The Enhanced Actinide Removal Plant (EARP) at the Sellafield site (Cumbria, UK) is an important facility for the decontamination of active effluents from the Sellafield nuclear licensed site. The EARP uses the coprecipitation of Fe(III) to form ferrihydrite which sequesters radionuclides and other contaminant species from effluent streams. The Sellafield site has now begun an estimated 120 year post-operational clean out (POCO) programme, this POCO programme may have implications for the EARP. Following the closure of the Magnox reprocessing plant (July 2022), the concentration of Fe(III) and phosphate in the bulk effluent is expected to fall. Furthermore, water based wash outs of plants at the Sellafield site will reduce the initial acidity of the bulk effluent feed. The concentration of the challenging radionuclide 90Sr is expected to increase during the POCO programme, it has been observed that the presence of phosphate increases Sr abatement by ferrihydrite. In this study, a range of techniques including XAS, FTIR, SEM, EDS and PXRD have been employed to investigate the sequestration of U by lower concentrations of Fe(III) and at a range of initial pH conditions previously not studied. Upon lowering the Fe(III) concentration it was observed that two removal mechanisms were responsible for the removal of U, complexation to the ferrihydrite surface as an inner sphere edge sharing species and the precipitation of a discrete U phase. At high Fe(III) concentrations the removal of U was solely due to the surface complexation of U to the ferrihydrite surface. The removal of Sr has also been investigated, geochemical solution data showed that a 100x reduction in the final ionic strength of the solution significantly improved Sr removal, in samples with appreciable Sr removal, EXAFS analysis showed an inner sphere monodentate complex of Sr to the ferrihydrite surface was responsible. The inclusion of phosphate also significantly improved Sr abatement by ferrihydrite, EXAFS analysis showed that a ferrihydrite-phosphate-Sr ternary complex formed at high phosphate concentrations. The precipitation of a discrete Sr phosphate solid can be ruled out owing to a lack of spectroscopic evidence and the use of ultra-trace 90Sr coprecipitation experiments. This thesis aims to improve the understanding of the EARP facility by investigating the effects of decommissioning on the EARP process as well as investigating new challenging radionuclides.
|Date of Award||31 Dec 2023|
- The University of Manchester
|Supervisor||Samuel Shaw (Supervisor) & Katherine Morris (Supervisor)|