Abstract
The accident at Tokyo Electric Power Company's Fukushima Dai-ichi Nuclear Power Plant (TF-1 accident) caused severe situations and resulted in a difficulty in measuring important parameters for monitoring plant conditions. Therefore, we have studied the TF-1 accident to select the important parameters that should be monitored at the severe accident (SA parameters) and are developing the Severe Accident Instrumentations and Monitoring Systems (SA-keisou) that could measure the parameters in severe accident conditions. For an example, reactor water level was selected as one of the SA parameters. The reactor water level is considered to be one of the most important parameters for reactor operators to understand the status of the reactor-core fuel. A new reactor water-level instrument was developed that complements the existing differential pressure method. The instrument is essentially a simple gamma thermometer which consists of several differential thermocouples and a heater. The gamma thermometer had been developed to monitor the in-core gamma-ray distribution, can be installed in the in-core housing, and measures the reactor water level by using the difference of the thermal conductivity of water and the air. This water-level instrument can measure the water-level condition of cooling reactor-core fuel. The instrument was manufactured, was verified at a testing facility, and the feasibility was confirmed. Another selected SA parameter was the hydrogen concentration in the primary containment vessel (PCV). An instrument that measures electric resistance of palladium was developed to monitor hydrogen concentration. In principle, the resistance of palladium increases with hydrogen absorption. The resistance of palladium is almost independent of vapor, whereas the temperature shift of resistance requires to be corrected. Platinum resistance was used for the correction: the hydrogen sensor consisted of palladium and platinum wires and is assembled in double spirals. This instrument was tested in our facility where its specification was confirmed. Also, the PCV water-level instrument and the Containment Atmospheric Monitoring System (ion chamber) were developed for the severe accident.
