Optimum Design Of Reactor Vessel Insulation For IVR-ERVC And Reliability Assessment Based On Passive System Functional Concept

IVR-ERVC (In-Vessel Retention-External Reactor Vessel Cooling) strategy is proposed to be adopted as one of the severe accident mitigation measures of China's 1000MW LWR technology. But there are still a lot of uncertainties related to IVR-ERVC system performance due mainly to the lack of operational and experimental data. It is necessary to evaluate ERVC system characteristic such as cooling mode, transient flow, heat transfer and heat transport etc.and their impact on heat-load effectiveness of ERVC. Passive system is more sensitive to environment and parameters, so it's necessary to evaluate the functional reliability of the ERVC system with the result of system characteristic and carry on optimum design. For this purpose, this paper analyses the steady and transient state system performance of two-phase flow natural circulation in IVR-ERVC of the 1000MW PWR, based on simulation tool and related methods. Further, a detailed study on designing critical insulation parameters of the passive ERVC is carried out and functional reliability of ERVC system is evaluated through systematic method.Firstly, in this paper, the 1000MW pressurized water reactor passive ERVC system is described and initial conditions and functions of the 1000MW pressurized water ERVC system are defined. Boundary conditions of ERVC system used in simulation are also identified and Relap5 modelling and nodalization for system thermohydraulic behavior simulation of IVR-ERVC is developed.Secondly, applying RELAP5/MOD3 system analysis code, flow and heat transfer characteristic of two-phase flow natural circulation of IVR-ERVC system has been analysed in case of severe accident. Based on parametric effect analysis for such parameters as decay heat power of molten corium, inlet resistance of annulus channel between RPV and insulation, angle of vent baffle, initial water level etc., relevant mechanism analysis of ERVC system is performed. Some meaningful results are provided for system design optimization, related procdure development and IVR-ERVC heat-load effectiveness evaluation.Furthermore, critical operational parameters are selected via Analytic Hierarchy Process (AHP). Key parameter ranges and probability distributions are identified according to engineering experience and expert judgment. Functional reliability criteria are defined by the function of ERVC system. Within the engineered possible range, sampling is carried out through Latin Hypercube Sampling (LHS) method with respect of the selected parameters. The sampled data are used as input of RELAP 5 for uncertainty propagation calculation. Ranges of nominal values of critical insulation design parameters are identified by sensitivity analysis and statistical analysis.Finally, with the acquired nominal value of design parameters, along with the ranges of uncertainty and probability distributions provided through the engineering knowledge about technique level of manufacturing and installation, a second LHS with respect to the critical parameters is performed. The sampling results are used as input for RELAP 5 uncertainty propogation calculation. And functional reliability of ERVC system is then statistically evaluated.