Longevity of Plutonium 238 Heat Sources
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Roberta N. Mulford, Paul D. Richardson, Joseph Hickey, and Rene Chavarria, Los Alamos National Laboratory, Los Alamos, NM 87545, USA ABSTRACT
Plutonium oxide heat sources are used to power space missions. The heat produced by alpha decay of the 238 isotope of Pu is converted to electricity in a thermopile, providing electricity during a substantial fraction of the 88 year half-life of the isotope. Decay of the Pu produces helium and uranium, and a fraction of the evolved helium is captured in the oxide matrix. All of the helium produced in decay can in principle be contained in the oxide lattice, where it occupies the tetrahedral sites. Some helium diffuses out at a rate that is somewhat dependent on the form and morphology of the fuel. Rates have previously been measured for oxide aged about 1 year. Current measurements on sealed heat sources as old as 34 years indicate that the rate of diffusion has changed only slightly over time. Possible mechanisms for helium release include bubble diffusion, point defect migration, agglomeration and movement of He at grain boundaries, and volume diffusion through the lattice sites. We observe primarily diffusion from site to site within the lattice, with an activation energy of 18.7 kcal/mole, independent of point defect movement, despite the rising concentration of helium in the lattice over time and the accumulation of radiation damage within the lattice. Because of the slow diffusion of helium from the fuel to the headspace, heat sources are anticipated to be stable over a long lifetime. INTRODUCTION
The radioisotope thermoelectric generator (RTG) provides a rugged, reliable, and lightweight power source with a long life for applications where an unattended and maintenancefree power supply is required. The thermal energy produced by decay of about 9 grams of plutonium 238 oxide granules maintains these sources at an operating temperature of about 350ºC. A silicon-germanium thermopile converts heat to electricity with an efficiency of between 3 and7%. 238Pu is used for its high specific power of 0.4 W/g, and long physical lifetime from the 238Pu half-life of 87.7 y, giving the power sources a minimum of 25 to 30 years of service life for tens of milliwatts of electrical power. The practical lifetime may be much longer, and is likely limited by the decay product helium building up within the casing. The disposition of radiolytically produced helium within the unit and the effect of helium production on microstructure, morphology, and the ultimate integrity of the fuel are of both scientific and practical interest. Helium can occupy interatomic holes in the PuO2 fluorite lattice, suggesting the possibility that all of the helium arising over time can be captured in a perfect lattice. In practice, some helium diffuses out of the solid. Helium release under both pulsed and sustained heating has been examined in multiple studies, to underpin analyses of capsule pressurization under thermal insult. Most of these studies were done betwee
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