Research On Hydrogen Source And Risk Mitigation Measures In AP1000 Nuclear Power Plant

During severe accident in AP1000 NPP, the cooling of the reactor core is not enough due to the loss of coolant, the temperature of core fuel elements and structural materials will rise continuously under the influence of fuel decay heat. The zirconium alloy cladding will react with water or water vapor under the conditions of high temperature and produce large amount of hydrogen, then the hydrogen will be released to containment through the break of reactor coolant system. Hydrogen collected inside the containment, where it exploded, will derectly threat the integrity of AP1000 containment. Therefore, some measures must be taken to deal with the hydrogen risk in the containment under serious accident. Currently, hydrogen igniters and passive auto-catalytic recombiners are mainly used to reduce hydrogen risk of containment in AP1000 NPP.Firstly, the LOCA accident with gravity injection invalided is considered as the initial accident condition. In order to study the sensitive of hydrogen source term under severe accident in AP1000 NPP, the integration accident analysis code is used. It shows that the amount of hydrogen and releasing rate of hydrogen depened on the break size and break position.Then, based on the study on sensitivity of hydrogen source term and hydrogen mitigation measures, such as hydrogen ignitions and passive auto-catalytic recombiners, are adopted by Westinghouse, selecting the large break with gravity injection invalided as severe accident condition, and the GASFLOW program is used to study the behavior of hydrogen flow, hydrogen volume fraction and hydrogen risk in different compartments under severe accident in AP1000 NPP. It shows that the main hydrogen flow path is 1#SG source term compartmentâ†'the upper compartmentâ†'the bottom compartment, a distinct layered distribution is formed inside the containment; The hydrogen igniters and passive auto-catalytic recombiners can eliminate large amount of hydrogen, which reduces the hydrogen risk of each compartment effectively inside the containment, but the deflagration to detonation transition (DDT) risk of 1#SG source term compartment is still exist. However, when the igniters eliminate hydrogen, it releases a lot of heat, which is more likely to cause the high temperature in local position. Therefore, some appropriate measures are should to be taken to prevent the local high temperature from damaging other devices in the compartment.Finally, assuming that two groups of the hydrogen igniters are losing power, but the passive auto-catalytic recombiners function well. The LB-LOCA with gravity injection failure in the reactor coolant system is considered as initial accident condition. This paper proposes a measure of post-inerting to reduce hydrogen risk of AP1 000 containment, then the gas type, gas injection location, gas injection rate, gas injection start time and so on are respectively investigated. It shows that the carbon dioxide chosen as inert gas for the accident is suitable and its injection type is gaseous state. In the process of post-inerting, the injection quality of carbon dioxide must be enough, it can ensure the containment in a state of inerted completely, the position of gas injection which located below the break source term is more appropriate, because it enhances the hydrogen diffusion and mixing and reduces the local hydrogen concentration inside the containment; the start time of gas injection is neither too late nor too late. Gas injection start time should be earlier than hydrogen release peak phase and the end time should be later than hydrogen release peak phase, and an appropriate injection rate should be selected to ensure the effective gas injection duration.