Advanced Transmission Electron Microscopy of Pu Alloys

Résumé du livre

The characterization of microstructural changes in Pu-Ga alloys resulting from storage and aging phenomena is an important technical challenge to the nuclear Stockpile Stewardship program. We have identified at least two age-related phenomena that may occur in Pu alloys, dimensional changes due to the initial transient, helium accumulation, and void swelling, and phase instability. The initial transient is a well-known effect that results from the initial cascade damage. This form of dimensional change tends to saturate within approximately two years. A second contributor to dimensional change is the build-up of helium as a result of the alpha decay. Helium is generated at a rate of approximately 40 parts per million per year. Positron annihilation results by Howell indicate that the helium atoms will quickly fill a nearby vacancy and diffuse through the lattice as a helium filled vacancy. Void swelling is potentially the most severe mechanism of dimensional change in Pu alloys. It has been observed in all-materials exposed to irradiation, but has yet to be seen in naturally aged Pu. Phase instability is a potential concern due to the fact that the {delta}-phase is thermodynamically metastable at room temperature. Timofeeva has shown that the {delta}-phase will decompose to {delta}-phase and Pu{sub 3}Ga given enough time at ambient temperature. At sub-ambient temperatures, the {delta}-phase undergoes a displacive or martensitic phase transformation to the monoclinic {alpha}{prime}-phase, which is approximately 20% more dense. Phase transformations such as these would result in density changes, dimensional changes, and Changes in mechanical properties. Traditional characterization techniques such as optical microscopy, x-ray diffraction and scanning electron microscopy are insensitive to many of the age-related microstructural changes. In this investigation, we have applied advanced transmission electron microscopy (TEM) to investigate the microstructure and bonding of Pu alloys. A 300 keV Phillips CM300FEG with a field emission gun electron source and Gatan Imaging Filter are used for the investigations.