Study On Hydrogen Control Under Severe Accident Of Small Integrated Pressurized Water Reactor

After the Fukushima nuclear accident,the hydrogen risk and control problem in the severe accident of nuclear power plant become the hot spot of the industry again.In recent years,the small integrated pressurized water reactor(PWR)for the diversified use of nuclear energy has been widely concerned by countries around the world because of its remarkable advantages in the construction cycle,modular layout and inherent safety.However,the design of small integrated reactors is very different from that of large reactors,and the serious accident process is quite different from that of traditional large pressurized water reactors,which makes the hydrogen source term,the migration of hydrogen in the containment and the requirements of hydrogen control system of small integrated reactors have remarkable characteristics.Therefore,the research on hydrogen source term,hydrogen behavior and hydrogen risk control under severe accident conditions for small integrated pressurized water reactor is one of the important topics to meet the requirements of small reactor serious accident management,which can provide technical basis for small reactor serious accident management.In this paper,the mechanical severe accident analysis program SCDAP/RELAP5 is used to systematically analyze the hydrogen source terms of typical severe accident sequences with the small integrated pressurized water reactor IP200 as the research object.In view of the shortage that SCDAP/RELAP5 cannot simulate serious accidents in the containment,a twodimensional hydrogen analysis program is developed in the containment to study the distribution and migration of hydrogen in the small reactor containment,analyze the risk of hydrogen in the small reactor containment under severe accidents,and study the hydrogen combustion load in the containment.Finally,the dehydrogenation capability of the hydrogen control system is studied from the point of view of serious accident management,and a hydrogen control scheme suitable for small reactor containment is proposed.The main research contents of this paper include:(1)Taking IP200 as the research object,RELAP5/SCDAP program is used to establish the analysis model of the serious accident in the reactor and the analysis model of the pressure inhibition pool outside the reactor suitable for IP200 reactor.The thermal hydraulic response,core degradation process,pressure response of suppression pool and hydrogen generation process of IP200 under typical high pressure fusion reactor and low pressure fusion reactor accident sequences were analyzed.The influence of breach area on core degradation process and hydrogen source term was discussed,which provided hydrogen source term and containment input boundary for subsequent research.(2)In view of the limitation that SCDAP/RELAP5 program cannot analyze serious accident phenomena in the containment,a two-dimensional hydrogen analysis program is developed for the analysis of hydrogen behavior in the containment with suppressor pool design.The accuracy of the program is verified by domestic and foreign experiments.Meanwhile,the calculation result of SCDAP/RELAP5 program above is the input boundary.The transmission,distribution and mixing processes of hydrogen,water vapor and air in small reactor containment under severe accidents are analyzed by using the developed two-dimensional hydrogen analysis program,and the changes of temperature,pressure and share of various components in time and space are calculated.The two-dimensional distribution field diagram during the release of hydrogen gas was obtained,which visually demonstrated the flow distribution process of hydrogen gas in the containment.A preliminary analysis of the hydrogen risk in different areas of the containment during hydrogen release based on the Shapiro diagram showed that due to the special structure of the small reactor containment,hydrogen is evenly distributed in the containment,and there is a risk of hydrogen combustion in the middle of the containment,the bottom ring and the suppression pool gas space.(3)Aiming at the hydrogen combustion problem in the containment space,the functions of the developed hydrogen analysis program are expanded.By adding the hydrogen combustion reaction mechanism and the turbulent flame velocity sealing model,a two-dimensional hydrogen analysis program suitable for the simulation calculation of hydrogen combustion in the containment space is completed.The accuracy of the program is demonstrated by using the experimental data of THAI-HD22 in Germany.On this basis,the pressure load and thermal load on the containment were studied for the accident condition of 100% zirconium-water hydrogen combustion in the containment,and it was found that the containment with suppressor pool design can cope with the pressure steep rise caused by the hydrogen combustion,so as to ensure the integrity of the containment.At the same time,the consequences of hydrogen combustion in the containment are analyzed from two perspectives of different hydrogen concentration and different combustion starting points.(4)In order to solve the problem of hydrogen control in the containment,based on the two-dimensional hydrogen analysis program of the containment,the single step chemical model based on hydrogen-oxygen recombination reaction was added with a new passive recombination function module,and the accuracy of the program was verified by Kali-H2 experiment in France.Subsequently,the program was used to study the hydrogen control scheme with only igniter installed.The study showed that igniter could effectively reduce the hydrogen concentration in the containment,but the flame combustion temperature exceeded2500 K,and there was a threat of excessive thermal load in the containment.In view of the shortcomings of the igniter control scheme,a control scheme was proposed to install Passive Autocatalystic Recombiner(PAR)only.The feasibility and improved design of the scheme were demonstrated.The study showed that PAR could carry out hydrogen elimination in a wider concentration range.But hydrogen combustion is still possible.In view of the shortage of PAR and igniter,an improved hydrogen control scheme for small reactor containment using igniter and igniter together is proposed.This scheme focuses on hydrogen release in the breach area.By first using PAR to reduce hydrogen concentration and then igniting igniter,hydrogen concentration can be effectively reduced.It also avoids overheating of the hydrogen combustion flame due to ignition,and the resulting load does not pose a threat to the integrity of the containment.