Probing thermodynamics of radiogenic helium and defects in δ-plutonium alloys and interactions with adsorbed environmental gases

Abstract

Differential scanning calorimetry coupled with simultaneous evolved gas analysis (DSC-EGA) on aged δ-Pu samples shows that most radiogenic helium remains trapped within the Pu matrix at temperatures very close to, or slightly above, the melting temperature. Our results indicate that helium release from 50-year-old δ-Pu occurs as a burst just below the melting temperature (>0.994 Tm), with subsequent pressure oscillations as temperature increases. Subordinate quantities of H2 were also released along with helium. The helium emission tails off and ceases above ∼720 – 750°C. In a δ-Pu alloy aged 6 years, the initial helium burst occurs slightly above melting (∼1.015 – 1.042 Tm), with a discrete, larger helium spike occurring between 670 and 686°C. The proximity of helium release to the liquidus transition presented challenges in the deconvolution of overlapping process enthalpies, the liquidus endotherm, and the exotherm resulting from bubble collapse, annealing and gas expulsion. Helium’s strong affinity for vacancy binding in a 2He-vac configuration is predicted by Density Functional Theory (DFT) modeling. The measured stored energy associated with the He release events in a 50-year-old δ alloy is on the order of ∼10–11 J/g, which is significantly higher than stored energies measured in the sub-solidus regimes (∼2 J/g) that are related to the solid-state annealing of processing- and radiation-induced defects. This implies that aged δ Pu alloys have a remarkable resilience to accommodate the lattice strain produced by the internal pressure of the helium bubbles and provides further insight into the thermodynamic behavior of aged δ Pu.