Analysis of reactor water level measurements during the Fukushima unit 2 accident

The reactor water level measurement system in Fukushima was based on measuring the pressure difference between two water-filled pipes that were connected to the reactor at different elevations. A Japanese accident investigation committee has presented a hypothesis that a leak of superheated steam from the unit 2 reactor to the drywell caused evaporation of the water in the measurement system, causing the indicated water level to be much higher than the actual level. This article presents an analysis of the measurement system with the MELCOR code. The results support the hypothesis of a small leak, starting 78.5 h after the earthquake. The best agreement with the measurements was obtained with 8 cm² leak area. The leak was probably located close to the water level measurement system, so that it caused localized heating in the drywell. The leak increased the fission product release to the environment because it bypassed the suppression pool. The calculated cesium release from unit 2 to the environment was 2.1% of the core inventory. Combined with the author's earlier MELCOR calculations of units 1 and 3, the current estimate of the total Cs-137 release from all three accident units at Fukushima is 14.3 PBq.