Detection of fuel break

The break of fuel rods occurred in Mihama No. 1 reactor should be detected by the change of radioactivity concentration in primary coolant, but actually, it was not able to be detected. It was claimed that significant signal did not appear in the monitors because the progress of break was slow. In this study, the change of iodine concentration shown in case of the break of fuel cladding tubes was evaluated independently, and the change of iodine concentration in primary coolant at the time of the Mihama No. 1 reactor was compared with it, thus the time of the occurrence of accident was estimated. The model for evaluating the rate of defects in fuel cladding tubes using the iodine concentration in primary coolant is explained. When pellets are exposed to primary coolant due to the break of fuel cladding tubes, the rate of transfer of I-131 produced in the pellets to the primary coolant is 5 times as much as that through a standard pin hole. The change of iodine concentration in primary coolant in case of Mihama No. 1 reactor was estimated, and a peak was found in the actual radioactivity concentration curve, which is in very good agreement with the estimated change. It occurred on January 10, 1973. The problems accompanying the evaluation of radioactivity concentration are pointed out. (Kako, I.)