Investigation of the on-line monitoring sensor for a pipe wall thinning with high accuracy

In Japan, the safety of nuclear power plants (NPP) has been secured by performing a time-based maintenance. However recently, NPPs were taking into consideration a new procedure using a condition-based maintenance, aiming to improve both reactors safety and operation availability. Therefore, it is needed to develop new sensors that can monitor, during reactor operation, the integrity of both piping system and pressure vessel. Also, a proper measurement method has to be chosen in order to carry out an effective on-line monitoring. In the present investigation, it was used the electromagnetic acoustic transducer (EMAT) because it does not need a copulation in a high-temperature environment (up to 200 degrees C). Based on this method, two measurements techniques have been employed using a new developed sensor, depending on the required purpose. The 1st technique is based on the Pulse Echo (PE) method. The paper analyses different methods for carrying out plates thickness measurement using EMAT and supplying a vertical ultrasonic wave. The 2nd technique is based on the electromagnetic acoustic resonance (EMAR) that is usually used for material fatigue assessment. The developed sensor measures, at 200 degrees C, a minimum 5 mm ± 0.04 mm plate thickness using PE, and 2 mm ± 0.01 mm using EMAR, respectively. Moreover, it was found no attenuation of the sensor signal even when the sensor performed a 200 hours continuously measurement. (author)