Numerical Study On Flow And Heat Transfer Characteristics Of A Rectangular Fuel Assembly With Blocked Channels

Rectangular fuel assemblies have the advantages of compact structure,high volume power and low temperature of the fuel,so they are widely used in integrated nuclear reactors and engineering test nuclear reactors.The coolant channels of a rectangular fuel assembly consist of many parallel narrow rectangular channels,and the gaps of these narrow rectangular channels are very small,usually about 1-3 mm.When a nuclear reactor runs for a long time,impurities may be mixed into the coolant.Therefore,when impurities flow through a rectangular fuel assembly with the coolant,the coolant channels of the fuel assembly may be blocked locally.The blockage will cause local high temperature area,a blockage accident may happen,and the blocked fuel assembly might be destroyed.Therefore,in order to evaluate the effect of partial blockage in coolant channels,it is necessary to conduct a research on the flow and heat transfer characteristics of an obstructed rectangular fuel assembly.In this paper,a single channel blockage model and a multi-channel blockage model are constructed using the CFD method.By analyzing the changes of blockage positions and blockage types,the flow and heat transfer characteristics of a rectangular fuel assembly are studied,which coolant channels are partially obstructed.The single channel blockage model only considers one coolant channel and adjacent fuel elements,while the multi-channel blockage model considers five coolant channels and adjacent fuel elements.The multi-channel blockage model is further divided into two cases,namely one channel blockage model and two adjacent channels blockage model.The blockage is set as a cuboid,in order to model the actual blockage.Three typical blockage positions are analyzed,namely inlet,middle and outlet.The blockage types consist of single side blockage and center blockage,and blockage ratio is 10%.The calculation results show that the temperature of the fuel assembly will increase at the local blockage zone,and the temperature increase margin is the largest,when the blockage is located at the middle of the rectangular fuel assembly.When the coolant bypasses the blockage,the coolant will detach from the wall of the blockage,then a retention zone with low coolant velocity will occur downstream of the blockage.The area of the retention zone in single side blockage case is larger than that in center blockage case.Increasing the velocity of the inlet coolant can significantly reduce the temperature of the fuel assembly,but the increase of the convective heat transfer coefficient caused by increasing the coolant flow velocity decreases,as the coolant flow velocity increases.Moreover,when there is a blockage in one of the parallel coolant channels,the flow rate of the fuel assembly will be redistributed.The flow rate of the blocked channel will decrease,and the flow rate of the non-blocked channel will increase.When two adjacent channels are blocked,the temperature of the blockage zone is always larger than that when one channel is blocked.As there are blockages on both sides of the fuel plate,the heat from the fuel at the blocked region is more difficult to transfer to the coolant.