A Study on Natural Circulation Flow Rate for Various Shaped Channels

Abstract

The severe accident in Fukushima daiichi nuclear power plant revealed the importance of molten core cooling system without the electric power, then cooling system by natural circulation flow has attracted attention. The heat removal performance of this system depends on the natural circulation flow rate, so it is essential to predict the flow rate accurately for the safety design. The purpose of this study is to develop the evaluation method for the natural circulation, and to examine its accuracy dependence on the channel shape. Experiments were carried out at atmospheric pressure, using room temperature air-water flow. We used seven kinds of channels with various shape and diameter, and measured natural circulation flow rate and pressure drop for each channel. Predictive analysis was conducted by our method based on balance between driving force and pressure drop in the loop channel. The major results are as follows: (1) It is possible to predict the natural circulation flow rate with an error of less than 20 percent for various shaped channels. The average and standard deviation for the ratio between calculated and experimental flow rate is 0.98 and 0.047. (2) Two phase prediction method for pressure drop has maximum 15 % error. To improve the prediction accuracy, it is important to improve the prediction of the void fraction and the two-phase multiplier.