Experimental Study Of Primary And Secondary Side Coupling Heat Transfer Characteristics Of The Passive Residual Heat Removal System In AP1000

Passive residual heat removal system(PRHRS),which is a newly designed system in the advanced reactors of AP1000 and CAP1400,plays an important role in the critical accidental conditions.In the station black-out accident(SBO),Passive residual heat removal exchanger(PRHR HX),In-containment refueling water storage tank(IRWST),flow channels of coolant in reactor core and corresponding pipes in PRHRS form the natural circulation loop and residual heat is removed from the core to the heat sink(IRWST)under the density contrast between the cold and hot leg in the loop.The effective and reliable operation of PRHRS are of great significance for the effective cooling down and depressurization of AP1000 plant.The most previous studies were made for engineering verification to prove that the heat transfer capacity of the PRHR HX satisfies the actual engineering requirement.However,there are few studies to focus on the local heat transfer mechanisms and primary and secondary side coupling natural circulation characteristics that are also vital to the safety of reactor.Therefore,the experiment bench is built to study the coupling and local heat transfer characteristics.Taking the AP1000 reactor design as the prototype,an overall scaled-down PRHR HX and IRWST model has been built to simulate the thermal hydraulic behavior in both the primary and secondary side.The key thermal hydraulic parameters including temperature distribution in the overall IRWST and PRHR HX and the local heat transfer coefficient are obtained in the experiment.Based on the experimental data,the heat transfer mechanism and primary and secondary side coupling natural circulation characteristics are analyzed.Based on the scaled PRHR HX model,the heat transfer characteristics of upper horizontal bundle,vertical bundle,and lower horizontal bundle of the C-shape tube bundle are carefully researched.The proposed correlations in literature are compared to the experimental data to validate theirs applicability for the specific natural convection condition and the modified correlations are given at the same time.The results also shows that the thermal stratification in the vertical condition is very obvious,but there is almost no temperature difference in lateral direction in the bulk.At the last,the primary and secondary coupling characteristics are carefully analyzed.In the natrual circulation process,the change of heat transfer mechnism from natural convection to nucleate boiling in the secondary side and the enhancement of mixture effect between tubes improve the heat transfer effect of the heat transfer exchanger,leading to better cooling effect and increasing of density in the primary side.The increasing of density contrast between hot and cold leg increases the flow rate in the loop and the the increasing of flow rate further improves the heat transfer effect and flow rate.But the flow rate cannot rise all the time because the improvement of the flow rate increases the flow resistance.In the process,the rising trend of the flow rate does not change.The researches take the depth investigation on the thermal hydraulic behaviors of the PRHRS used in AP1000,which provide essential references for the PRHRS operation and further optimized design.