Power coefficient anomaly in JOYO (2nd report); Presumption about the causing mechanism

not registered; not registered; Sasaki, Makoto; not registered

This report describes the presumption about the causing mechanism of the power Coefficient Anomaly in JOYO, which occurred in 75 MW Power Ascension Test, summer 1979. After the previous report, the new information about the results of the post irradiation examination and the analyses about JOYO power coefficient could be obtained. The causing mechanism of the anomaly could be presumed from these information as follows. [Step 1]-50 MW Power Operation Period- The fuel burn-up reached about ten thousand MWD/ton at the end of 50 MW duty 2nd cycle, and the produced fission gas was almost retained in the fuel pellets. [Step 2]-Fission Gas Release during Initial Power Ascension from 50 MW to 75 MW- On the occasion of the initial power ascension from 50 MW to 75 MW, the fission gas release began suddenly at the power level of some above 50 MW. [Step 3]-Rising of Fuel Pellet Temperature and Appearance of Pellet Crack due to Fission Gas Release- The fission gas release reduced the gap conductance and caused the rising of fuel pellet temperature and appearance of pellet crack. These phenomina induced the fuel stack length to elongate abruptly. [Step 4]-Abnormal Behavior of Power Coefficient- The fuel temperature rising and the fuel stack length elongation induced the fuel expansion reactivity effect and Doppler reactivity effect to enlarge, and caused the anomaly behavior of power coefficient. [Step 5]-Power Coefficient Changing and Reactivity Loss of 40 cent- After the initial 75 MW, the fuel stack length didn't respond nomaly to the reactor power change by some reason. Therefore, the magnitude of power coefficient became smaller and the power level dependency changed after 75 MW. At the shut down of reactor, the core average stack length elongated a few mm and the reactivity of about 40 cent was lost from the core after the power coefficient anomaly. As mentioned above, it is presumed that the causing mechanism of the power coefficient anomaly in JOYO was the ...