Aging Effects in Plutonium Metal Pose an Interesting Puzzle

Aging effects in plutonium metal are of metallurgical and practical interest. The Dismantlement Program has provided a wide variety of plutonium samples from a large number of retired nuclear weapons , enabling the development of a comprehensive database of materials properties as a function of age. Data are providing a substantive statistical foundation for these properties as well. The primary cause of aging in plutonium is radioactive decay, which alters composition and deposits energy into the lattice. Three major consequences of this decay are annealing, changes in phase, and in-growth of helium. Daughter species produced during radioactive decay alter the composition of the metal directly with age, in a known way. Composition may in turn affect phase, or lead to metastability. Impurities can affect mechanical properties by controlling the mobility and distribution of defects. Ingrowth of daughter species during radioactive decay may restabilize a metastable condition, as some of the daughters are known to serve the same function as gallium in stabilizing the delta phase. Energy deposited in the metal lattice produces disordered regions, including dislocations and vacancies. The number and movement of dislocations govern the plastic properties of a metal. Impurities or lattice defects produced by radioactive decay can block the passage of a dislocation, making the crystal harder to deform. Vacancies in the crystal can migrate to form small loop-shaped dislocations that can take part in plastic deformation. Thus, radioactive decay can alter the mechanical properties of a metal through a complicated set of competing processes. Age-induced changes are measured by a variety of techniques, as part of the Enhanced Surveillance Program and surveillance studies on plutonium metal of various ages. LIBS (laser-induced breakdown spectroscopy) gives a measure of annealing by indicating how far impurities have moved within the metal. Precision density and indentation testing (hardness measurement) may be sensitive enough to indicate the degree of annealing. Metallography reveals any large-scale structural perturbations resulting from annealing. Changes in phase are best revealed by x-ray diffraction measurements and by examination of the density of the material. Indentation testing may also indicate phase changes. Helium bubbles within the metal reduce the