Understanding surface species and interactions between adsorbed chloride and water on stored PuO2

Abstract

5% of plutonium dioxide (PuO2) in storage at the Sellafield nuclear site is contaminated with high levels of adsorbed chloride, derived from degraded PVC bags, and water species, derived from the air atmosphere, and requires repackaging for long term storage. The PuO2 powder must be dried and the chloride treated before repacking into welded cans. Heat treatments of contaminated PuO2 powders have been investigated over a range of temperatures, to remove the adsorbed chloride and water species. Heat treatments have also been carried out on powders of artificially contaminated PuO2 and the less toxic analogue CeO2, as well as on CeO2, ThO2 and UO2 thin films. The sorption mechanisms of chloride and water on aged PuO2 are linked, as HCl(g) dissociatively adsorbs to form OH- and Cl-, but does not form new crystal phases. Adsorbed chloride occurs in three fractions: leachable in caustic solution, non-leachable in caustic solution and volatilised. The higher the surface area of the PuO2 particles, the more chloride and water adsorb. Heat treatments at temperatures less than 700 °C volatilise some of the leachable chloride fraction and convert another fraction into non-leachable chloride. Heat treatments at temperatures greater than 700 °C are high enough to volatilise most of the leachable chloride fraction, as well as some of the non-leachable chloride fraction. Co-adsorbed chloride alters the water sorption profile for the PuO2 as it is heated to low temperatures. Once the PuO2 has been heat treated at high temperatures, a reduced amount of water re-adsorbs to the surface, provided subsequent exposure to a humid atmosphere is limited. At least 1 monolayer of water adsorbs tenaciously to the PuO2 surface. Simulant PuO2 powders follow the same trends as aged PuO2, in terms of surface-area controlled chloride sorption and changes in crystal structure with heat treatment. Variations in adsorbed chloride monolayers, crystallite and particle sizes and particle morphology between the simulant and aged PuO2 are due to differences in the original calcination temperature, storage history and contamination mechanisms. Heat treatment of contaminated PuO2 powders above their original calcination temperature leads to sintering, accompanied by decreases in both the lattice parameter and the specific surface area, as the self-irradiated, defect-filled, aged PuO2 lattice anneals. Characterisation of chloride-contaminated CeO2 powder, as an analogue for PuO2, demonstrates the link between chloride sorption and water sorption. Chloride does not directly bond to cerium, and cerium is not reduced upon chloride-contamination. Adsorbed chloride spreads homogeneously on all available surfaces, including in pores beneath the surface, but is not trapped inside the pores. When the CeO2 is heat treated, the chloride volatilises from all available surface area, including particle surfaces and interior pores, with increasing temperature. Differences in sorption behaviour between CeO2 and PuO2 powders are due to particle morphology and the stability of Ce3+ complexes, conferred by the electronic structure of cerium. As a result, a cerium chloride hydrate phase forms on CeO2, following storage of contaminated samples in air for three years, or chloride-contamination of α- or γ-irradiated CeO2, which does not form on PuO2. The sorption behaviour of chloride and water on the analogue thin films was very different to their powder counterparts due to morphological and surface area differences. No chloride was detected on contaminated UO2 films. Chloride does not volatilise in significant quantities from any of the films, and a reduction of Ce4+ to Ce3+ occurs when chloride-contaminated CeO2, calcined at low temperatures, is heat treated. These differences are such that no direct comparison between contaminated analogue thin films and aged PuO2 powder can be made. This work will form the basis for the design of a PuO2 treatment plant at the Sellafield site.

Date of Award	1 Aug 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Francis Livens (Supervisor), Nikolas Kaltsoyannis (Supervisor) & Robin Taylor (Supervisor)