Compositional and Structural Evolution of Plutonium Dioxide: Underpinning Future Decisions

Plutonium dioxide is a very dynamic material. Radioactive decay damages the lattice and also forms other elements in the material. Helium, an inert gas, may be localised or trapped in the lattice, or maybe released. Uranium isotopes (formed from the decay of plutonium-238, 239, 240) and americium-241 (formed from decay of plutonium-241) are formed atom-by-atom within the plutonium dioxide lattice.

The UK has 140 tonnes of separated plutonium in the form of plutonium dioxide, the World's largest civil stockpile. This has been separated over the last half century and will need to be stored for several decades into the future before its end use. Currently, Government intends most of this material to be made into nuclear reactor fuel ('mixed oxide fuel'), with a small proportion, which cannot be made into fuel, being disposed of as waste, although policy changes could lead to more of it being designated as waste. Whatever the final fate of the plutonium, the material will need to be processed into a suitable form for its end use, and its evolution while it is being stored will affect its suitability for processing. We therefore need to be able to predict how plutonium dioxide will change in storage, so we know whether it will be suitable for its final use. The purpose of this project is to understand how plutonium dioxide changes so we can make these predictions.

We will make experimental measurements with plutonium dioxide to define the effects of radiation damage, helium formation and decay product formation on the material over timescales up to several decades. The evolution of plutonium dioxide will be explored using both a series of model samples and materials drawn from the UK stockpile. Behaviour of decay products will be determined using the stockpile materials. We will use synchrotron techniques (X-ray absorption spectroscopy, diffraction and tomography), electron microscopy and specific surface area measurements to characterise the materials. The results of these experiments will be used to develop computational models of plutonium dioxide evolution. Because decay products form atom-by-atom, and decay processes affect the electronic structure of the material, we need to model all these processes at the scale of individual atoms and small aggregates of atoms, but because the properties we are interested in are manifest at the lattice scale, we also need to understand how the atomic-scale effects carry across to this larger scale, and we will also develop models which we can use at this larger scale.

There are three proposed impact activities, focused on: the UK plutonium programme and particularly the organisations responsible for stewardship of plutonium stocks; the nuclear workforce and development of next generation Subject Matter Experts; and the wider public.

UK Plutonium Programme. The UK has to ensure that both its civil and military plutonium stockpiles are managed safely for decades into the future and that, whatever option is chosen for final disposition, the material is fit for that purpose. Ongoing plutonium stewardship is costly (£ 73 M p.a.) and Nuclear Sector Deal cost reduction targets are driving a search for efficiency savings, which have to be underpinned by robust understanding of the material, as we aim to provide. There are four key stakeholder organisations in the UK:

the Nuclear Decommissioning Authority (NDA) is responsible for implementing Government policy on management of the UK nuclear legacy, including "the safe, secure and cost-effective lifecycle management of nuclear materials";
Sellafield Ltd (SL), the operator of the Sellafield site in west Cumbria, where the UK civil plutonium stockpile is stored, is responsible for the day-to-day stewardship of this material;
the National Nuclear Laboratory (NNL), the UK's national capability for nuclear fission technology including nuclear materials management, operates unique R&D facilities, including plutonium facilities in its Central Laboratory on Sellafield site, and both supports the UK plutonium programme and provides expert advice to Government; and
the Atomic Weapons Establishment (AWE) is responsible for maintenance of the UK's independent nuclear deterrent and is hence the custodian of the UK's military plutonium stockpile.

Under the auspices of this project, we will convene a Stakeholder Group (SG), comprising the four key stakeholders, plus the relevant academic partners from the EPSRC TRANSCEND consortium (EP/S01019X/1), which has a complementary plutonium work package. This group will focus on two-way technical exchange, feeding research results from the project to stakeholders to support their programmes, and providing a route for stakeholder input to the direction of the project.

In addition to the four UK stakeholders above, who have a technical interest in this project, the Department of Business, Energy and Industrial Strategy (BEIS) sets UK policy on plutonium management, and the Office of Nuclear Regulation (ONR) is the independent regulator responsible for oversight of all UK nuclear facilities and programmes. We will use existing bodies for wider dissemination of research findings, including to BEIS and ONR through the NDA Research Board, the Nuclear Waste and Decommissioning Research Forum; Government's Nuclear Innovation and Research Advisory Board; and the ONR Independent Advisory Panel.

Nuclear Workforce. Nearly 70% of subject matter experts (SMEs) are due to retire by 2025 and, given the scale and duration of the UK's forward Pu management programme, a cross-industry 'Alpha Resilience & Capability' (ARC) initiative is being put in place, with a particular focus on SME development. We are closely connected to this initiative through the Nuclear Skills Strategy Group, and Livens contributes to the ARC technical workstreams.

Wider Public. We will use the EPSRC's Next Generation Nuclear (NGN) CDT and the Imperial College-led CDT in Nuclear Engineering, and their successors, as vehicles for public impact through activities including the Nuclear Hitchhiker blog run by our students and PDRAs; regular school and public events; the annual Pint of Science Festival; our annual weekend-long event 'The Nuclear Journey-Keeping it Clean' at the Museum of Science & Industry, Manchester; and public lectures by the project investigators.