A review on hydrogen concentration measuring system designed for severe accident

A large quantity of hydrogen will be released due to zirconium-water reaction in a pressurized water reactor during severe accident. And the hydrogen will diffuse into containment vessel, and thus increases the risk of explosion. Since the containment vessel is exactly the last barrier for fatal irradiated species, being damaged by hydrogen detonation or an uncontrollable leak of gas are unacceptable. Accordingly, efficient strategies should be adopted to address this issue. Hydrogen elimination and measuring devices, therefore, are widely applied in nuclear power plant all over the world. Because of harsh environment inside the containment vessel during severe accident, always under high temperature, high pressure, and being inundated with steam, irradiated dust and chemical species, it is a big challenge to design a reliable hydrogen concentration measuring system. Generally, the hydrogen concentration measuring systems developed presently can be divided into two categories including non-in situ measurement and in situ measurement; the first integrates with withdraw parts and the hydrogen sensor can only be set outside the containment vessel, while the second based on in-situ hydrogen sensor which can be set in the containment vessel. The hydrogen sensors for the non-in situ measurement are always thermal conductivity detectors. Despite lots of controversies on the safety under severe accident, such as a possible leak of the irradiated mixture, the non-in situ measurement has been a mature technique for design basis accident. In-situ measurement is state-of-the-art technique due to direct detection and high accuracy. Hydrogen concentration measuring systems by in-situ measurement and with hydrogen sensor originated from catalytic recombination, thermal conductivity, resistance, and electrochemistry, have produced outstanding performance and attracted a lot of attentions in the world. In this study, we will introduce the most recently development of hydrogen concentration measuring systems designed for severe accident. Furthermore, relevant technique requirements as well as verifications will also be contained. (author)