Abstract
AbstractRadial diffusion experiments have been carried out to assess the migration of 36Cl, as chloride, through a cementitious backfill material. Further experiments in the presence of cellulose degradation products were performed to assess the effect of organic ligands on the extent and rate of chloride diffusion. Results show that breakthrough of 36Cl is dependent on chloride concentration: as the carrier concentration increases, both breakthrough time and the quantity retained by the cement matrix decreases. Experiments in the presence of cellulose degradation products also show a decrease in time to initial breakthrough. However, uptake at various carrier concentrations in the presence of organic ligands converges at 45% of the initial concentration as equilibrium is reached. The results are consistent with organic ligands blocking sites on the cement that would otherwise be available for chloride binding, though further work is required to confirm that this is the case. Post-experimental digital autoradiographs of the cement cylinders, and elemental mapping showed evidence of increased 36Cl activity associated with black ash-like particles in the matrix, believed to correspond to partially hydrated glassy calcium-silicate-sulfate-rich clinker.
Highlights
THE UK concept for geological disposal of intermediate level (ILW) and some long-lived low-level (LLW) radioactive waste is based on a multi-barrier system
The aim of the present work is to assess the retention of chloride in a cementitious admixture, namely NRVB, which is a candidate backfill for a geological disposal facility (GDF)
The breakthrough curve for the diffusion of 36Cl through NRVB cylinders shows a clear dependence on NaCl carrier concentrations (Fig. 2)
Summary
THE UK concept for geological disposal of intermediate level (ILW) and some long-lived low-level (LLW) radioactive waste is based on a multi-barrier system. According to the generic design (Hicks et al, 2008), the waste will be grouted in steel drums, transported to a vault 300–1000 m underground and backfilled with a porous cement matrix. One potential candidate material is Nirex Reference Vault Backfill or NRVB (Francis et al, 1997), comprising a mixture of Ordinary Portland Cement, Reduced solubility arises from the formation of radionuclide-containing solid phases, commonly the corresponding oxides and hydroxides for metallic species at high pH. There is much less evidence in support of the precipitation or coprecipitation of anionic species and conventionally.
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