Investigation into the Cause of an Increase in the Measured Values from the Gaseous Radionuclide Monitors in HANARO

The measured values from gaseous radionuclide monitors in HANARO were increased slightly right after the shutdown of the 43rd operation period, in May 2006. Though the amount of increase is not at issue, it was important to establish the initial actions to be taken, and to examine the cause of the event quickly and accurately. In this report, the actions taken to cope with the situation and the investigation procedure for the origin of this phenomenon are described. A leakage of the Xe-133 generated inside the nuclear fuel, as a fission fragment, was considered to be the origin of an increase in the monitor values. From the sipping and 5 MW operation tests, it was confirmed that the pressure difference due to the pool water depth would affect the Xe-133 release. Therefore, the response of the monitor during the fuel movement was carefully surveyed. It was finally concluded that the HFH-085R fuel had a deficiency. This Xe-133 release was confirmed to be a new type of deficiency which has not been experienced or reported during the use of metallic fuel like HANARO fuel. In this procedure, when searching for the defective fuel, various analysis methods were utilized. These methods would be very useful for the safe operation of the research reactor