Radiolytic Recombination of H2, O2 and N2 over PuO2 Surrogate oxides

Abstract

Radiolysis of H2O adsorbed onto PuO2 leads to the formation of gaseous H2 and potentially O2. Despite H2 forming at low levels of H2O coverage of PuO2, pressurisation is rarely observed; this is hypothesised to be a result of radiolytic recombination of H2 and O2 upon exposure to the radiation field. This thesis reports experimental and modelling studies of the radiolysis of gas mixtures and gas/thoria mixtures as proxies for radiation chemistry in stored PuO2 canisters. The evolution of gas composition following exposure to radiation was both measured and simulated, with comparisons between the two used to infer reaction mechanisms and critical steps. H2 and O2 consumption was measured for gas phase samples exposed to γ-rays and the radiolytic yield of H2 consumption was found to increase upon raising the ratio of H2:O2. 229 rate constants were used within a chemical kinetic model to simultaneously solve couple rate equations and was validated against experimental results. A gas phase recombination mechanism was elucidated and shows a two-step mechanism for H2O formation: a primary Ar-initiated, H2O forming pathway and a secondary H2O-mediated pathway forming further reactive species to produce further H2O via the primary pathway. The compensation of H2 loss by H2O mediation offers a suggestion as to why the kinetics of H2-O2 radiolytic recombination are debated within the literature. Under similar atmospheres and radiation exposure, ThO2 is suggested to irreversibly consume O2 and slow the rate of H2 consumption. As radiation exposure continues, less O2 is consumed by the surface and only gas phase recombination proceeds. This has implications for the use of ThO2 as both a PuO2 surrogate and for storage as a potential future nuclear fuel.

Date of Award	31 Dec 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Francis Livens (Supervisor) & Andrew Horn (Supervisor)