Simulating radiation damage in δ-plutonium

Radiation events in δδ-Pu (fcc) have been simulated in an attempt to understand the fundamental mechanisms that contribute to the Pu ageing process. The Pu interactions are modelled using a potential based on the modified embedded atom method (MEAM). The energetics of point defects have been investigated using static calculations together with molecular dynamics (MD) to simulate radiation events. All MD simulations were carried out with Pu initially in the face-centred-cubic (fcc) structure, although this is not the lowest energy configuration for the pure metal.The point defect study suggests that the mono-vacancy has the lowest formation energy (0.46 eV), with interstitial defects favouring the 〈100〉 – split orientation over occupation of the native fcc octahedral site. Displacement threshold energy calculations at room temperature give a minimum value of between 5 and 6 eV, increasing to 8–14 eV along the major crystallographic directions.Low energy collision cascades, initiated with energies in the range of 0.4–1 keV, show that the cascades form in a similar manner to other fcc metals with a vacancy rich zone at the cascade core, surrounded by isolated interstitial defects. Higher energy cascades show similar features but with occasional channelling of energetic atoms and sub-cascade branching which significantly reduces defect production. A common trait observed across all the cascades was the relatively slow annealing period, compared to cascades in other fcc metals, with simulations at energies above 5 keV requiring many 10’s of picoseconds before the ballistic phase was completed.