Irradiation performance of HTGR fertile fuel in HFIR target capsules HT-12 through HT-15. Part I. Experiment description and fission product behavior

Sixteen types of Biso-coated designs, on ThO2 kernels, were irradiated in High Flux Isotope Reactor target capsules HT-12 through HT-15. The report addresses the description of the experiment and extensive postirradiation analyses and experiments to determine fertile-particle burnup, fuel coating failures, and fission product behavior. Several low-temperature isotropic (LTI) pyrocarbon coatings, which ''survived'' according to visual inspection, were shown to have developed permeability during irradiation. These particles were irradiated at temperatures approximately equal to 12500C and to burnups equal to or greater than 8 percent fission per initial heavy-metal atom (FIMA). No evidence of permeability was found in similar particles irradiated at temperatures approximately equal to 15500C and burnups approximately equal to 16 percent FIMA. Failures due to permeability were not detectable by visual inspection but required a more extensive investigation by the 10000C gaseous chlorine leaching technique. Maximum particle surface operating temperatures were found to be approximately 3000C in excess of design limits of 9000C (low-temperature magazines) and 12500C (high-temperature magazines). The extremes of high temperatures and fast neutron fluences up to 1.6 x 1022 neutrons/cm2 produced severe degradation and swelling of the Poco graphite magazines and sample holders