| After a severe accident in a nuclear power plant,the core and core support components will melt due to high temperature,and form a hemispherical multi-layer molten pool in the tower head of the pressure vessel over time.The core melt will continuously release decay heat through the pressure tower head.The third-generation plant adopts the measure of External Reactor Vessel Cooling(ERVC)to achieve the purpose of In-Vessel corium Retention(IVR)in response to severe accidents.So far,IVR-ERVC has become an important severe accident mitigation stratgy in the third-generation nuclear plant.The principle of the IVR-ERVC strategy is when a severe accident occurs,the cooling water in IRWST will be used to flood the reactor cavity.There is an annular flow channel between the outer wall of the pressure vessel and the insulation.Water in the annular gap will absorb the heat from the lower head and form bubbles.Natural circulation will take place by the density difference and take out decay heat to prevent the core debris from melting through the lower head.In this process,when the actual heat flux of lower head does not exceed the external cooling limit,the IVR-ERVC strategy can successfully cool the lower head and maintain the integrity of the pressure vessel Critical Heat Flux(CHF)is a key parameter which characterizes the limit of external cooling capacity,so it is particularly important to study the CHF mechanism on the outer surface of reactor lower head.At present,many researchers have conducted many studies and experiments on IVR-ERVC strategy.However,due to the large size of prototypical reactor vessel lower head,it is difficult to solve the manufacture process of test section and heating problems.At present,two-dimensional slice test sections are used for experimental research to represent the reactor vessel lower head.However,it is somewhat different from the actual shape of the reactor vessel lower head.In order to explore the influence of geometrical effect of the test section slice on the measurement of CHF,this paper uses a spherical slice to carry out tests,and compares results with the two-dimensional slice test results when other conditions are same.In order to solve the problems of heating and manufacture,this test bench adopts a 12 radius ratio.The test section is a 90° arc with aradius of 0.9975m and a circumferential opening angle of 15°.The flow channel has a thickness of 80mm.The heater copper block in this test section is divided into nine parts,and heating power of each part can be individually adjusted to achieve different heat flux designed for this test.Deionized water was used as working fluid in this test and natural circulation tests were carried out under conditions of 98℃、90℃、82℃ inlet water temperature and 3.3m,4.3m,5.3m natural circulation height.The experiment data indicate that the natural circulation flow is greatly affected by the inlet water temperature and is not sensitive to the change of natural circulation height and the two-dimensional test flow is much larger than this test,which is up to 47.8%;on the whole,CHF of this test section increases with the increase of the inlet subcooling degree and the natural circulation height while the magnitude of change gradually decreases.Compare with CHF obtained from two-dimensional slice test,it is found that the CHF in the cylindrical slice is significantly higher than that is this test.When the inlet water temperature is 98℃ the difference between the two is 10.3%,and when the inlet temperature is 82℃,the gap reaches 28.4%.For the first time,this experiment creatively uses the same proportion of spherical slice to reveal the uncertainty of CHF caused by geometric effects when cylindrical slices are used to simulate the pressure vessel lower head. |
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