling these result in the precipitationof amorphous silica, causing strong cementation and theformation of a clay stone layer adjacent to the metal overpack.There is also significant silicitation as far as the outer clayboundary. These phenomena were observed in publishedexperiments, and are shown to arise robustly within a suitablecoupled model of thermal, hydration and chemical alteration.Their relevance to barrier performance and potential impact uponperformance assessment will be discussed.