Study On Model's Uncertainty Of Passive Containment Cooling System In AP1000 Nuclear Power Plant

As one of the most important safety systems in AP1000 design,Passive containment cooling system(PCS)plays an important role in postulated accidents.During reactor accidents,PCS can provide the sufficient heat removal capability for the core and containment via the natural circulation cooling mechanism for the sake of preventing the over-heating damage and subsequently the radionuclide release.In addition,PCS could significantly reduce the reliance of electrical power supplies and accurately perform their safety functions during the lose of offsite power supply(LOOP)or station blackout(SBO)accidents.So it is of great significance for the study of its uncertainty.This paper analyzes the parameter uncertainty of thermal hydraulic model.Using Matlab software,the PCS model is established based on lumped parameter method.Comparing with the results calculated by WGOTHIC,it shows that the point model is reliable.Then filter the input parameters,the air temperature,wind speed and the discharge data of the break are selected as research targets.The results based on different combinations of input parameter are calculated and analyzed.The Monte Carlo Method is employed in order to study the impact of parameter uncertainty on the system reliability.By analyzing the calculations,it shows that under LOCA,the air temperature and wind speed will influent the time of peak pressure's occurrence.The lower the temperature is,the later peak pressure occurs.However,this phenomenon is not very obvious in the accident of MSLB.In LOCA accident,PCS can keep the integrity of the system under the condition of "95/95 confidence level/probability level".In MSLB accident,without considering the maximum error of measurement,PCS can also maintain the containment's integrity.The measurement error of steam flow has a big impact on the results,so improving the measurement accuracy can help do better analysis of the system.Then using fault tree technology,the probability safety assessment of PCS under normal condition and earthquake is analyzed.The effect of failure probability caused by value of common cause failure(CCF)factor is studied under different earthquake conditions.Different ratios of a/Am(peak ground acceleration/median ground acceleration capacity)have impact on the system failure probability.With the increase of CCF factor,when the peak ground acceleration is much lower than the component seismic resistance capacity,difference between the results of CCF factor is about one magnitude,while the results are very similar when the peak ground acceleration is close or even higher than the component resistance capacity.Finally,based on Monte Carlo Method,the failure probability of PCS under two different seismic hazard curves is calculated,which is compared with the result of fault tree method.Under the same peak ground acceleration,if the annual probability of exceedance is lower,the failure probability is lower.