Water distribution test designed to study the relationship between water film coverage and water flow rate and containment structure, is an important part of the passive containment cooling test. In the process of developing CAP1400 Pressurized Water Reactor (PWR), water distribution test facility was built by SNPTRD (State Nuclear Power Technology Research & Development Center). Water distribution test facility is a separate-effect test facility, based on CAP1400 containment as the prototype. Test body contains 1/8 sector of the containment dome and a certain length (3m) vertical wall, with the ratio of 1:1 of the prototype. Water film identification algorithm is used to extract water film image information from the photo of measured area obtained by Matrix CCD and to determine the effective water film region in order to calculate the water film coverage. The starting point of designing water film identification algorithm was to find out the boundaries between water film zone and no water film region based on the gray difference. In order to ensure that the difference identification can be achieved, median filtering and mean value block were conducted on gray value matrix to remove high-frequency fluctuations in the gray value curve. The algorithm was implemented with MATLAB and sensitivity studies were performed to investigate the effects of block size and difference magnitude on the identified coverage of water film. It was found out that block size and difference magnitude had little influence on the identification of water film coverage. According to the corresponding degree between identified images and original photos, 2-by-2 block and gray-difference function were selected in the algorithm. Then the program identified results were compared with artificial identified results, the results showed that: water film identification algorithm can identify water film effectively. Coverage deviation between calculated and artificial identified results is less than 5%, and nearly not affected by brightness, laying foundation for the analysis of water distribution test.
Scaled down thermal-hydraulic test facility is widely used in the nuclear reactor safety analysis. The wall stored energy is one of the common problem in a scaled down test facility, which will cause distortions in the simulation of the transients or local phenomena. For the natural-circulation mode in the Core Makeup Tank (CMT), the cold wall will work as heat sink and absorb heat from the heated water, and then has an influence in energy balance on the simulation of the prototype phenomena with scaled down test facility. In order to study the influence of the wall stored energy on the natural-circulation mode in the CMT test facility, this paper established the CMT test facility model with RELAP5/MOD 3.4, and studied the influence caused by the wall stored energy distortion. The simulation results show that the natural-circulation process in the CMT test facility can be divided into two stages: 1. Before the thermal stratification reaches the outlet, the natural-circulation flow rate decreases gradually. 2. Once the thermal stratification reaches the outlet, the natural-circulation flow rate has an apparent turning point and decreases faster. That is because the heated water reaches the outlet and fills the discharge line quickly, which reduces the density difference between the hot leg and cold leg, thus causing the natural-circulation flow rate decrease faster. Besides, before the turning point, the wall stored energy basically has no influence on the natural-circulation process, and the influence can be neglected in the design of test facility. However, after the turning point, the wall stored energy distortion results in a slower descent speed of natural-circulation flow rate, which is not conservative. Therefore, we can conclude that the scaled down CMT test facility can only simulate the prototype properly and conduct the natural-circulation simulation experiment before the thermal stratification reaches the exit, while afterwards, the simulation of the test facility won’t be accurate and conservative.
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