Accelerated dynamics study of vacancy mobility in δ-plutonium

Using parallel-replica dynamics and temperature accelerated dynamics, we extract the rates for mono- and di-vacancy diffusion in δ-plutonium (Pu) using two parameterizations of the modified embedded atom method (MEAM). We find that mono-vacancy diffusion is faster in “pure” Pu than in δ-stabilized Pu. Also, at higher temperatures, the rate of double jumps is nearly the same as single jumps in pure Pu. Since these double jumps contribute four times as much as single jumps to the diffusion constant, models incorporating mono-vacancy diffusion must account for this mechanism to predict mass transport in Pu. While di-vacancies are energetically only slightly preferred compared to mono-vacancies, they are significantly more mobile. Surprisingly, this enhanced mobility is due to the prefactor; the migration barrier is essentially identical. The di-vacancy dissociates at a rate similar to the mono-vacancy hop rate.