Physical Property Changes in Plutonium Observed from Accelerated Aging using Pu-238 Enrichment
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Physical Property Changes in Plutonium Observed from Accelerated Aging using Pu-238 Enrichment Brandon W. Chung, Bill Choi, Cheng Saw, Stephen Thompson, Conrad Woods, David Hopkins, and Bartley Ebbinghaus Lawrence Livermore National Laboratory, Livermore, CA, 94551
ABSTRACT We present changes in volume, immersion density, and tensile properties observed from accelerated aged plutonium alloys. Accelerated alloys (or spiked alloys) are plutonium alloys enriched with approximately 7.5 weight percent of the faster-decaying 238Pu to accelerate the aging process by approximately 17 times the rate of unaged weapons-grade plutonium. After sixty equivalent years of aging on spiked alloys, the dilatometry shows the samples at 35°C have swelled in volume by 0.15 to 0.17 % and now exhibit a near linear volume increase due to helium in-growth. The immersion density of spiked alloys shows a decrease in density, similar normalized volumetric changes (expansion) for spiked alloys. Tensile tests show increasing yield and engineering ultimate strength as spiked alloys are aged. INTRODUCTION Plutonium, because of its radioactive nature, ages from the “inside out” by means of selfirradiation damage and thus produces Frenkel-type defects (vacancies and self-interstitial atoms) and defect clusters [1, 2]. The self-radiation damage in Plutonium-239 occurs mainly by alphaparticle decay, where most of the damage comes from the 235U recoil nucleus. The defects resulting from the residual lattice damage and helium in-growth could result in microstructural and physical property changes. Because these self-irradiation effects would normally require decades to measure, a fraction (7.5 wt%) of more active isotope 238Pu is added to the reference plutonium alloy thus accelerating the aging process by approximately 17 times. By monitoring the properties of the Pu-238 spiked alloy over a period of about 3.5 years, the properties of plutonium can be projected for periods up to about 60 years. This paper presents density, volume, and tensile changes observed from accelerated aged plutonium alloys.
EXPERIMENT Each dilatometer unit consists of a small vacuum controlled-atmosphere sample chamber fitted with three linear variable differential transducers (LVDTs). The schematic and detailed operations of the dilatometer system are presented elsewhere [3, 4], 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. In the current design, the
detector coil is placed outside of the sample chamber. Two different lengths (2 and 3 cm) of alloy specimens are used to differentiate between surface oxidation and volumetric swelling in the materials. These alloys have nominal gallium concentration of 0.5 weight percentage. These specimens are placed in the copper well located inside the dilatometer system at 35°C storage temperatures. A reference low thermal expansion glass (Zerodur) is also placed in the copper well to
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