Development of fuel failure detection system for a high temperature gas cooled reactor. II

For pt.I see ibid., vol.34, p.567 (1987). A fuel failure detection (FFD) method based on selective detection of short-life and gaseous fission products, developed for a high-temperature gas-cooled reactor, is described. An improved precipitator was used as a detector for the fission products and the performance of the FFD system was tested using an irradiation rig at the Japan Material Testing Reactor. In the rig, three kinds of samples of coated-particle fuels were irradiated and each sample of the primary helium gas was fed to the FFD system. Failure rates of the three fuel samples called intact, normal, and slightly failed, were estimated at about 10^-6, 10^-5, and 10 ^-4, respectively. The FFD system showed a significantly increased response in counting rate for the sample gas with a failure rate of 10^-4. The FFD system did not respond to the sample gas with the smaller failure rate of 10^-5 even when the background level of long-life fission products in the primary coolant gas increased with fuel temperature and reactor power