Evaluation of the nucleation and growth of helium bubbles in aged plutonium

The processes of radioactive decay that take place in plutonium (Pu) can result in the generation of lattice damage and helium (He) bubbles, which can lead to significant changes to its physical, chemical and mechanical properties. As part of efforts to understand the ageing behaviour of Pu, dilatometry has been used to quantify lattice damage and the nucleation and growth of helium bubbles. Pu specimens were subjected to isothermal heat treatments for extended durations followed by metallographic examination in order to study the microstructures. The dilatometry data have enabled lattice damage and He bubble growth to be measured and quantified, as well as the calculation of diffusion coefficient of He in Pu. The values of the diffusion coefficients and activation energies suggest that the diffusion of He in Pu occurs via a vacancy mechanism, while bubble nucleation and growth at elevated temperatures takes place via the process of Ostwald ripening.