| The numerical calculation of the sodium-cooled fast reactor faces the following problems:the complex geometry of the wire;the applicability of the turbulence model;the special properties of the liquid sodium,etc.At the same time,because of the tight arrangement of fuel rods,the block accident is easy to occur,In the thesis,the steady state of a 7-pin sodium-cooled fast reactor wire-wrapped fuel bundle was numerically simulated,and two kinds of inlet blockage accidents were analyzed.The convergence standard,contact model,boundary layer model,turbulence model and turbulent Prandtl number model were analyzed respectively.The economical and accurate calculation models were obtained,and the velocity and temperature fields were analyzed.The results show that the Reynolds stress model and the standard k-εmodel are in good agreement with the experimental correlations,and there is little difference in the velocity and secondary flow distribution between the two models.The velocity boundary layer conforms to the law-of-the-wall,but the temperature boundary layer no longer conforms to it,and the standard wall function in Fluent is no longer applicable.The contact model of the wire and rod has a great influence on the heat transfer.If the wire is embedded too much into the fuel rod,the flow velocity in the gap between the wire and fuel rod will increase,the intensity of secondary flow will weaken,and the cross-mixing effect will weaken,which will underestimate the flow resistance and overestimate the heat transfer performance.Since the turbulent Prandtl number of liquid sodium is no longer approximate to a constant,if the turbulent Prandtl number model is not used,the heat transfer performance of sodium cooled fast reactor will be overestimated.Due to the influence of the wire the coolant rotates clockwise.In the cross section,a pair of vortexes will be formed before and after the wire,but in the longitudinal direction,no obvious vortexes are found before and after the wire.Due to the obstruction of the wire,the surface temperature of the fuel rod in front of the wire and facing the incoming flow direction is low,while the surface temperature of the fuel rod behind the wire is high.The heat transfer on the fuel rod surface is uneven,and the highest temperature appears at the corner of the contact between the wire and the rod.In the transient calculation of inlet blockage accident,the variation of flow field and temperature field after blockage is analyzed respectively.The calculation of central blockage accident shows that the flow field reaches the steady state in about 0.02 seconds and the temperature field reaches the steady state in about 0.15 seconds.The maximum temperature in the fluid domain reaches 718.4K.In the transient process,the temperature starts to rise from the rod surface near the downstream of the block,and gradually extends outwards and downstream.The velocity behind the blockage is low,the length of the wake zone with high temperature is about 60 mm,and the highest temperature occurs about 5 mm downstream of the blockage.The flow velocity at the outlet is not much different from that of the normal working condition,and the temperature distribution at the outlet is less sensitive to the blockage accident.When small changes in flow velocity and temperature are detected at the outlet,it is likely that a serious blockage accident has been formed in the component.The heat transfer in the low velocity zone behind the blockage mainly depends on the fluid from the nearby unblocked flow channel,and the secondary flow generated by the wire can relieve the blockage accident to a certain extent.The analysis of the corner blocking accident shows that:After the flow blockage occurs,the flow field reaches the steady state in about 0.02 seconds,but the temperature field reaches the steady state in about 0.08 seconds.After reaching the steady state,the highest temperature is 676.0K,which is lower than that of the central flow blockage accident.The wake area behind the blockage extends to 70 mm,which is longer than that of the central blockage accident.Compared with the central blockage accident,the corner block flow accident has a greater change in velocity at outlet.This may be due to the corner block flow being close to the fuel box,which makes it more difficult for the wake area after the blockage position to return to normal.At the same time,due to no inner heat source in the fuel box the maximum temperature of the corner blockage accident is lower than that of the center blockage accident.In this study,the meshwork method of the wire and the analysis results of the turbulent Prandtl number provide a reference for the core design and safety analysis of the sodium-cooled fast reactor.The analysis results show that the open structure of sodium-cooled fast reactor is beneficial to alleviate the blockage accidents,but more fuel rods may need to be analyzed to clarify the influences of the fuel box and the rod number. |
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