Density changes in plutonium observed from accelerated aging using Pu-238 enrichment

Abstract

In support of Stockpile Stewardship activities, accelerated aging tests on a plutonium alloy enriched with 7.3 atomic percentage of 238 Pu is underway using dilatometry at 35, 50, and 65°C and immersion density measurements of material stored at 50°C. Changes in density are expected from radiation damage in the lattice and helium in-growth. After twenty-five equivalent years of aging, the dilatometry data shows that the alloys at 35°C have expanded in volume by 0.11% to 0.12% and have started to exhibit a near linear expansion behavior primarily caused by the helium accumulation. The average He-tovacancy ratio from tested specimens was determined to be around 2.3. The model for the lattice damage and helium in-growth accurately represents the volume swelling at 35°C.The density converted from the dilatometry corresponds well to the decreasing density trend of reference plutonium alloys as a function of time.

Physical Property Changes in Aging Plutonium Alloys

“…Changes in quasi-static density and mechanical properties at ambient temperature and pressure are expected due to the aging process. Our past results have shown gradual increase in strength, decrease in density and ductility of plutonium alloys due to aging [3][4][5]. In this paper we update changes in density and mechanical properties of plutonium alloys due to aging.…”

“…The schematic and detailed operations of the dilatometer system are presented elsewhere [3][4][5], so only a brief description is provided here. An LVDT measures minute changes, 0.1 micron or less, in the position of a push-rod by monitoring changes in the inductance of a detector coil.…”

“…A correction needs to be applied to the measured density to compensate for the heat generated by the 238 Pu-spiked sample. Details of density correction are described elsewhere [5].…”

Opportunities in Plutonium Metallurgical Research*

“…Research on natural aging of Pu has received significant attention over the past seven years [6][7][8][9][10][11][12][20][21][22][23][24][25][26]. Many of these investigations have focused on the natural self-irradiation caused by the decaying Pu isotopes and the resultant age-related changes in the microstructure.…”

“…Chung et al [25] have used dilatometry to investigate the continuous change in length of an accelerated aging specimen as a function of age. The technique of dilatometry measures the length change of the specimens and takes into account both the increase in volume due to the lattice parameter expansion from the initial transient as well as the free volume increase due to the accumulation of helium.…”