Investigation On Transient Heat Transfer Characteristic In Pressure Vessel Lower Head In Advanced Water Cooled Nuclear Reactors During Severe Accident

After Fukushima accident,more and more attention has been paid on severe accident analysis.In order to mitigate severe accident,several severe accident management strategies have been proposed.In vessel retention(IVR)is one of these candidate strategies.Within this strategy,reactor pit would be submerged when severe accident occurs.It is expected that decay heat from melt pool relocated in the lower head could be removed from the coolant from external reactor vessel.The integrity of lower head could be ensured provided heat flux at outer surface vessel wall is smaller than local critical heat flux.Extensive work has been done on the effectiveness of IVR strategy,however simplified assumption has been applied for lower head in most of these work.Seldom study has been done on heat transfer characteristics in the lower head wall,which makes it hard to obtain accurate safety margin during severe accident analysis.It is required by IAEA that severe accident scenario and strategies to mitigate it should be considered in new nuclear power plant.Besides heat transfer behavior in advanced water cooled reactors could be different from ordinary pressurized water reactor due to different core design.Thus severe accident analysis for advanced water cooled reactor is nessary and transient heat transfer behavior in the lower head in this process should be studied.Considering the state-of-art study of severe accident analysis aroud the world,the objects of this thesis are:(i)to develop a module which is feasible to simulate transient heat transfer behavior in the lower head during severe accident;(ii)to verify and validate this module by analytical solution and experiment data;(iii)to investigate system behavior and to propose suggestions for optimization design in two different supercritical water reactors during accident;(iv)to study heat transfer behavior and parameter sensitivity in the lower head in large scale advanced pressurized water reactor during severe accident.The content of this thesis includes:1.Base on two-dimensional spherical coordinate,a module with aim to investigate transient heat transfer behavior in the lower head is developed.To be specific,the variation of heat transfer area within the vessel wall at different position in normal direction and heat transfer in longtitude direction are considered.As a result,relative accurate heat flux profile at outer vessel wall could be predicted;Heat transfer model for melt vessel is developed in order to obtain rational wall temperature at inner surface of lower head;Wide range of correlations including pre and post-CHF region are applied,so that the transition of heat transfer regime could be predicted from spatial and temperal point of view.Simulation results are verified and validated to analyitical solution for uniform input heat flux and experiment data from LIVE experiment with non-uniform input heat flux.After compared with benchmart result,it could be found that heat flux distribution at outer vessel wall is significantly different from inner one.Maximum heat flux is reduced due to surface area difference and heat transfer in longitude direction.Thus notably higher safety margin could be obtained.2.Safety analysis for design basis accident and severe accident are performed on two supercritical water reactors.Five different design basis accidents(loss of feedwater heating accident,loss of turbine load without bypass accident,feedwater pump seizure accident,control rod ejection accident and loss of coolant accident)in thermal spectrum supercritical water reactor are chosen to study.The accident which is most likely developed into severe accident is identified.After severe accident analysis and sensitivity analysis,the parameter which strongly affects heat flux profile is identified.Besides,system response in mix spectrum supercritical water reactor during loss of coolant accident is compared.Severe accident analysis in mix spectrum supercritical water reactor is performed.After optimizing reactor design,the integrity of lower head during postulated severe accident could be ensured.3.Postulated severe accident analysis is peformed on large scale advanced pressurized water reactor.Heat flux profile predicted by one-dimensional model and two-dimensional model in the lower head is compared.Sensitive analysis on heat transfer correlation for melt pool,mass flow rate for external vessel mass flow rate,inlet water temperature for external vessel channel,transition boiling heat transfer correlation and heat transfer model for molten vessel wall is studied.Sensitive parameters are identified.This thesis aims to develop a module which is capable to predict transient heat transfer characteristics in the lower head during severe accident more accurately.Then this module is applied to severe accident analyisis in advanced water cooled reactor,in which researchers in China is actively involved.The result obtained could provide useful information on reactor design and accident analysis.