Discussion of accident progression of Fukushima Daiichi Unit 1 based on behavior of fuel range water level indicator readings

As a part of a challenge to the unsolved issues of Fukushima Daiichi accident, the accident progression of Fukushima Daiichi unit 1 which can explain the behavior of the readings of the water level indicator was estimated based on the measured values (the readings of the water level indicator, the reactor and containment pressure) and the knowledge obtained by existing accident analyses. Based on the estimated accident progression, a simulation of the water level in the water level indicator pipes during the accident was conducted by using the three-dimensional thermal hydraulic analysis code GOTHIC to check the probability of estimated accident progression. As the boundary condition required for the analysis, the reactor pressure and the conditions concerning the containment temperature (the flow rate and the temperature of the leakage gas from the reactor pressure vessel, etc.) were assumed based on the estimated accident progression. The evaluated result approximately reproduced the entire trend of the measured readings of the water level indicator including the difference in the behavior of the channel A and B, and the measured containment pressure. These results indicate that the trend of the measured readings of the water level indicator can be understood by assuming the estimated accident progression. (author)