In BWRs, differential pressure-type water level gauges are commonly used to monitor the water level in the reactor pressure vessel (RPV). However, during the 2011 Fukushima Daiichi nuclear accident, decrease in the reference surface led to an incorrect estimation of the reactor water level, posing serious challenges to accident management. To address this issue, we propose a novel water level measurement method utilizing ultrasonic reflection characteristics. This method transmits ultrasonic waves obliquely through the reactor wall, exploiting the difference in reflectivity between water and air interfaces to detect the water level. Experiments were conducted using a small-scale rectangular acrylic tank equipped with a 20 mm thick SUS303 steel plate to simulate the reactor wall. Ultrasonic transducers with frequencies of 2 MHz and 1 MHz were used, and the received signal amplitudes were analyzed in relation to water level changes. Although the measurement accuracy decreased as the container became thicker, it was confirmed that measurement accuracy for full-scale water level measurment length of 280 mm was ±7.5 %FS using 2 MHz ultrasonic wave and ±4.1 %FS mm using 1 MHz for a vessel with a thickness of 20 mm. These findings demonstrate the feasibility of the proposed method for robust water level monitoring in severe accident conditions.
SASA et al. (2025) studied this question.