Numerical Simulation And Analysis On Flow Blockage Accident Of Fuel Subassembly For Sodium Cooled Fast Reactor

At present,most sodium cooled fast reactors(SFR)generally utilize wire-wrapped spacers to fix the fuel assembly,which makes the slender and narrow flow channels prone to blockage accidents,resulting in deterioration of the heat transfer of cladding and possible damage,and even partial boiling of sodium.However,flow blockage of coolant channels has negligible effect on the total mass flow rate and it is difficult for reactor protect system to detect the effect caused by flow blockage.Hence,the fuel assembly flow blockage accident is considered to be one of the important research topics of the SFR thermal hydraulic analysis.A larger number of works have been carried out in the literature.However,it remains a great challenge to accurately predict the thermal hydraulic characteristics of the fuel assembly under flow blockage accident due to the complex mechanism involved.In this paper,the numerical simulation method of computational fluid dynamics(CFD)and sub-channel analysis was used to investigate the flow blockage accident of China experimental fast reactor(CEFR)fuel assembly.The numerical models for both porous media and solid media blockage have been established.The influence from the blockage area and its porosity was investigated as well as the axial length of blockage and mass flow rate.The cross flow between sub-channels through gap has been evaluated.The simulation results that a stagnant recirculation region is formed at the of downstream blockage.It was found that the size of recirculation region is related to the blockage area.The behavior of cross flow at the downstream of blockage is the interaction of wire-wrapped spacers and blockage,and its intensity is enhanced by the blockage area or mass flow rate,weakened as the blockage porosity increases.Under the influence of the blockage condition,the profile of coolant mass flow rate on the cross section of fuel assembly may be redistributed.In terms of the heat transfer characteristics of the fuel assembly,the temperature of cladding rises rapidly in the blockage region and then descend at the downstream of blockage.It was found that the maximum cladding temperature difference is higher as blockage area increases and decreases as the blockage porosity or coolant mass flow rate increases.It can be concluded that it is difficult to detect this temperature variation with thermocouples when the location of blockage is not close enough to thermocouples,even if the cladding temperature varies in hundreds of degrees Celsius,which makes blockage accident more elusive.This paper aims to explore the analytical method of numerical simulation research on SFR fuel assembly blockage accidents.The result achieved in this study can provide certain enlightenment and practical significance for the future work.