Studies Of Accident Mitigation Capability Analysis For Small Modular Reactor Passive Safety System

Nuclear energy has the advantages of high energy density,clean and pollution-free,etc.Using nuclear power to generate electricity can effectively alleviate the current energy shortage and environmental problems facing our country.With the development of the nuclear power industry,the nuclear industry has begun to research and develop various new types of reactors in the future in order to further improve the safety and economics of nuclear power.Among them,small modular reactors have gained widespread attention in recent years due to the characteristics of low initial cost,short construction period,flexible plant layout and wide range of uses.In recent years,small modular reactors have achieved certain accomplishments in both design and research and development,However,the effectiveness of their safety systems under complex working conditions still needs extensive verification to ensure their safety.This research aims to verify the safety of small modular reactors by establishing a complete thermalhydraulics model and exploring the accident mitigation capabilities of its safety systems,especially passive safety systems,under various complex accident conditions.This project selected the Westinghouse Small Modular Reactor(WSMR)designed by Westinghouse Electric Company as a prototype,and established a complete severe accident analysis program MELCOR model based on its reactor system design.After performing the steady-state verification on the model,it was used to carry out various types of accident sequence analysis based on the two major initiating accidents,including the station blackout accident and the loss of coolant accident,and analyzed and compared the calculation results.In addition,uncertainty calculations and analysis were performed based on the Best Estimate Plus Uncertainty(BEPU)method and Tolerance Interval Statistical Techniques(TIST)under the sequence of the loss of coolant accident.Taking the core water level as the key systematic index,the correlation analysis was carried out using Spearman’s Correlation Model.The research results indicated that:1.The small modular reactor has a variety of residual heat removal mechanisms.Under the accident conditions,if the corresponding safety system,especially the passive safety system,works normally,it can ensure suficient core cooling capacity.2.The passive core makeup tanks designed in the small modular reactor has a fast and excellent mitigation effect on the station blackout accident sequence.However,two or more core makeup tanks must work normally in order to provide sufficient residual heat removal efficiency.If the initial injection signal fails,the small modular reactor can still use its natural circulation capability to achieve residual heat removal over a period of time.As long as the core makeup tank is reinvested before the failure of the fuel cladding,the accident can be effectively mitigated,and the possible damage to the core structure can be avoided.3.Compared with the station blackout accident,the loss of coolant accident is a more frequent type of accident.The integrated design of the small modular reactor limits coolant inventory inside of the reactor pressure vessel,when the main system boundary is breached,a brief core uncovery might happen.In the process of mitigating the loss of coolant accident,more passive safety features are involved,and system operations are also more complicated.The small modular reactor can comprehensively utilize passive safety injection,automatic depressurization,heat transfer and other means to effectively mitigate the accident.The good thermal conductivity of the containment wall helps avoid the potential risk of overpressurization during the accident.4.The results of uncertainty and sensitivity analysis on a series of influencing factors in the loss of coolant accident show that the core water level decline is related to the flow rate at the break.The break size has the most significant effect on the flow rate at the break,the increase of the break size will directly lead to more coolant loss.The second most influential factor is the heat transfer coefficient of the containment vessel.By strengthening the heat gradient between two sides of the break,the coolant release rate is indirectly enhanced,and the core water level is further reduced.This research used the severe accident analysis code MELCOR to successfully create a comprehensive system model for WSMR.The research work can provide some experience and guidance for digital simulation and analysis related to small modular reactors.This paper conducted a detailed accident safety analysis specifically for the WSMR passive safety system design,and conducted in-depth research on the accident process and mechanisms.Moreover,uncertainty analysis and sensitivity analysis were carried out on relevant parameters and factors.The research results can provide guidance for the formulation and improvement of the Severe Accident Management Guidance(SAMG)related to small modular reactors,safety design of new reactors,and engineering designs and applications related to small modular reactors.