Study On Hydrogen Control For Severe Accident Conditions In The Large Dry Containment

During certain severe accidents in the pressurized water reactor (PWR) nuclear power plant (NPP), a great amount of hydrogen is generated due to zirconium-steam reaction (in-vessel period) and molten corium concrete interaction (ex-vessel period) and released into the containment. A flammable mixture (hydrogen/steam/air) is gradually formed in containment atmosphere, under the effects of hydrogen diffusion and flow. Deflagration or detonation might occur to produce high thermal and pressure loads, which may threaten the integrity of the containment, and the resulting large quantity of radioactive materials are eventually leaked into the environment.For the special hydrogen safety issues of current NPP's safety enhancement and advanced reactors'design, one of the most significant questions for discussion is how to design an effectual hydrogen control strategy to satisfy the regulatory requirements. Base on the safety analysis theories and methods, a new framework of hydrogen safety analysis and hydrogen management is presented in this paper, for the hydrogen control and management in large dry containment under severe accident conditions. The key content of the framework is one systematic analysis process, including:①, how to identify and verify the analytical tools;②, How to establish and verify the hydrogen analysis model to make the calculation results correct;③, how to choose the most typical severe accident condition;④, the characteristics of hydrogen sources;⑤, hydrogen flow and distribution in the containment atmosphere;⑥, assessment of hydrogen combustion risk;⑦, efficiency analysis of hydrogen mitigative messures, such as catalytic recombiner or igniter;⑧, optimization design of hydrogen mitigative messures.The significant research fields are included as follows:(1) On the basis of a new method of accident analysis, where deterministic analysis and level 1 probabilistic safety assessment (PSA) results are integrated, typical severe accident sequences are calculated, such as large-break loss-of coolant-accident (LB-LOCA), middle-break loss-of coolant-accident (MB-LOCA), small-break loss-of coolant-accident (SB-LOCA), station blackout (SBO) and steam generator tube rupture (SGTR), and the characteristics of hydrogen sources are investigated. The research results show that severe accident scenarios may be ranked into three types: the fast hydrogen releasing (LB-LOCA, MB-LOCA), the intermediate hydrogen releasing (SB-LOCA, SBO) and the slow hydrogen releasing (SGTR), where LB-LOCA is the most typical accident scenario, considering that the amount of hydrogen in containment is equivalent to that generated from a 100% zirconium-steam reaction.(2) Hydrogen flow and hydrogen concentration distribution in containment atmosphere under LB-LOCA as the basic calculating accident scenario are investigated. The study shows that the containment spray has a great side effect to hydrogen concentration distribution, because of the phenomenon of steam condensation and the resulting hydrogen concentration increase. The analysis of hydrogen combustion risk under different atmosphere conditions shows that the spray would increase greatly the probability of hydrogen detonation, and threaten the integrity of the containment. Therefore, to assess detailedly the containment spray which impacts on hydrogen concentration distribution and hydrogen combustion is a significant step in the framework of hydrogen safety analysis and hydrogen management.(3) PARs, igniters and the combination of PARs and igniters are three important hydrogen management strategies for the nuclear power plant with a large dry containment. The analytical models of PARs and igniters, based on the complex containment structure, are developed to evaluate the efficiency of the hydrogen mitigative messures, including hydrogen removal efficiency, time of ignition, position of igniters, pressure and temperature loads to containment due to hydrogen combustion, etc. On the base of the research work above, a criterion of optimization design for hydrogen management strategies is proposed in this paper.(4) The local deflagration or detonation is one of most important issues in the hydrogen risk analysis, and the lumped-parameter (LP) method does not simulate detailedly the local hydrogen distribution, while the computational fluid dynamics (CFD) method could conquer the shortcoming above. Based on the foregoing research results,"containment hydrogen distribution in the in-vessel period"is analyzed by the computational fluid dynamics (CFD) method at the end of this paper, as an important complementarity for the calculating results of lumped parameter (LP) code. The combination of LP and CFD is a new application in hydrogen safety anlaysis.This study presents an integral analytical framework of hydrogen safety, and provides a new technique of hydrogen control and management during severe accidents, which is of engineering significance and reference value for nuclear power plants in China.