Resonant ultrasound, thermodynamics, and bad assumptions about plutonium aging

239Pu is one primary fuel for nuclear weapons. With a 1/e time constant of 1.1 × 1012 s, it undergoes significant radioactive decay over year-like time scales. What happens as it ages is an important question. Common wisdom is that it gets less dense with time. In a little over 300 years, 1% of this isotope has decayed, each decay releasing 239Pu 86 keV of recoil energy to the uranium daughter product and 5.2 MeV to the alpha particle produced. Most of the decay energy is converted immediately to heat, releasing about 1.9 mW/g. However with age, some thermally recoverable energy remains trapped in local distortions of the lattice. We show here how Resonant Ultrasound Spectroscopy combine with Differential Scanning Calorimetry yields a surprise that can be explained with a short-bond defect model. Such a model can only apply to plutonium, providing an “escape route” for radiation damage not present in the materials whose radiation damage behavior is assumed to apply to plutonium. The short-bond impurities are expected to act like any other impurity, but also increase the density.