PuO₂ and Fuel Residues

Over 100 tonnes of plutonium, in the form of PuO2, is currently stored at the Sellafield site. Radiation induced degradation of chlorinated polymers, such as poly(vinyl chloride) (PVC), used historically in packaging of PuO2, has led to chloride-contaminated PuO2, which constitutes a significant proportion of the Pu inventory. The Government’s preferred option for plutonium disposal is re-use as mixed oxide fuel but further work is needed to underpin this option. In the interim period before any disposal route is implemented, safe and secure storage of PuO2 will need to be maintained in stores at Sellafield. Reuse of the chloride contaminated plutonium requires treatment of the PuO2 to reduce Cl- contamination to acceptable levels for fuel manufacture; whereas storage requires stabilisation coupled with a predictive understanding of the evolution of Cl–contaminated PuO2 over time. The PuO2 in storage containers is subject to a range of conditions from the centre of the container, where it will be hot and dry with the potential for gas formation as steam, to the outside where it is cooler and water may be present that can undergo radiolysis to produce H2; therefore, interrogation of the behaviour of Cl–contaminated PuO2 under these conditions is required. Furthermore, one potential treatment for the chloride contaminated PuO2 is the use of pyrohydrolysis – reaction with steam at elevated temperatures. Hence, the interactions between chloride species and water/hydroxyl ions are key to understanding and managing these materials. The project involves a detailed study of Cl- adsorption/desorption behaviour on PuO2 surfaces, in parallel with analogue studies on CeO2, UO2 and ThO2 to determine the combined effect of physical (particle size/morphology/porosity), chemical (e.g. formation of PuCl3), and radiation-induced phenomena. Interaction/competition of Cl- with surface bound OH/H2O will also be a key variable. To simulate the Cl–contaminated PuO2 in the storage containers, samples of PuO2 and Ce/U/Th-analogues with varying particle size and different amounts of chemisorbed H2O will be synthesised and exposed to HCl vapour under a controlled atmosphere. The Cl–exposed samples will then be heated to a range of temperatures and the amount of Cl- removed will be quantified in terms of the fraction evolved as HCl or Cl2 gas, the leachable fraction and the ‘non-leachable’ fraction that remains associated with the solid. The reacted product will be analysed using pyro-hydrolysis, TGA, TEM/SEM-EDX, along with surface spectroscopy techniques such as XPS, to determine the chemical form of any residual Cl-. Radiation-induced phenomena will be studied using the Pelletron at University of Manchester’s Dalton Cumbrian Facility, capable of supplying 15 MeV helium ions to investigate the effects of; (i) radiation-induced defect sites in CeO2 and other analogue phases; and (ii) Cl–contamination on hydrogen production from water radiolysis. The results from this fundamental research will provide a greater understanding of PuO2 surface species and interactions, complementing more empirical studies at the NNL aimed at defining flowsheet conditions for a heat treatment process, ultimately informing plant design for the treatment or storage of Cl–contaminated PuO2. This challenging project will involve access to national/international user facilities including the Dalton Cumbrian Facility for radiation science experiments, National Nuclear Laboratory Central Labs at Sellafield to carry out the Pu chemistry and the Rad Annex at the Environmental Molecular Science Laboratory, WA, USA to analyse reacted Pu products.