Thermal-hydraulic Safety Analysis Of Primary Cooling System For Small Modular Natural Circulation LFR SNCLFR-100

Lead or Lead-alloy cooled fast reactors(LFRs)have been proved to be the most promising reactor which will be commercially realized firstly owing to its outstanding fuel breeding and waste transmutation performance,small modular LFRs have attracted particular attentions in recent years,and small modular natural circulation LFR is one of the potential candidates for LFRs development,as there is no main pump,the reactor safety and economy have gained remarkable improvement.A small modular natural circulation LFR named SNCLFR-100 is being developed by University of Science and Technology of China(USTC).In the present work,a series of key thermal-hydraulic safety issues of SNCLFR-100 primary cooling system were analyzed,such as steady-state thermal-hydraulic analysis code development,3D thermal-hydraulic phenomenal analysis and transient security features research.A series of thermal-hydraulic models for typical small modular natural circulation LFR were established,including the thermal physical property models of coolant and materials,pressure drop and heat transfer models,fuel pin heat conduction model,hot plenum and cold plenum model,single channel model,closed parallel multi-channel model,hottest channel model,etc.Based on those models,a steady-state thermal-hydraulic analysis code named STAC for small modular natural circulation LFR has been developed.A preliminary validation to STAC with the results of sub-channel code named KMC-Sub and steady-state thermal-hydraulic analysis results of URANUS that developed by Seoul National University were carried out.The validation results show that STAC had a high accuracy and reliability.With this analysis code,the rough temperature distribution of SNCLFR-100 primary cooling system,the core temperature distribution at BOL and EOL,and the natural circulation of primary cooling system were carried out.Meanwhile,several correlations were proposed,which can be used to evaluate the natural circulation characters of SNCLFR-100 primary cooling system under different operating conditions quickly.A quarter of overall analysis model for SNCLFR-100 primary cooling system were established using ANSYS ICEM,which was used to simulate the Lead flow and heat transfer in hot pool and cold pool under rated condition of SNCLFR-100.Meanwhile,a complicated core model was also built based on some reasonable simplifications and assumptions,which was mainly used to study the core mass flow distribution characteristics and got the boundaries from the overall simulation results.What more,in order to carry out the above CFD simulation successfully,additional CFD models were developed,including pin thermal model and heat exchanger model.With the primary simulation results,several optimization recommendations were proposed for SNCLFR-100 primary cooling system design,and a primary exploration was made to optimize the core mass flow distribution in order to get a uniform core outlet temperature distribution.The steady-state and transient thermal-hydraulic characteristics of SNCLFR-100 primary cooling system have been studied with system code ATHLET,aiming at analyzing the dynamic response characteristic of SNCLFR-100 primary cooling system under four severe reactor accidents,including unprotected transient overpower(UTOP),unprotected loss of heat sink(ULOHS),simultaneous UTOP and ULOHS.The analysis results show that SNCLFR-100 has excellent intrinsic safety feature,the biggest challenge for core safety in those accidents is the failure of cladding material to withstand high temperature.Considering the limitations of system codes and CFD codes in analyzing the complex three-dimensional thermal-hydraulic phenomena for typical pool-type fast reactor primary cooling system,a coupling calculation with ATHLET and ANSYS FLUENT was studied,in this way,three-dimensional thermal-hydraulic phenomena are considered in carrying out the transient analysis.Based on this method,the accuracy of transient safety analysis will be improved.The station black-out accident(SBO)was studied with this coupling method,and the research is focus on the generation and development of thermal stratification in hot plenum and the coupling feedback of one-dimensional and three-dimensional thermal and hydraulic phenomena.The analytical code that developed in this thesis can be used to supporting the future LFR research and design in China,and the research results that obtained have a certain academic significance and engineering application value.