Luke Jones2 5 Jamie Southworth2 5 Robin Orr3 Howard Sims4 Simon Pimblott1

2, The University of Manchester, Moor Row, Cumbria, United Kingdom
5, The University of Manchester, Manchester, Greater Manchester, United Kingdom
3, National Nuclear Laboratory, Seascale, Cumbria, United Kingdom
4, National Nuclear Laboratory, Abingdon, Oxfordshire, United Kingdom
1, Idaho National Laboratory, Idaho Falls, Idaho, United States

During the packaging of PuO2 for storage the conditions are closely controlled to limit water uptake by the oxide as radiolytic decomposition of any adsorbed water will lead to the formation of both a potentially flammable atmosphere containing molecular hydrogen as well as reactive oxygen species which may be incorporated into the oxide phase. To underpin the safety case for the long term storage of PuO2, a better understanding of the fundamental physical, chemical and materials degradation processes occurring at the water-oxide interface inside the storage canisters is desirable.
This study investigates the effect of the radiolysis of adsorbed water on PuO2 and various surrogate oxides. Samples of baked oxide powder were equilibrated in humidity chambers to attain a wide range of masses of water adsorbed onto the powder surface. In the case of PuO2, once the surface water on the oxide had equilibrated with the humid atmosphere, the samples were sealed into a glass vessel held at constant humidity with either an argon or a nitrogen over-gas and left for up to 3 months. Periodic sampling of the headspace was undertaken and the atmosphere above the oxide analysed using gas chromatography. In the case of the surrogate oxides, once the oxides had adsorbed the maximum quantity of water, the samples were flame sealed under argon into a glass tube and Co-60 gamma irradiated. Following gamma irradiation, the headspace was analysed using gas chromatography. Prior to and after gamma irradiation, the oxide powders were characterised using a variety of different microscopy and spectroscopy methods.
In nearly all of the experiments a linear production of molecular hydrogen as a function of radiation dos was observed. For PuO2, the rate at which H2 was produced increased with increasing water loading. This result contrasts with the experimental data in this study and from similar experiments in the literature utilising UO2, CeO2 and ZrO2 which showed a higher rate of production for lower water coverages. There was no evidence of a steady state hydrogen concentration being reached in the PuO2 experiments, an outcome opposite of that observed in storage canisters and in experiments performed by other groups in which the PuO2 was not held under constant humidity. Radiolytic production of molecular oxygen was not observed. Comparison of the properties of the surrogate oxide (ZrO2) powder prior to and post irradiation did not reveal any differences in stoichiometry, surface functionalization or the formation of oxygen-centered free radicals, so the location of the sibling oxidant to the evolved molecular hydrogen currently remains unanswered.

Acknowledgement: This work was supported by the Dalton Cumbrian Facility Project, a joint initiative of the University of Manchester and the Nuclear Decommissioning Authority, by the UK Engineering and Physical Sciences Research Council and by the US Department of Energy, Office of Nuclear Energy.