Simulation Study Of In-vessel Retention Of Melting Material For PWR

According to defense in depth and successive barriers concept for nuclear power plant,during severe accidents,if in-vessel retention(IVR)of melting material could be achieved,the interaction between core melting material and cavity concrete could be prevented so as to alleviate a series severe accident sequences,such as succedent hydrogen explosion and containment overpressure.Presently,external reactor vessel cooling(ERVC)is one of important strategy for achieving IVR.Passive ERVC system is also an important feature of third generation nuclear power system.In this work some primary issues are investigated:based on evaluating the applicability of RELAP5 code to calculate ERVC,the characteristics of two-phase flow under IVR-ERVC condition;the relative phenomenon of IVR-ERVC during typical severe accdent sequences;three-dimensional full-scale ERVC with heat soure simulation studies;comparison study of convection heat transfer correlations for outside lower head.Firstly,the characteristic of two-phase natural circulation flow between RPV and insulation is studied.The modeling of using RELAP5 code are employing to study the capability of two-phase natural flow and the sensitivity of thermal-hydraulic and configuration parameters of ERVC.The results show that for AP1000 ERVC design,the molten pool could be cooling down by natural circulation flow outside lower head as long as flooding level is higher than about 5.5m.Besides,the instability boundaries for ERVC system are found.The AP1000 nuclear power plant model including ERVC system mentioned above is set up by RELAP5/SCDAP code to study the relative phenomenon of IVR-ERVC in typical severe accidents.Some typical severe accident sequences which can lead to core melting are selected to investigate the accident progression and phenomenon for IVR-ERVC,including core damage progression,behaviors of core molten materials and molten pool in lower head,behaviors of lower head wall and validity of IVR-ERVC.The results show that:the effect of Inconel grid on progression in relative slow progression is great than fast accident progression.The bare top area of core does not melt quickly,and the middle part of core will melt first.IVR could achieve through ERVC in the severe accidents which own high contribution to core damage and happening probability.And as long as the cavity is flooded before molten materials fall into lower head,the influence of the speed of core molten progression on ERVC is not great.For studying three-dimensional ERVC system,the three-dimensional model of ERVC system is set up by RELAP5-3D code.Because the multi-dimensional control volume in RELAP5-3D code can not simulate three-dimensional hemisphere gap channel,so the three-dimensional ERVC system is established by RELAP5-3D code with only inlet and riser simulated by MD control volumes.The gap channel between lower head and insulator is simulated by detailed nodalization.And the quasi-three-dimensional model is established by RELAP5 code for comparing study with RELAP5-3D code modeling.The results show that the onset of instability computed by quasi-three-dimensional model by RELAP5 is lag than three-dimensional model by RELAP5-3D.And the results difference between the two models is affected by thermal-hydraulic and configuration parameters.At last,the program for calculating ERVC heat transfer process is written by FORTRAN language.And various convection heat transfer models of lower head exterior wall are chosen for comparing study.The results show that the ACOPO experiment formula underestimates the ERVC heat transfer capability..The convection heat transfer correlation for ERVC condition in RELAP5 code is relative conservative than correlations obtained from AP1000 slice experiment facility and narrow channel for turbulent-turbulent flow.