13-18 May 2018
Casino Conference Centre
Europe/Prague timezone

Heat treatments of Cl-- and water-contaminated PuO2 and its analogues

17 May 2018, 14:00
15m
Marble Hall (Casino Conference Centre)

Marble Hall

Casino Conference Centre

Reitenbergerova 4/95, Mariánské Lázně, Czech Republic
Verbal Chemistry of Nuclear Fuel Cycle, Radiochemical Problems in Nuclear Waste Management NFC 5

Speaker

Sophie Sutherland-Harper (University of Manchester)

Description

Magnox PuO2 packaged before 1975 has become contaminated with Cl-, from thermally degraded PVC bags it was stored in, and water, from exposure to the atmosphere. Heat treatment is the preferred method for decontaminating the PuO2 for storage before either subsequent reuse as Mixed Oxide (MOX) fuel or safe disposal in a Geological Disposal Facility (GDF). Ion chromatography experiments carried out on the contaminated PuO2 show that leachable chloride on the solid decreases and volatilised chloride increases with increasing heat treatment temperature. Powder XRD shows no change in fcc Fm3 ̅m crystal phase with heat treatment, although high heat treatment temperatures (> 600°C) sinter the crystals, increasing the crystal size, and anneal the lattice damage caused by self-irradiation, decreasing the lattice parameter. When PuO2 is heated to 225°C and cooled in a sealed (Baskerville) vessel, a non-condensable mixture of gases (including hydrogen, helium, NO and CO), ideal above ~113°C, is produced, and the monolayers of water on the surface of the PuO2 decrease. Repeating this experiment with 0.05 mL water in a side chamber increases the number of water monolayers on the PuO2 surface.
PuO2 artificially chloride-contaminated, by exposure to dry HCl vapour, and humidified in a sealed pot with H2SO4 (conc.) shows similar results to the Magnox PuO2 when heat treated at various temperatures. CeO2 analogue nanocrystals have also shown an increase in size when exposed to HCl vapour and following heat treatment at temperatures higher than the original calcination temperature. Studies on CeO2 analogues also showed that the sorption mechanisms of chloride and water are linked. Upon heat treatment of the contaminated CeO2 at 900 °C, the nanocrystal morphology resembles that of PuO2 in TEM images, and the chloride, which was homogeneously spread on and within the CeO2 particles, is no longer detectable by EDX.
XPS and UPS studies on CeO2, CexO2-x, ThO2, UO2 and U metal thin films, involving in situ sputter deposition synthesis, HCl/Cl2 contamination and Thermal Programmed Desorption-Mass Spectrometry (TPD-MS), showed that adsorbed Cl- is not volatilised up to 800°C, but diffuses beneath the surfaces of the films. No change in oxidation state is observed for the metals in the metal oxides, but U metal oxidises upon contamination and TPD, with Cl- the only Cl species detected. XPS studies on spin coated CeO2 thin films contaminated with HCl vapour ex situ show a change in Ce oxidation state at high heat treatment temperatures from +4 to +3 for the thin films originally calcined at low temperatures, but Cl- remains the only Cl species present and also decreases on the CeO2 surface, following heat treatment at high temperatures.

Primary authors

Sophie Sutherland-Harper (University of Manchester) Dr Carolyn Pearce (Pacific Northwest National Laboratory) Dr Robin Taylor (National Nuclear Laboratory) Dr Jeff Hobbs (Sellafield Ltd.) Dr Robin Orr (National Nuclear Laboratory) Dr Kevin Webb (National Nuclear Laboratory) Dr Thomas Gouder (Institute for Transuranium Elements) Dr Alice Seibert (Institute for Transuranium Elements) Dr Helen Steele (Sellafield Ltd.) Prof. Francis Livens (University of Manchester) Prof. Nikolas Kaltsoyannis (University of Manchester)

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