Performance of AGR-1 high-temperature reactor fuel during post-irradiation heating tests

The fission product retention of irradiated low-enriched uranium oxide/uranium carbide tri-structural isotropic (TRISO) fuel compacts from the Advanced Gas-Cooled Reactor 1 (AGR-1) experiment has been evaluated at temperatures of 1600-1800 degrees C during post-irradiation safety tests. Fourteen compacts (a total of 58,000 particles) with a burnup ranging from 13.4% to 19.1% fissions per initial metal atom (FIMA) have been tested using dedicated furnace systems at Idaho National Laboratory and Oak Ridge National Laboratory. The release of fission products Ag-110m, Cs-134, Cs-137, Eu-154, Eu-155, Sr-90, and Kr-85 was monitored while heating the fuel specimens in flowing helium. The behavior of silver, europium, and strontium appears to be dominated by inventory that was originally released through intact SiC coating layers during irradiation, but was retained in the compact at the end of irradiation and subsequently released during the safety tests. However, at a test temperature of 1800 degrees C, the data suggest that release of these elements through intact coatings may become significant after similar to 100 h. Cesium was very well retained by intact SiC layers, with a fractional release <5 x 10(-6) after 300 h at 1600 degrees C or 100 h at 1800 degrees C. However, it was rapidly released from individual particles if the SiC layer failed, and therefore the overall cesium release fraction was dominated by the SiC defect and failure fractions in the fuel compacts. No complete TRISO coating layer failures were observed after 300 h at 1600 or 1700 degrees C, and Kr-85 release was very low during the tests (particles with failed SiC, but intact outer pyrocarbon, retained most of their krypton). Krypton release from TRISO failures was only observed after 210 h at 1800 degrees C in one compact. Post-safety-test examination of fuel compacts and particles has focused on identifying specific particles from each compact with notable fission product release and detailed analysis of the coating layers to understand particle behavior. (C) 2016 Elsevier B.V. All rights reserved.