Irradiation behavior of rock-like oxide fuels

Two irradiation tests were performed on the rock-like oxide (ROX) fuels in order to clarify in-pile irradiation stabilities. In the first test small disk-shape fuel targets were irradiated in the JRR-3 in JAERI. In the second test pellet-type fuels were employed. Irradiation behaviors such as swelling, fractional fission gas release (FGR) and phase change were examined by puncture test, profilometry and ceramography. Swelling and FGR behavior of the pellet-type fuels improved considerably compared with the disk-type fuels. Yttria stabilized zirconia (YSZ) single-phase fuel showed an excellent irradiation behavior, i.e. low FGR (<3%), negligible swelling and no appreciable restructuring. The particle dispersed fuels showed lower swelling and higher FGR than those of mechanically blended fuels. Spinel decomposition and subsequence restructuring in the spinel matrix fuels was observed for the first time in the present investigation. It would be possible to reduce the FGR of the spinel matrix fuels to that of the corundum ones, if the maximum fuels temperature is limited below 1700 K where neither spinel decomposition nor restructuring was observed. Damaged area of spinel matrix due to fission fragment irradiation seemed to be confined to thin layers around the surface of YSZ particles as expected.