Heat Transfer Research Of Flow Blockage In The Inlet Of Plate-type Fuel Assembly Based On CFD Simulation

The plate-type fuel assembly is composed of parallel rectangular fuel plates.The coolant channels between the plates are narrow.The structure is dense.The heat transfer intensity and the heat transfer efficiency are high.Due to these heat transfer characteristics,the plate-type fuel is now mostly used for experiments nuclear reactors and integrated reactors.In the coolant channels of the plate-type fuel assembly,the coolant adopts a cycle cooling method from the bottom to the up.Due to its narrow flow channel,when the fuel element swells by radiation or material fragments inside the core flow into the flow channel,a coolant flow blockage accident may occur.Once the flow blockage accident occurs,the flow field and temperature field of the coolant will change abruptly,and the heat exchange mechanism will change,which may cause the fuel plate to lose cooling and increase the plate temperature.Therefore,the study of the thermal and hydraulic characteristics of the plate-type fuel assembly inlet blockage accident has important safety significance for this type of reactor.In this paper,the software STAR-CCM+commonly used in CFD is used to simulate the single-channel operation of single-phase flow and multi-channels operation of single-phase flow and two-phase flow under steady state.In single-channel conditions,the existing experiments are firstly numerically simulated,and the Realizable k-εturbulence model is selected through simulation and comparison analysis.At the same time,the modeling method used in this paper is used to calculate the plate-type fuel assembly blockage accident is feasible.This simulation method is used to simulate the normal working conditions and 50%inlet flow blockage conditions of a single channel under single-phase flow.It is found that after the inlet flow blockage occurs,the coolant will form the backflow behind the blockage to generate flowing vortex.Velocity,temperature and turbulent kinetic energy will undergo a series of changes with the flowing vortex.Multi-channels conditions are divided into single-phase flow multi-channels conditions and two-phase flow multi-channels conditions.Under single-phase flow conditions,the normal conditions and the inlet blockage conditions of two plates and three channels were simulated,and the inlet blockages were compared at 25%,50%,and 75%.With the increase of the degree of blockage,the impact of the blockage accident on the coolant channel will also increase.The greater of the degree of blockage,the more severe of the coolant disturbance,and the higher of the frequency with flow vortices.At the same time,the fuel plates in the multi-channels working condition will conduct heat to the blockage channel,so the entire system is in a relatively safe operating condition.Under two-phase flow,using the same 50%inlet blockage model as single-phase flow condition,different heat flux were simulated and analyzed.It was found that when the heat flux was 700 k W/m~2,the coolant in the fluid domain reached the maximum temperature of saturated boiling.At the temperature point,the cavitation fraction begins to appear in the calculation domain.When the heat flux reaches 800k W/m~2,the flow blockage condition reaches CHF(critical heat flux point).This paper discusses the research results in the event of flow blockage in the inlet of plate-type fuel assembly from the aspects of flow field,temperature field,energy field,and mass flow distribution.The flow and heat transfer mechanism of the coolant in the single-phase and two-phase,single-channel and multi-channels flow are analyzed and discussed.