Analyzing reactor water level measurements in the Fukushima Daiichi 1 accident

Measuring the water level in the Fukushima Daiichi reactors was based on measuring the pressure difference between two pipes, connected to the reactor at different elevations and filled with water. Boiling of the water in the measurement system distorted the measurements. The water level readings during the unit 1 accident were analyzed with the MELCOR code. The behavior of the measurement system can be reproduced by assuming a small leak of superheated steam from a steam line to the drywell before 6.5 h after the earthquake. This confirms earlier conclusions of a Japanese accident investigation committee and analyses performed by TEPCO. The steam leak did not have any significant effect on the cesium release to the environment.