First-level Probabilistic Safety Analysis Of Internal Events Under CMRR Reactor Rated Power Conditions

In the safety analysis of nuclear reactors,the method of probabilistic analysis is commonly adopted regarding severe accidents.As the method of Probabilistic Safety Analysis(PSA)develops and matures,it provides measures of probing into severe accidents.In order to enhance the safety standard of research reactors,China National Nuclear Safety Administration has carried out relevant laws and regulations propelling the application of PSA methods into the safety analysis of research reactors.Consequently,China Mianyang Research Reactor(CMRR)plans to develop PSA models at all levels while the level 1 PSA research with respect to internal events under full power lays the foundation of PSA research at other levels.In this research,40 initiating events and sorted 10 initiating event groups are determined,and frequencies for occurrences of initiating event groups are calculated via building initiating event fault trees,generic data and the operating history of CMRR.In the fault tree modeling of safety-related systems of CMRR,the concept of standardized derivative fault trees is applied in order to take into account the impact of the independent failure of components,the common cause failure of components and the human error.Accordingly,11 system fault trees and 169 device and component fault trees are built.To mirror the severity degrees of accident consequences,8 accident end states are defined in terms of the commonly-used end state CD.In the modeling of event trees,9 event trees and corresponding 62 accident sequences are gained.Results about the Core Damage Frequency(CDF)show that the point estimation of CDF regarding internal events happening under full power at CMRR is 1.22E-07/RY,which is less than the threshold value of CDF for severe accidents,1.00E-07/RY,as well as the more advanced goal suggested by the International Atomic Energy Agency(IAEA),1.00E-05/RY.The total CDF calculated proves the high safety standard of CMRR in its design and operation.CDFs corresponding to all initiating event groups manifest that the greatest contributions to the total CDF originate from initiating event groups of the Loss of Flow Accident(LOFA)and the Insertion of Excessive Reactivity(IOER),the portions of which are 80.5%and 10.9%respectively.CDFs corresponding to all end states defined in this research demonstrate that the contribution to the total CDF of the most severe core damage which happens at a high thermal power level without a successful scram is 0.4%.The system reliability analysis of the means of reactivity control shows that the point estimation of the failure probability of the means of reactivity control is 9.78E-07/demand and the corresponding point estimation of the CDF is 1.63E-07/RY unless the contributions of the ATWS mitigation system and the Heavy Water Shutdown System(HWSS)are considered.Under such circumstance,the total CDF of CMRR still meets the both safety goals mentioned above,and the calculation proves the excellent inherent safety of CMRR.When the contribution of ATWS mitigation system is taken into consideration,the point estimation of the failure probability of the means of reactivity control is 7.70E-07/demand,which signifies the reliability of the means of reactivity control is enhanced by 21.3%.Under such circumstance,the corresponding point estimation of CDF is 1.54E-07/RY,showing that the risk of core damage at CMRR decreases by 5.5%.In addition,the CDF with respect to core damage happening at a level of thermal power decreases from 4.15E-08/RY to 3.26E-08/RY demonstrating that the most severe core damage risk diminishes by 21.4%.When the contribution of HWSS is considered,the point estimation of the failure probability of the means of reactivity control is 1.16E-08/demand signifying the reliability of the means of reactivity control is augmented by 98.5%.Under such circumstance,the corresponding point estimation of the total CDF is 1.22E-07/RY,which shows that the core damage frequency of CMRR decreases by 20.8%.Additionally,the CDF representing core damage that happens at the level of high thermal power is 4.92E-10/RY,showing that the most severe core damage risk diminishes by 98.5%.The calculation about the system reliability of the means of reactivity proves that the ATWS mitigation system and HWSS can not only improve the reliability of the means of reactivity control,but also can mitigate the severity of accident consequences.