Approximately 125 tonnes of separated Pu is in long term storage at Sellafield as calcined PuO2 powder in nested, sealed steel storage cans. Under certain circumstances, gas generation may occur with consequent storage package pressurisation. In practice, this is rarely seen and empirically derived criteria are used to ensure safe storage conditions remain and account for the release of known gases into the package . Fundamental mechanisms that could lead to pressurisation must be understood. The 5 main routes suggested are:
(i) Helium accumulation from α decay;
(ii) Decomposition of polymeric packing material;
(iii) Steam produced by H2O desorption from hygroscopic PuO2 due to self-heating or loss of cooling in stores;
(iv) Radiolysis of adsorbed water; and,
(v) Generation of H2 by a postulated chemical reaction of PuO2 with H2O.
Mechanisms (i) and (iii)-(v) are currently under study by the EPSRC-funded TRANSCEND Consortium, with the last 3 mechanisms (iii)-(v), all involving PuO2/H2O interactions, being especially complex, inter-connected and poorly understood. The PDRA project is focussed on those three mechanisms.
The physico-chemical state and amount of the water ad-/absorbed at the surface, or wettability of the surface, will be determined not only by the PuO2 crystal faces presented at the surface of the powders, but also radiation damage to the surface and (bulk) defect structure. Such radiation damage will also affect the susceptibility of the PuO2 to corrosion – potentially by mechanism (v) but more likely by hydroxyl radicals/hydrogen peroxide generated by the radiolysis of water at or close to the plutonium dioxide surface.
Pre-DISTINCTIVE, we developed a contact angle technique for measuring metal oxide surface wettability – effectively a measure of chemisorbed OH group concentration at the oxide surface.
Experimental work in DISTINCTIVE focussed on the use of direct nanogravimetric measurements to quantify the total amount of physisorbed H2O and associated enthalpies of water sorption on unirradiated single element CeO2, UO2 and ThO2 in Lancaster’s UTGARD Lab and PuO2 in Central Lab.
Work in DISTINCTIVE has also involved preliminary studies of PuO2 electrochemistry and corrosion behaviour at JRC Karlsruhe. Parallel computational work has focussed on the bulk electronic structures of UO2 and PuO2, and the geometric structure and energetics of water adsorption on stoichiometric surfaces.
Thus, using a combined experimental and computational approach, this WP will concentrate on unravelling the surface phenomena underpinning mechanisms (ii)-(v) and the effects on these processes of (bulk) defects and surface radiation damage.
Academic Lead: Colin Boxall
Researcher: Dominic Laventine
Location: Lancaster University