Postirradiation examination of capsules HRB-4, HRB-5, and HRB-6

A description is given of an experiment to evaluate the irradiation performance of fuel rods fabricated using candidate processes and materials for the large high-temperature gas-cooled reactor (LHTGR) start-up and recycle fuel systems. The GA fuel specimens tested in each capsule consisted of six bonded fuel rods fabricated on prototype production equipment with preproduction candidate matrix and graphite shim materials. The fuel rods were fabricated by the admix-compaction and hot-injection processes and were carbonized in graphite tubes to simulate the in-block curing process envisioned for LHTGR fuel fabrication. The irradiation tests were conducted in the removable beryllium facility of the ORNL High-Flux Isotope Reactor (HFIR), and the capsules were monitored for in-pile fission gas release. The post-irradiation integrity of the 18 fuel rods tested was excellent. Irradiation-induced fuel rod dimensional changes were compatible with the dimensional change behavior of LHTGR near-isotropic moderator graphite. Fertile particle coating failure was attributed to excessive internal gas pressures generated during irradiation to those severe conditions, although matrix/coating interaction was a contributing failure mode. Fissile particles exhibited outer pyrocarbon (OPyC) coating failure levels up to 72.6 percent. High OPyC coating failure levels were attributed to excessive matrix/coating interactions resulting from the surface porosity of the OPyC coatings and the high matrix coke weight to fuel rod surface area ratios. As a result of these and other tests, LHTGR-type fuel rod matrix compositions with low coke weight to fuel rod surface area ratios have been developed to eliminate matrix/coating interactions. These matrix compositions are being tested in subsequent capsule irradiation tests