Simulation Study On The In-Vessel Melt Retention For A Small Integral Pressurized Water Reactor

In the design of IP200(a small integrated pressurized water reactor),IVR(In Vessel melt Retention)measure is the most important defense to avoid the radioactive leakage.However,whether the true heat load of molten pool can be restrained lower than the safety limit,there are still three factors that should be considered carefully.Firstly,the characteristics of the integrated reactor(such as core power,coolant storage)and accident sequence(such as corium collapse,safety system)will affect the early process of severe accident,and then change the formation of the molten pool in the lower head.Secondly,how to reasonably quantify the coupling effect of above phenomena,which will directly determine the results of IVR thermal evaluation.Thirdly,the influence by the boiling pattern in ERVC(External Reactor Vessel Cooling)on the cooling efficiency cannot be ignored.Thus,it is necessary discuss the flow boiling condition for independent evaluation,which is separated from the internal molten pool.In order to solve the above problems,this paper carries out the following five topic researches.Firstly,this paper conducts a simulation study on the early accident process of IP200 reactor.This part work aims to investigate the effect of early accident progression on IVR.The analysis code SCDAP/RELAP5 is used to establish to model for reactor and safety system.The severe accident scenario of SLOCA(Small break Loss Of Coolant Accident)along with ESBO(Extended Station Black Out)is simulated.The effects of melting and collapsing on the initial state of molten pool are considered by calculating the continuous phenomenon.Comparing with the similar studies about the small power reactors,the reason to determine the maximum thermal load is pointed out.Next,the flow and heat transfer characteristics of IVR molten pool are studied by simulations.In the analysis of SCDAP code,it cannot be hundred-percent to get the most serious scenario for molten pool,that all the core corium can collapse to the lower head.Therefore,three special models for molten pool are created to evaluate the different IVR characteristics of IP200 reactor when it completely collapsed.Models includes: transient heat transfer characteristics,steady-state flow field characteristics,and layered configuration characteristics.For the transient heat transfer characteristics,a transient heat transfer analysis program of molten pool is established,based on the natural convection model of empirical relation and the moving boundary model with the assumption of isothermal solidification,The LIVE-L5 L experiment is selected for the benchmark validation.In particular,the growth rates of the solidified crust during the dynamic process are emphatically discussed.For steady state flow field characteristics,a fine steady-state simulation program for heat transfer and flow in a molten pool is constructed,based on the CFD model of single-phase natural convection and the enthalpy conservation transformation model.The applicability of the model and algorithm is verified with two different steady-state conditions in BALI benchmark experiment.Then,the molten pool in IP200 reactor is calculated as the object.The influence of the internal heat power on the flow field partition,is discussed with the cloud diagram and the vector diagram.In some cases,the metal phase and oxidation phase may be stratified in the molten mixture,thus changing the layered structure of the molten pool.Based on the concept of FIBS(FInal Bounding State),an estimation model for the steady-state heat transfer of a layered molten pool was developed based on the characteristics of the layered molten pool.The molten pool of IP200 with two-layer,three-layer and water-layer configurations are calculated respectively.The thermal load distributions with the effects of internal power and metal mass are analyzed.Besides,the changes of peak heat flux,which are caused by the heavy metal's appearance and the water layer's boiling,are discussed by the comparison of local CHF.Finally,the flow instability of natural circulation in the ERVC loop,which located outside the molten pool,is simulated.In this part,the open natural circulation transient flow characteristics of ERVC loop are simulated by using the thermo-hydraulic code RELAP5.The steady-state verification of the low heating conditions in the REPEC experiment is carried out,to evaluate the model applicability for the two-phase flow.Then,RELAP5 model is used to establish a slicing model for the ERVC system of IP200 reactor,and the transient flow of natural circulation,which is caused by sub-cooled boiling,is simulated.The high and low undercooling boundary of the unstable flow is divided.The mechanism of the flow instability is explained according to the oscillation regularity.By the model creating and system simulating,this paper not only studies the influence of structure characteristics and phenomena characteristics on the heat transfer of molten pool,but also gives out some specific values of key parameters in IVR.Besides,this paper also preliminarily establishes a set of systematic scheme for the IVR research of small integrated reactor,which aims to provide reference for the engineering design.