Toward an understanding of aging in plutonium from direct measurements of stored energy

We present here the first direct measurement of the radiation-damage-induced energy stored in δ-phase plutonium. The primary mode of radioactive decay of 239Pu occurs with a time constant of τ=1.1 × 1012 s. Each decay imparts about 85 keV of recoil energy to the uranium byproduct, 5.2 Mev to the alpha particle, and a spectrum of mostly low energy gamma rays with the most probable at 51 keV [1]. Most of the decay energy is converted immediately to heat, releasing about 1.9 mW/g. However, some thermally-recoverable energy remains trapped. Reported here are measurements of that stored energy using differential scanning calorimetry (DSC) applied to 239Pu-2.0 at.%Ga δ-phase alloy. Retained energy of ∼2 J/g saturates at about 5 months and is unchanged after 30 years. The magnitude of the stored energy agrees with a short-bond defect model that that we present here. This model treats radiation damage as a Pu impurity with shortened bond lengths. It explains the change in known properties with age and predicts that density increases with age, contrary to current thinking. The short-bond impurities proposed are expected to act like other impurities, affecting strength, phase transitions, grain boundaries and other metallurgical properties.