First principles simulation of cement for nuclear waste disposal
The immobilization of wastes that arise from the decommissioning of the UK civil nuclear program is a significant challenge facing both industry and researchers. The current plan for nuclear industry is to dispose of wastes within a deep underground facility and to use cement to immobilise many of these waste forms. However, there is a lack of understanding regarding the true structure of cement as well as the properties of cement-like materials under the effects of radiation. We demonstrate that the nanoscale structure of calcium silicate hydrate does not notably influence the structure of the defects that form when cement interacts with ionizing radiation. In particular, we show that electrons locate in the water- and calcium-rich interlayer region, where they may act as precursors to radicals and H2 gas. Holes, on the other hand, appear to be benign. The holes point into the intralayer region where they are unlikely to further react. We then utilize thermodynamic integration to study the thermochemistry associated with reactions that involve replacing atoms in cement with those of interest to the nuclear industry. Comparison with experimental findings as well as results from other theoretical studies, we find that strontium, yttrium, zirconium and lanthanum are likely to spontaneously replace calcium atoms within the intralayer of cement where they are chemically immobilised. By testing the same technique using a second crystalline model of calcium silicate hydrate we show that the results of both models agree that the strontium decay chain is immobilised within the intralayer region of cements. The techniques outlined present a simple framework for atomistic models of cement-like minerals to study the storage of nuclear wastes within cement.
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