| In sodium fast reactor,after being stored for 2～3 reload cycles in storage pit in the reactor vessel,spent fuel assemblies(SFAs)need to be transferred out of the reactor.The SFA will be transferred in the inert argon area in reactor building after being removed from the sodium pool.SFA handling machine consists of handling mechanism and a cover tube,it is the most complex component during defueling.Failure of fuel handling machine can result in overstay of SFA in argon atmosphere and consequently,lead to overheating of SFA due to the decay power.It should be assured that the maximum temperature of fuel rod cladding does not exceed the design limit.For the sake of investigating heat dissipation of SFA in the argon atmosphere,typical transport locations are selected throng all the transportation conditions by analysis,steady-state experiments are carried out to reproduce the enveloping scenario of fuel handling system failure,i.e.experiments without and with the cover tube,are investigated.Heat transfer experiments of a full-size simulant fuel assembly(FA)without cover tube are carried out to simulate the shutdown condition of the spent fuel assembly up the sodium pool.The test section is a 169-rods simulant FA.It consists of an assembly wrapper,1 center rod,156 heating rods and 7 temperature measuring rods.The center rod can be heated and measured the temperature at the same time.The heating power of the heating rods is supplied and adjusted by 4 multiple-channel low voltage DC power supplies and temperature measuring rods are not heated.The test section(cover tube)is installed in a sealed vessel where the temperature can be adjusted by external heating.The argon gas is injected into the vessel to simulate the atmosphere in the reactor building.The results show that the effect on the temperature distributions in the simulant FA,especially in the center rod,of the environment temperature fluctuation can be neglected when the fluctuation range is-20℃～20℃.Based on the heat transfer mechanism analysis inside the simulant FA,Manteufel heat transfer model is proved to be reliable when the emissivity is in the range of 0.3 ～ 0.35.Based on the heat transfer mechanism analysis outside the simulant FA,it can be seen that the convective heat transfer coefficient is increasing,but the share is decreasing with the increase of heating power,and the radiation is always dominant.It can be concluded that enhancing the emissivity is the most effective approach to reduce the thermal resistance and the peak temperature in the fuel assembly.To simulate the shutdown condition of the spent fuel assembly in the transport chamber,firstly,heat transfer experiments of a scaled-down simulant FA with different kinds of cover tubes,i.e.irregular cover tube and regular cover tube,are carried out.The scaled-down simulant fuel assembly is a scaled-down 37-rods simulant fuel assembly,it consists of an assembly wrapper,30 heating rods and 7 temperature measuring rods,temperature measuring rods are not heated.Test procedures are as the same as those in the heat transfer experiment which is used to simulate the condition up the sodium pool.The results show that temperature filed in the FA is still symmetric with irregular cover tube,and as the same as the results with circular cover tube,so the irregular cover tube can be replaced by a circle cover tube.And consequently,experiments of a full-size simulant FA with cover tube are carried out,the irregular cover tube is replaced by a circle one during the experiment.The results show that the effect on the temperature distributions in the simulant FA,especially in the center rod,of the environment temperature fluctuation can be neglected when the fluctuation range is-20℃～20℃.Manteufel heat transfer model is still reliable when the emissivity is in the range of 0.3 ～ 0.35,enhancing the emissivity is still the most effective approach to reduce the thermal resistance and the peak temperature in the fuel assembly. |
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