Analysis Of The Accident Of Blockage Of The Flow Passage Of The Plate Fuel Element Stack

There are some obvious advantages in using plate-type fuel plate,such as the compact structure,low temperature in fuel meat,high heat release rate,heat transfer efficiency,increased power volume ratio.So,it is used widely in research reactor and integrated reactor.In reactor core,fuel plates are arranged parallel to each other.The narrow rectangular channels are arranged between the plates for coolant flow,and the coolant channels are not connected with each other.In some accidental cases(extraneous matter falls into the reactor core,fuel plate swells and bends due to radiation,mechanical stress),the flow blockage accident will be occurred.Coolant flow rate in the blocked channel will decrease rapidly,and the fuel plate temperature will increase sharply because of the loss of part cooling.During the accident,steam may generate and departure nucleate boiling may occur.The fuel integrity is threaten.On the other hand,due to the lack of proprietary monitoring and mitigation measures for flow blockage accidents,indirect monitoring is generally carried out by low-power shutdown,radioactive measurement.In addition,when the channel blockage ratio is very low,the accident does not have a significant impact on the core,so it cannot be detected immediately.The accident will exist for a long time,which makes the superposition of the accident possible.According to different accident situations,the accidents are divides into three cases,single channel blockage,superposition of blockage accidents and blockage of fuel assembly(multi-flow channel blockage).In the paper,JRR-3M 20MW research reactor which is also the typical plate-type fuel reactor is chosen.The system analysis code RELAP5/MOD3.4 and the fluid dynamics program FLUENT are adopted respectively to investigate the flow and heat transfer characteristics on single channel blockage,superposition of blockage accidents and blockage of fuel assembly.Firstly,RELAP5/MOD3.4 code is used to model the main coolant system ofJRR-3M reactor.A multi-channel core model is also established to execute the analysis of single channel blockage and accident superposition.In order to solve the problem of fuel assembly blockage,FLUENT code is adopted for 3D modeling of fuel assembly,and the coupling analysis between RELAP5 and FLUENT is carried out to discuss the flow and heat transfer characteristics between each subchannel and fuel plate in detail.According to simulation,it can be found that the accident result is closely related to the degree of blockage and power density distribution.The higher the blockage ratio,the greater the power density,and the more serious the accident consequence.To be specific,when a single channel blockage occurs and the blockage ratio is 60%,the minimum departure from nuclear boiling ratio(MDNBR)in the core will reach the allowable limit of 1.5.If the blockage ratio of the channel increases,departure from nuclear boiling will occur in the blocked channel,worsening heat transfer.Even if the coolant channel is completely blocked,the heat from the blocked fuel plate can be removed by coolant from the adjacent flow channel.The fuel plate will not melt down.However,when the channel is totally obstructed,the fuel plate will undergo strong thermal stress and compressive stress,which may cause mechanical damage to the fuel plate.When loss of flow accidents are selected for the superposition simulation of accidents,the calculation shows that if the flow channel blockage superimposed loss of flow accident occurs,the accident condition will further deteriorate,and the allowable blockage ratio will also be reduced to 50%.Then,due to the low flow emergency shutdown operation of the loss of flow accident,the consequences of the accident can be significantly alleviated.During the whole process,the fuel plate temperature is far below the allowable safety limit,and the fuel will not melt down.In the subsequent natural circulation,the core temperature will increases again when coolant flow reversed.The core cannot be maintained in a safe quasi-steady state until a stable natural circulation is established.As for flow blockage on fuel assembly,the results indicate that three dimensional calculation is important for accident simulation to get the more realistic results,and the spatial distribution of power density in the assembly needs to be considered in detail.When the blockage rate is 64%,the coolant outlet temperature in the hottest subchannel in the assembly will approach the saturation temperature.If the blockage ratio increases,the departure from nucleated boiling will occur and the fuel plate will melt down.In order to obtain the real flow heat transfer situation in the assembly blockage under the extreme accident,thermal and hydraulic experiments are also required,and FLUENT program will no longer be applicable.