Research On Thermal Hydraulic Characteristics Of In-containment Refueling Water Storage Tank Used In AP1000

In the Third Generation advanced nuclear power plant AP1000,the Passive Residual Heat Removal System(PRHRs)is implemented.The In-containment Refueling Water Storage Tank(IRWST)is one of the key equipment in PRHRs.Both the C-shape Passive Residual Heat Removal Heat Exchanger(PRHR HX)and the Automatic Depressurization System(ADS)Stage 1-3 sparger are immerged in IRWST.In accidental conditions,the PRHR HX and ADS sparger operate to remov e the residual heat in the primary side to the secondary heat sink IRWST.The effective and reliable operation of both the PRHR HX and ADS sparger are of great significance for the effective cooling down and depressurization of AP1000 plant.In the present work,taking the AP1000 reactor design as the prototype,the overall scaled-down separated-effects IRWST&PRHR HX&ADS test facilities are built to simulate the thermal hydraulic behavior in the residual heat removal accident,including the PRHR HX wall heating transfer and the ADS steam spraying condensation performances.The key thermal hydraulics parameters including the 3-D temperature,velocity distributions in overall IRWST and the local heat transfer coefficient are obtained based on the experimental data.Then the key heat transfer characteristics and mechanism are analyzed to evaluate both the overall and local thermal hydraulic characteristics in IRWST.For the special C-shape PRHR HX,the separated-effects scaled PRHR HX model is used,and the heat transfer characteristics of the vertical bundle,upper horizontal bundle,and the lower horizontal bundle in both the single-phase natural convection stage and the two-phase boiling heat transfer stage are investigated.Based on the experimental results,the applicability of the traditional heat transfer empirical correlations for different sections of the PRHR HX bundle is evaluated.In addition,the experimental results indicate that the thermal stratification in IRWST is very obvious during the operation of the PRHR HX,and the corresponding Stratification number approaches 1.As a result,the innovative optimized baffles designs are proposed in order to reduce the thermal stratification in IRWST.The temperature and velocity distributions,as well as the heat transfer effects are also evaluated by the experimental data.The experimental data validate that the baffles designs work effectively to reduce the thermal stratification in IRWST for more than 30%,which is advantageous for the long stage cooling down and depressurization of the reactor primary side during the accidental conditions.For the complicate shape and multi-holes sparger in the Automatic Depressurization System(ADS)Stage 1-3,the steam direct contact condensation(DCC)phenomenon is mainly researched.Based on the separated-effects scalded IRWST&ADS sparger experiment,both the local steam jets condensation phenomena and the overall thermal behavior including 3-D temperature distribution,velocity distribution,and natural circulation,etc.are investigated.The innovative “steam condensation column” lumped model is proposed using characteristic parameters to describe the special shape and multi-holes sparger steam performance.Then the heat transfer effects are estimated based on the theoretical model and experimental data.The experimental results show that the steam spraying heat tranfer coefficient ranges from 0.6-2.9 MW/(m2·?)under the relatively large subcooling condition.In addition,the experimental results indicate that the thermal stratification phenomenon is very obvious in IRWST.So an optimized ADS arrangement design is further proposed to reduce the thermal stratification extent in IRWST.The height of the sparger is adjusted to change the overall flow and temperature distributions in IRWST,and the stratification criteria including the Richardson Number and Stratification Number are utilized to predict and evaluate the thermal stratification extent,respectively.The thermal stratification is reduced significantly in the optimized ADS deisgn,which is advantageous for the long term cooling down,depressurization,and safe operation of the reactor.The present work takes the depth investigation on the thermal hydraulic characteristics of the IRWST used in AP1000,and the experimental data can be used for the validation and evaluation of the Core and System INtegrated Engine for design and analysis(COSINE)software,which is of great value for the application.The experimental results provide practical engineering reference for the operation and optimized design of the passive safety system in AP1000 plant.