Chapter 8: A molecular dynamics study of helium clustering in high temperature plutonium dioxide

The behaviour of helium in high temperature PuO₂ has been examined via molecular dynamics calculations. In an initially defect free lattice helium is mobile at temperatures above 1500 K, making inter-site hops via thermally generated oxygen vacancies. Over a 5 ns time scale, helium clustering does not occur until above 2400 K, at which temperature temporary plutonium Frenkel pairs are possible. The created Pu vacancy can be pinned by either oxygen vacancies (as a Schottky trio), or by the addition of extra He before Pu recombination occurs. This site then acts as a helium cluster seed location to which further He can be attracted. The time evolution of helium clusters was analysed and a mean He:vacancy ratio of 0.75:1 was obtained. Throughout the dynamical evolution of the cluster, a He:vacancy ratio below 1:1 was always found in contrast to previous static calculations which predicted a theoretical maximum ratio of 3.5:1.