Fission-product release during postirradiation annealing of several types of coated fuel particles

Postirradiation annealing tests were conducted at temperatures of 12000, 13500, and 15000C on five designs of TRISO-coated fuel particles for use in high-temperature gas-cooled reactors. The only fuel type that retained all monitors radioisotopes (Cs, Ag, Eu, and Ce) over the full range of annealing temperature was a particle design with a UO2 kernel having a thin overcoating of ZrC to serve as an oxygen getter. The other four fuel candidates (UO2, UC2, UCO, and UO2 with ZrC dispersed in the inner buffer layer) all released Ag and Eu at 15000C. The last three of these four fuels also released Ce at 15000C, and the first two fuels released Cs as well. At the two lower annealing temperatures, only the highly diffusive Ag and Eu isotopes were released, the former only from UO2 with dispersed ZrC (at both 13500 and 12000C), and the latter from the same particle design at 13500C and from UCO at both temperatures. (orig.)