An ageing stockpile: atomistic simulation of helium behaviour in plutonium dioxide

Murray, Elanor Rose (2023). An ageing stockpile: atomistic simulation of helium behaviour in plutonium dioxide. University of Birmingham. Ph.D.

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Abstract

Until the UK Government arrives at a decision regarding the final treatment and disposition of plutonium, the NDA remain responsible for the safe and secure storage of the PuO2 stockpile currently in inert steel cans at Sellafield. Radiogenic helium gas generation naturally occurs in the ageing of PuO2 due to the spontaneous alpha decay of Pu isotopes, creating self-radiation damage to the lattice. The risk of helium gas pressurisation is a safety concern for long-term storage of PuO2; hence, fundamental understanding of helium behaviour, including its incorporation, diffusion, and clustering, is needed. Atomistic simulation techniques are ideally suited to provide fundamental insight into helium behaviour in PuO2. Initial defect calculations from atomistic simulations of bulk PuO2 were performed, identifying the Schottky trio and oxygen Frenkel pair as favourable defects. The (111) surface was predicted to have the lowest surface energy and therefore dominate the PuO2 growth morphology. Intrinsic defects were simulated for the (100), (110) and (111) surfaces. It was found that there is an energetic drive for plutonium Frenkel pairs to form at the surface rather than the bulk of PuO2. Atomistic simulations using interatomic potentials were performed to model helium incorporation in bulk PuO2. In defect-free PuO2 the octahedral interstitial site was determined to be the most energetically favourable site for helium. When defects were present, the plutonium vacancy had a negative helium incorporation energy, indicating it may act as a trap for helium atoms. Helium clustering around plutonium vacancies and Schottky trios was observed. However, Pu vacancies and Pu Frenkel pairs are energetically unstable and so may not be available for helium accommodation. Molecular dynamics and energy barrier searching methods were employed to investigate the diffusion and clustering of helium in PuO2. The results show that in perfect PuO2, interstitial He is not mobile over nanosecond time scales at temperatures below ∼ 1500 K, with the lowest energy diffusion barrier being 2.4 eV. When oxygen vacancies are present the He diffusion barrier drops to 0.6 eV, enabling helium diffusion at lower temperatures. The dominant He diffusion mechanism is shown to be oxygen vacancy assisted inter-site hopping rather than the direct path between adjacent interstitial sites. Thus, oxygen vacancies are key to the diffusion of helium; such vacancies are generated either via self-radiation damage, or thermally at high temperatures. Unlike oxygen vacancies, plutonium vacancies act as helium traps. In molecular dynamics simulations it was observed that helium occupying a Pu vacancy can easily be ejected via Frenkel pair recombination. As such, plutonium vacancies must be stabilised to nucleate helium clusters, such as in Schottky trios. High temperature molecular dynamics simulations show that helium can diffuse via oxygen vacancies into clusters with the majority of helium clusters which form over nanosecond time scales having a He:Vacancy ratio below 1:1. Further static calculations show that a ∼ 3.5:1 He:Vacancy ratio is the largest possible for an energetically stable helium cluster. Schottky defects are found to act as seed points for He cluster growth and high local concentrations of He can create such defects, which then pin the growing He cluster. iv

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):