Estimates of helium gas release in 238PuO2 fuel particles for radioisotope heat sources and heater units

Release data of noble gases (Xe and Kr) from small-grain (7-40 mu m), large-grain ( greater than or equal to 300 mu m), and monocrystal UO2 fuel particles, during isothermal irradiation up to 6.4 at.% and 2030 K are reviewed and their applicability to estimate helium release from (PuO2)-Pu-238 fuel particles(greater than or equal to 300 mu m in diameter) is examined. Coated (PuO2)-Pu-238 particles have recently been proposed for use in radioisotope power systems and heater units employed in planetary exploration missions. These fuel particles are intentionally sized and designed to prevent any adverse radiological effect and retain the helium gas generated by the radioactive decay of (238)pu, a desired feature for some planetary missions. Results suggest that helium release from large-grain ( greater than or equal to 300 mu m) particles of K could be <7% at 1723 K, <0.6% at 1042 K, and even less for polycrystalline particles fabricated using sol-gel processes. Results also suggest that helium release from small-grain plutonia particles at 1723 K could be >80% but less than 7% at 1042 K, which is in general agreement with the experiments conducted at Los Alamos National Laboratory more than two decades ago. In these experiments, the helium gas release from small-grain (7-40 mu m) (PuO2)-Pu-238 fuel pellets has been measured during steady-state heating at temperatures up to 1886 K and ramp heating to 1723 K. (C) 2000 Elsevier Science B.V. All rights reserved.