Experimental Study On Flow Characteristics And Adhesion Of Spray Cooling Liquid Film For Spent Fuel Rod In PWR

The third generation pressurized water reactor is one of the main directions of nuclear power development in China.In the design of the AP1000 reactor type introduced from Westinghouse and the CAP 1400 reactor type independently developed in China,the safety design scheme of spray cooling spent fuel assemblies is adopted to avoid overheating of spent fuel rods and diffusion of radioactive substances under the extreme condition of water loss in the spent fuel pool.The spent fuel assembly belongs to a dense rod bundle arrangement,and there is adhesion of liquid film between adjacent spent fuel rods.This severe disturbance can seriously affect the local liquid film,leading to heat transfer deterioration and affecting the safety of the spent fuel.Therefore,conducting research on the falling liquid film formed by spray cooling of spent fuel rods has important engineering value.At present,domestic and foreign scholars are paying more attention to the falling film flow on the plate,the falling film flow outside the horizontal tube,and the falling film flow inside the vertical tube.The research on the falling film flow characteristics outside the vertical tube is not clear enough,and there is a lack of research on the falling film flow characteristics between dense rod bundles.Therefore,this study focuses on the flow characteristics and adhesion phenomenon of spray cooling liquid film,which has important academic value.This article uses optical measurement methods to conduct experimental research on the spray cooling liquid film outside the spent fuel rod.Firstly,the spatiotemporal variation and fluctuation characteristics outside the single spent fuel rod were studied.The experimental results show that the liquid film thickness and the amplitude and frequency of liquid film fluctuation increase with the increase of Reynolds number.When Re is in the range of 608-7 538,the liquid film thickness ranges from 0.457 2 mm to 1.428 2 mm,and the mean square deviation ranges from 0.018 84 mm to 0.1383 mm.Moreover,the fluctuation of the liquid film exhibits significant multi frequency characteristics,but the transient flow fluctuation frequency of the liquid film is concentrated in the range of 0-20Hz.The liquid film flow outside a single spent fuel rod will accumulate in the top area of the rod,causing the liquid film thickness to gradually decrease and then flatten out as the distance from the rod top increases.Secondly,the impact of liquid film adhesion between spent fuel rods was studied.The experimental results show that the liquid film adhesion between two spent fuel rods may lead to the breakup of the liquid film flow layer,and its impact on the flow characteristics of the liquid film is mainly as follows:at a low Reynolds number,the liquid film thickness will decrease with the increase of Reynolds number,which is due to the increase of the fluid adsorbed due to the liquid film adhesion;When the Reynolds number is large,the liquid film thickness will increase with the increase of Reynolds number,which is due to the air entering the liquid film due to the rupture of the liquid film flow layer,leading to the virtual height of the liquid film thickness;When the Reynolds number is too large,the liquid film thickness will decrease with the increase of Reynolds number,because the axial velocity of the fluid is far greater than the radial velocity,and the fluid at the fracture tends to decline directly in the axial direction.The phenomenon of liquid film adhesion will cause the instantaneous thinnest liquid film thickness to be thinned to 30.3%～70.7%of the thickness without liquid film adhesion,and at the same time,the average thickness of the thinnest liquid film will be thinned to 62.2%～88.3%.Finally,based on the flow characteristics of the liquid film outside a single spent fuel rod and the flow characteristics of liquid film adhesion between two spent fuel rods,an equation for the fluctuation coefficient of the liquid film and an empirical range of the adhesive coefficient were proposed.The equation for the thickness of the outer liquid film of the spent fuel rod was established,providing basic data for determining the heat transfer characteristics and critical heat flux density of the spent fuel spray cooling.