Study On Uncertainty Evaluation Method Of Typical Accident Analysis In Nuclear Power Plant

Compared with the traditional conservative accident analysis method,the best estimate plus uncertainty(BEPU)accident analysis method can obtain the real analysis result and the safety margin,which can improve the economy and operational flexibility of the nuclear power plant under the premise of ensuring the safety of the nuclear power plant.The BEPU method for accident analysis has become an international trend,but China's current BEPUaccidentanalysis method is still in its infancy.It is important to analyse the uncertainty of peak cladding temperature(PCT)under large break loss of coolant accident(LBLOCA)and the peak radial average fuel enthalpy(PRAFE)under rod ejection accident(REA)based on the developed uncertainty evaluation procedure by using multiple uncertainty analysis methods.Three-dimensional thermohydraulics /point reactor kinetics coupled full-loop analysis model for CNP600 LBLOCA and three-dimensional thermohydraulics /space-independent reactor kinetics coupled full-loop analysis model for CNP600 REA were established by using RELAP5-3D code.The steady-state debugging and transient calculation of the established accident analysis modelswere carried out to confirm the rationality of the models,and the sensitivity analysis of some model parameters and power plant parameters were carried out on the basis of rational accident analysis models.The safety margins of PCT under LBLOCA and PRAFE under REA were calculated by using the Owen factor method,parametric statistical method,nonparametric statistical method and sensitivity analysis methodbased on the uncertainty evaluation procedure.The calculation results show that for the LBLOCA,the Owen factor method,parameter statistical method and nonparametric statistical method can gain theclose PCT safety margins,which are much lager than PCT safety margin that the sensitivity analysis method can gain.For the REA under the condition of hot full power and hot zero power,the Owen factor method,parameter statistical method,nonparametric statistical method and sensitivity analysis method can gain the close PRAFE safety margins.By comparing the four methods of uncertainty analysis,it is showing that the nonparametric statistical is most suitable for the calculation of the key safety parameter uncertainty under LBLOCA and REA.Spearman rank correlation coefficient method is adopted to calculate the sensitivity of input parameters under LBLOCA and REA to identify the input parameters which are more sensitive to the key safety parameters.The calculation results show that for the LBLOCA,PCT is sensitive to the decay heat,pumps resistance,pellet conductivity,gap conductivity,break area and two-phase discharge coefficient.For the REA under hot full power,PRAFE is sensitive to pellet size,gap size,pellet conductivity and gap conductivity.For the REA under hot zero power,PRAFE is sensitive to the ejected control rod worth,fuel temperature feedback and delay-neutron fraction.