Numerical Analysis Of Thermal Stratification In Pool Type Lead Based Reactor

Lead based reactor is considered to be the most promising reactor option for the generation ? reactor system.China LEAd-based Reactor(CLEAR)was selected as the reference reactor in CAS lead based reactor project,led by Institute of Nuclear Energy Safety Technology(INEST),CAS.In the first stage,the China lead-based research reactor CLEAR-? will be constructed.Lead-based reactor engineering validation facility CLEAR-S is a large scale integrated non-nuclear pool type facility which has been built to provide a unique experimental platform for full-scale prototype components test of lead based reactor.Thermal stratification analysis is important in lead cooled fast reactor because it can induce thermal stress and fatigue,which is one of the major safety concern for the lead cooled fast reactor.In this thesis,3D CFD steady state and transient analysis of flow of coolant of CLEAR-S was carried out to study thermal stratification in the hot and cold pools by using commercial code ANSYS FLUENT during different operating conditions.Simplified computational model of CLEAR-S was built.Mesh convergence achieved around 35 million elements and Standard k-epsilon model with standard wall function was found appropriate for simulations of flow of LBE in CLEAR-S.Appropriate flow resistance coefficients were selected in the simplified model of CLEAR-S which will provide almost similar pressure drop with porosity model as calculated by considering detailed model of each component.The steady state results showed that the simulation had a good agreement with the design parameters of CLEAR-S and maximum deviation is less than 2%,which indicates that the calculation model had good agreement with the design of CLEAR-S.Study state forced and natural circulation of LBE in CLEAR-S was simulated in order to study thermal stratification during study state and to analyze the roll of coupled wall conduction in circulation of coolant.The temperature profile was uniform in the whole domain of hot and cold pools without considering any conduction through the coupled walls.The temperature distribution in the hot and cold pools became non-uniform with the coupled wall conduction and thermal stratification was observed in the cold and hot pools near the bottom wall of hot pool.During natural circulation conditions coupled wall conduction enhanced natural circulation and kept the hot pool at relatively low temperature,as the cold pool cooled the hot pool through heat transfer between the walls of hot and cold pools.So wall conduction had some advantages and disadvantages as it enhances the natural circulation on one hand but disturbs temperature uniformity on the other hand.Loss of flow transient initiated at 100%,50%and 25%of rated mass flow value*This research was supported by the project of HIPS(KP-2017-19)and Special Project of Youth Innovation Promotion Association of Chinese Academy of Sciences of CLEAR-S was simulated in order to characterize thermal stratification on the basis of initial operating conditions.The simulation was carried out by removing the momentum and energy sources from pump and heat exchanger(HX)zones respectively,and DHR will remove the decay heat generated by core simulator during shutdown at t=0 sec.There is a small increase in mass flow rate after an initial sharp dip due to the start of natural circulation.During natural circulation mass flow rate was 3%of rated mass flow rate.After loss of flow transient from 100%mass flow rate LBE in the hot pool was distributed into six layers of different temperatures and temperature difference between each layer was about 5 to 7 K,which increased 6 K and 18 K with the decrease of mass flow rate of LBE during startup of transient of about 50%and 25%respectively.As the time progressed stratification layer moved upward in hot pool and the width of top most high temperature layer of LBE reduced which ultimately vanished after giving birth to a new layer at the bottom of hot pool.Thermal stratification started from DHR outlet in the cold pool after loss of flow transient from 100%mass flow rate.Stratification layer settled in the bottom and moved upward as the time progressed and temperature difference between LBE on both sides of the layer was about 10 K,which increased 7 K and 17 K with the decrease of mass flow rate of LBE during startup of transient of about 50%and 25%respectively.With the reduction of mass flow rate of LBE during the start of loss of flow transient the temperature difference of the LBE on both sides of stratification layer was increased.This research of thermal stratification in the pool type lead based facility is significant for the development of pool type lead based reactor technology because thermal stratification could cause thermal stresses on the walls and components inside the pools of reactor which is dangerous for the safety and integrity of the system.This study will also provide a guideline for the planning and execution of CLEAR-S experiment which will be held in near future.This research will be used as reference for the selection of simulation methods for thermal stratification study in lead cooled fast reactors.