The Recombination of Hydrogen and Oxygen on Metal Oxide Surfaces

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 account for the release of known gases into the package and so ensure safe storage conditions. The purpose of this proposed PhD project is to contribute to a fundamental understanding of the factors influencing the empirical criteria.

There are a number of fundamental mechanisms that could lead to pressurisation, and all 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 (hydrothermal) chemical reaction of PuO2 with H2O.
Mechanisms (i) and (iii)-(v) are currently under study by the EPSRC-funded TRANSCEND Consortium. The last 3 mechanisms (iii)-(v), all involve PuO2/H2O interactions and are especially complex, inter-connected and poorly understood.
The scope for this PhD is focussed on mechanisms (iv) and (v). Previous experience has shown that packages sealed in storage cans under uncontrolled or none ideal conditions can potentially have headspace atmospheres that are hydrogen rich but rarely flammable i.e. contain hydrogen and oxygen.
Small scale studies of PuO2 packages at Los Alamos National Laboratory suggest that gaseous hydrogen and oxygen may be formed in such packages. However, the study also found that the pressure is limited by a recombination process. This may be through a gas phase recombination process of molecular hydrogen and oxygen, or hydrogen and a surface oxygen species on the PuO2 surface, and could be thermally or radiolytically driven processes.
If the hydrogen is produced primarily by the radiolysis of water, i.e. mechanism (iv), the comparative absence of oxygen in the can headspace raises questions as to whether this is due to PuO2+x formation, i.e. mechanism (v) or some other oxidative process. Gas phase recombination, with or without PuO2 acting as a catalyst, could prevent the coincident observation of the two gases and limit the extent of package pressurisation, but not fully explain why a number of packages have been shown to contain hydrogen.
Preliminary studies conducted at the Dalton Cumbria Facility indicate that irradiation of gas phase mixtures of hydrogen and oxygen with helium ions or gamma rays can lead to loss of hydrogen, presumably through radiation-induced reaction with oxygen to form water. This loss of hydrogen is found to be accelerated by the presence of zirconium and cerium oxides. Other radiolytic reactions also remove oxygen such as its reaction with nitrogen, although the potential role of metal oxide surfaces in promoting this reaction is not clear.
Thus, questions arise as to whether this putative catalysis exists on PuO2. Sellafield Ltd have started a programme of work at NNL to investigate what conditions would challenge any catalytic performance of the PuO2 surface. For instance, is it possible that surface saturation by moisture or adsorbed gases can reduce the PuO2 catalytic efficacy? Further, does aged PuO2 material behave in a similar way to fresh material in terms of its putative catalytic performance.
The proposed PhD, which will involve a significant period of placement at NNL’s Central Laboratory, will be working to address these questions. The student will work alongside NNL to further the understanding of the efficacy of PuO2 as a catalyst, and understand dependencies of the composition of the gas-phase on the surface activity of the metal oxide.
Parallel/preliminary work at the University will focus on method development for the on-line sampling of both hydrogen and oxygen and potentially other species as a function of T, P, water content, dose rate, specific surface area, co-adsorbed species etc. during recombination / catalytic reaction studies.

Academic Lead: Colin Boxall
Researcher: Cameron Williams
Location: Lancaster University