Numerical Study On Fuel And Coolant Interaction And Melt Hydrodynamic Breakup Behavior

In the area of nuclear industry,once a severe accident occurs,molten fuel may interact with liquid coolant(FCI),which may lead to steam explosion.Large pressure shock may threaten the integrity of surrounding structures,causing radioactive substance released into atmosphere.Therefore,steam explosion has been the research focus in nuclear safty.FCI process is widely studied by experimental and numerical method.However,due to the complexity of FCI process,experiment meets several limitations in process control and data collection.With the development of numerical technology,several steam explosion analysis codes were developed.However,FCI is a complex multiphase flow process,involving the flow pattern in which multi-scale interfaces coexist.For interface tracking model,it can only track the large scale interface.Multi-fluid model is only used to simulate the dispersed phase.Therefore these models will meet difficulties in FCI’s simulation.Besides,mechanisms of the melt hydrodynamic breakup is still under investigations and current incomplete models lead to difficulty in accurate prediction.Additionally,with the development of China’s 3rd generation of nuclear power technology and independent software of nuclear power plant,the steam explosion analysis code meets an urgent need.Based on the characteristics of the FCI’s multiphase flow,a multi-scale numerical model is developed.In this model,the interface tracking model and the multi-fluid model are coupled into a unify solution structure.The current model has both the advantages of the interface tracking model and the multi-fluid model.It can used to simulate the complex multiphase problem,in which the large-scale phase and small-scale phase coexist.For this coupled model,a multiphase all speed algorithm with multi-scale interfaces is built.With the new model and the algorithm,a steam explosion analysis code METRIC(Melt and Water Interaction Code)is developed.Through typical verifications,the code is certified to be capable to simulate the key physical processes in FCI.For melt hydrodynamic behavior’s details,direct numerical simulation is carried out to reveal the mechanism.Research indicates that the jet breakup process includes front breakup and main body breakup;the processes are quite different under different Weber numbers.When Weber number is high,it is found that the instability and side stripping are the dominant factors.A new jet breakup model is presented based on Epstein’s correlation.For drop hydrodynamic breakup,study results show that process can be divided into two stages: deformation and breakup;the breakup modes are very different under different Weber numbers.For the catastrophic mode,deformation and Rayleigh-Taylor(R-T)instability dominate the breakup process.A new drop hydrodynamic breakup model considering deformation effect is obtained.With the new jet breakup model and the new drop hydrodynamic breakup model,this paper simulates the typical experimental conditions and validates the code’s capabilities.Simulation results of KROTOS 37 and KROTOS 42’s premixing stage indicate that current code is capable to simulate the key processes including jet breakup and coolant boling at the premixing stage.It is also found that Meignen model over estimates the evaporation compared with Saito model and Epstein model.The new model can give a closer prediction with experimental data.In addition,current analysis suggests that the fraction of molten drop is one of the most important factors to trigger the explosion.Simulation results of KROTOS 21 and KROTS 42’s explosion stage indicate that current code is capable to simulate the key processes including pressure spread and escalation,drop breakup at the explosion stage.Results reveal that the new model can obtain a closer pressure magnitude and pressure speed compared with the experiment,and can also improve the prediction result of fragmentation ratio.In addition,a typical ex-vessel steam explosion of pressurized water reactor(PWR)is also simulated with METRIC.It is found that at 1.1s,melt fully premixes with liquid coolant.This time could be the possible time to trigger the explosion.With evaluation of the explosion stage in PWR,it indicates that the pressure impulse on the cavity wall is larger than 0.12 MPa.s.It will possibly threaten integrity of the cavity structure.In summary,based on the new numerical method presented by current paper,a steam explosion analysis code is developed to provide a numerical tool to study steam explosion in nuclear reactor.Through the verification,validation and application,it is suggested that the current code has the equivalent capablities and accuracy of other steam explosion analysis codes developed by foreign countries.Additionally,the paper studies the details of melt hydrodynamic behavior with direct numerical simulation and new melt hydrodynamic breakup models are carried out,which can improve the steam explosion analysis code’s prediction results.