Analysis Of Countercurrent Flow Limitation In PWR Downcomer Based On RELAP5 Program

Reactor safety is the key problem to nuclear reactor operation.In nuclear power plant accidents,loss of coolant accident(LOCA)may lead to reactor core melting and even the leakage of radioactive material,so the research on this accident has attracted much more attention.When LOCA occurs in PWR,counter current flow(CCF)usually accompanies with this accident.In most cases the liquidflows down and steam flows up,and they interact at the phase interface.The shear at the interfaceincreases with the increase of gas flow rate while the liquid flows down.This limits the liquid down flow rate.In this case,there is a flow rate limit for the downward liquid,which is called counter current flow limit(CCFL).CCFL is the limit of this liquid flow rate-due to opposing gas flow.It has safety important in obtaining liquid coolant to the reactor core and preventing heating up of the core.Under large break loss of coolant accident(LBLOCA),CCFL will lead to the blockage of emergency cooling water injection into the downcomer of pressure vessel.Part or all of the cooling water may be taken away by the reverse steam generated in the core,and then flow back to the break loop,resulting in insufficient cooling water flow into the lower plenum,thus delaying the cooling time of the core.This result has a direct impact on the reactor safety.In this thesis,CCFL in the downcomer of pressure vessel is analyzed by using RELAP5 software,the best estimate codes.The main contents of this study are as follows.(1)The best estimate codes RELAP5 was used to establish the model of UPTF Test6.Under certain conditions,the sensitivity analysis of CCFL calculation ability based on RELAP5 was carried out according to the correlation parameters,resistance coefficient and node division scheme.The results show that RELAP5 can reasonably simulate the experimental conditions,and the recommended correlation parameter combination in RELAP5 is more suitable for the calculation of CCFL in downcomer;when the resistance coefficient is increased in the vertical direction.The time of the first injection in the lower plenum was decreasing from 54.4 s to 51 s,and the relative error of the injection flow rate in the lower chamber was reduced from 44.8% to 4.2% These results can well reflect the large-scale circumferential heterogeneity.In the calculation,the condensing of steam is overestimated under the condition of subcooling.The over estimated condensate water quantity has obvious influence on the pressure in the pressure vessel,but it has little influence on the flow rate in the lower plenum.At300kg/s steam injection rate condition,the real total steam flow rate of break is 1.83 t,and the RELAP5 calculation result value is 1.46 t.(2)Based on the numerical analysis of RELAP5 ability on CCFL simulation,the non-uniform water injection experiment UPTF Test 7 and scaling facility MIDAS used as reference were calculated,and the working conditions of variable flow rate conditions were studied and analyzed.The results show that the oscillation response in the calculated pressure is more sensitive and faster than that of the experimental data due to the existence of the discharge valve.The experimental data and simulation of the pressure variation at the initial stage of water injection reflected the water plug phenomenon observed in the experiment.The phenomenon of each stage strongly depends on the results of the previous stages;the circumferential heterogeneity of the scaled test facility is smaller than that of the UPTF facility.Different steam flow rate variations will lead to different trends of steam condensation ratio.(3)Because the scaling method is a commonly used as research method for this problem,on the basis of the previous studies,the scale of UPTF Test 6 was modeled and calculated.The applicability of the three scaling methods,namely three-level scaling analysis method,hierarchical two tiered scaling analysis method(H2TS)and modified linear method,is analyzed.The calculation results show that Ishii three-level scaling analysis and H2 TS method are more advantage on the study of the consistency of pressure,and the modified linear method has better similarity with that of the prototype in ECC bypass phenomenon,CCFL and flow rate.