PuO₂ and Fuel Residues

More than 100 tonnes of Pu are stored at Sellafield as PuO₂ powder in sealed steel storage cans. Under certain circumstances, gas generation may occur within the can with consequent can pressurisation.

This comprises one of the most serious fault scenarios to be considered in the safety cases for PuO₂ storage and avoided in practice. 5 routes to gas production having been suggested:

• Helium accumulation from alpha decay;
• Decomposition of polymeric packing material;
• Steam produced by H₂O desorption from hygroscopic PuO₂ due to self-heating
• Radiolysis of adsorbed water;
• Generation of H₂ by chemical reaction of PuO₂ with H₂O, producing a postulated PuO_2+x phase.

The last 3 mechanisms, all involving the interaction of PuO₂ with H₂O, are complex and poorly understood, not least because of the interplay between them. An additional challenge to the safe storage and eventual disposition of Pu is that a substantial portion of the inventory is “out-of-specification” because of impurities (specifically chlorine or carbon) or unfavourable powder properties (moisture content, unusually high/low specific surface area). These must be treated to stabilise them for storage in welded cans in new stores.

Thus, within the DISTINCTIVE theme of PuO₂ behaviour during extended storage, this programme, a collaboration between the universities of Lancaster and Manchester, and UCL, will seek to:

• Understand how the structure and properties of PuO₂ change with time in the presence of H₂O
• Attribute these to the fundamental chemical, physical & radiation driven processes at the PuO₂ surface;
• Understand the roles these processes play in gaseous product evolution at Pu oxide surfaces;
• Understand how the overall ‘system’ as well as the specific processes are affected by the ageing of the Pu including variations in Pu isotopics;
• Generate data sets for the better underpinning of the Pu storage safety cases.

Additionally, we will study the surface adsorption mechanisms of chloride on PuO₂ and how effects such as radiation, T and adsorbed water affect the surface speciation and consequently desorption of chloride species under conditions to be employed in likely treatment processes.