Hydrogen Concentration Measuring System Designed for Severe Accidents in Containment

Extensively released hydrogen due to zirconium-water reaction during severe accidents in containment of pressurized water reactor raises explosion crisis. Since the containment is the last barrier for fatal irradiation species, efficient measures should be implemented to restrain the hydrogen. Hence, hydrogen elimination and monitor devices are wildly applied to address this issue. Detection of hydrogen once has been conducted by a traditional hydrogen concentration measuring system with sampling devices and hydrogen sensors that located outside the containment. This arrangement, however, is a compromise between actual requirements for hydrogen measuring device and absence of favorable hydrogen sensors which could be applied in the harsh environment under severe accidents. Most recently, R&D of hydrogen concentration measuring system with in-situ hydrogen sensor has attracted increasing attention. Mitsubishi Heavy Industries, Ltd is focusing their job on an electrochemical hydrogen sensor based on solid state electrolyte. Besides, AREVA has developed a system depending on thermal conductivity detector associated with catalytic combustion sensor which requires external power supply to heat the assembly. In PERIC, we have developed a hydrogen concentration measuring system with in-situ hydrogen sensor which can be set in accident confident area. The hydrogen sensor is originally based on catalytic recombination of hydrogen and oxygen. Generally, catalyst prepared using noble metals such as platinum and palladium is scientifically loaded in the hydrogen sensor to serve as hydrogen sensitive material. On the event of severe accidents, mixture of hydrogen and air can spontaneously diffuse into the hydrogen sensor, where, part of the mixture is involved in a chemical exothermic reaction on the catalyst to generate water and heat. Generally, an increased concentration of hydrogen will raise relatively higher reaction temperature of the hydrogen sensor. The hydrogen concentration related temperature of the hydrogen sensor is detected using steel armored thermocouple. Besides, environmental temperature and pressure in the containment are also acquired to assist calculation. All the data are transferred to a signal processing cabinet, which, performs the calculation and indication functions using programmable logic controller and digital display device, respectively. There is no organic material, mechanical moving and power consumption part in the hydrogen sensor and thermocouple. The system indicated reliable performance in simulated containment under condition of high temperature, pressure, steam, and etc. The hydrogen concentration measuring system illustrated excellent endurance to poisoning species such as iodine and aerosol. Furthermore, the hydrogen sensor also suggested high resistance to irradiation. The system can survive a severe earthquake, and its seismic certification toward to safety shutdown earthquake is class I. Over 80 systems so far have be applied in pressurized water reactor in China and or Pakistan. The latest model is designed according to requirements of CAP1400.