Abstract
AbstractSamples of artificially aged Nirex reference vault backfill (NRVB) were prepared by progressive leaching with deionized water, after which some of the samples underwent hydrothermal treatment. Compared to unaltered NRVB, X-ray diffraction (XRD) and differential scanning calorimetry (DSC) provided evidence for the ageing resulting in alteration of the mineralogy, in particular the absence of portlandite was observed. The specific surface area of NRVB initially increased due to leaching, but then decreased after further leaching.Sorption distribution ratios (RD values) of uranium(VI), neptunium(IV), tin and zirconium onto aged NRVB samples were measured using the batch sorption technique. For all four elements, there was little difference between RD values for aged NRVB and those for untreated material. The most probable explanation for these findings is that even though the ageing treatments altered the NRVB mineralogy, calcium silicate hydrate (C-S-H) phases are responsible for most of the radionuclide uptake and their transformation to more crystalline forms has little effect on the bulk sorption capacity of the aged material.
Highlights
IN a future UK geological disposal facility (GDF), the current concept for intermediate-level wastes (ILWs) is disposal in vaults backfilled with cement (Nuclear Decommissioning Authority, 2010)
The major difference between the leached and untreated Nirex reference vault backfill (NRVB) was the absence of portlandite in the aged samples, as shown by X-ray diffraction (XRD) and differential scanning calorimetry (DSC) analysis
The removal of portlandite is consistent with a drop in leachate pH and lower calcium concentration observed with the leached NRVB
Summary
IN a future UK geological disposal facility (GDF), the current concept for intermediate-level wastes (ILWs) is disposal in vaults backfilled with cement (Nuclear Decommissioning Authority, 2010). Nirex reference vault backfill (NRVB) was developed as a potential cement and was designed to maintain a high near-field pH that minimizes the solubility of many radioelements and to provide a high sorption capacity. Together, these combine to retard the migration of many radionuclides in a GDF (Francis et al, 1994). The selection and justification of distribution coefficients for the sorption of radionuclides onto the backfill forms an important part of the process providing input data for the postÀclosure.
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