The electrical conductivity of two ceramic insulators, alumina, and silicon nitride was measured in a fission reactor, JMTR under a dynamic vacuum condition. The instrumented irradiation rig was dynamically evacuated during fission reactor operation. The uppermost vacuum level attained was better than 10^ Torr. In this experiment, we attempted to reveal effects of the gaseous environment on the measurement of electrical conductivity under ionizing irradiation. The results showed that the gaseous environment has a most hazardous effect in a certain gas pressure range. In the second experiment, an electrical resistivity of polycrystal α-alumina was measured at about 680K in a dynamic pumping condition. In a good vacuum, we observed smaller RIC(radiation induced conductivity) than our previous results. We observed RIED(radiation induced electrical degradation)-like behavior. The results suggested that RIED would take off faster at a higher ionizing dose rate. In the meantime, the take-off occurred at about the same displacement damage of about 0.03-0.05dpa in the range of fast neutron flux of 3.4-15.1x10^n/cm^2s
