Maintenance of fuel failure detector system of the HTTR

Noji, Kiyoshi; Kameyama, Yasuhiko; Emori, Koichi; Aono, Tetsuya

The FFD (Fuel Failure Detection) System has been installed in the HTTR in order to detects the abnormal release of fission products from the fuel during the operations. The FFD system samples the primary coolant from the high-temperature plenum division of the reactor core divided into seven regions. The system detects short life fission product(FP) gases from each region. The damaged region can be specified by the FFD system. In the design, it was considered that the change in the sampling flow rate during operation was not necessary. However, it became clear that the measured value became unstable because of a fluctuation of the sampling flow rate due to change in the primary coolant pressure during operation. Moreover, it was difficult to change the sampling flow rate during operation. The sampling flow rate was controlled by manual valves located in the service area where the entry is limited during operation. Therefore, an improvement was carried out to control the sampling flow rate from the outside of the service area. The stable measured value was obtained by the improvement. Moreover, noise reduction, improvement of oil level gauge of compressors gives excellent operation of the FFD. This report summarizes the maintenance work of detectors (precipitator), equipment and improvement items of the system.