PHD – Funded by the Nuclear Decommissioning Authority (NDA) at iCASE level for 4 years
Approximately 138 tonnes of separated Pu is in long term storage at Sellafield as 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 this gas release 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 to generate gaseous hydrogen and oxygen; and,
(v) Generation of H2 by a postulated (hydrothermal) chemical reaction of PuO2 with H2O.
The scope for this PhD is focussed on mechanisms (iv) and (v).
Small scale studies of PuO2 packages suggest that gaseous hydrogen and oxygen may be formed in such packages. However, these studies also found that the pressure is limited by a H2/O2 recombination process. This may be through a recombination process of hydrogen and oxygen and could be thermally or radiolytically driven processes. Experience has also shown that cans sealed under non-ideal conditions can have headspace atmospheres that, as expected, contain hydrogen but, mysteriously, contain little to no oxygen.
Preliminary studies 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. The potential role of metal oxide surfaces in promoting this reaction is not clear.
If the hydrogen is produced primarily by the radiolysis of water the comparative absence of oxygen in the can headspace raises questions as to whether this is due to the formation of a suggested PuO2+x phase or some other oxidative process or H2/O2 recombination. 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 with oxygen being mysteriously absent.
Thus, questions arise as to whether this putative recombination catalysis exists on PuO2 and the fate of the oxygen. Sellafield Ltd have started a programme of work at NNL to investigate this. The proposed PhD, which will involve a significant period of placement at NNL’s Central Laboratory, will be working to address these questions.
Preliminary work at Lancaster will focus on the development of methods for 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. The student will then 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.
The project is intellectually challenging and involves well-integrated elements of chemistry, engineering and materials science. The successful applicant will become familiar with techniques needed to tackle major problems in the nuclear industry and be part of a well-established team of nuclear researchers within Lancaster’s Engineering Department that seeks to address industrial problems while maintaining a strong science and technology base.
The appointee will interact with both Sellafield Ltd and the National Nuclear Laboratory (NNL), the UK’s largest nuclear research facility for the conduct of radioactive experiments. There will be a substantial period of placement at the NNL’s Central Laboratory in Cumbria.
How to apply
To apply, send a copy of your CV to Professor Colin Boxall, Engineering Department, Lancaster University, Lancaster LA1 4YR, or by email to email@example.com. Informal enquiries to this address are also very welcome.