In the Fukushima Daiichi NPP accident, loss of cooling capabilities for spent fuel pool (SFP) occurred. As a result, a water level of the SFP decreased and it was concerned that the spent fuels were damaged. In one of the safety measures for the SFP, a portable spray system is considered as a device to cool the spent fuels.

In this research, the numerical simulation method for evaluating the capability of spray cooling has been developed. To develop this method, we mainly focus on the thermal-hydraulic behavior of two-phase flow generated by the spray cooling systems at the top and inside of the fuel assembly. And an experiment for validating the numerical simulation method also has been performed. In the experiment, by using the experimental apparatus with 5x5 bundle and in order to evaluate the counter current flow limiting (CCFL), visualization of two-phase flow behavior at the upper tie plate is performed and the liquid volume entering into the simulated fuel assembly is measured. In this paper, visualization results of CCFL and CCFL condition of the simulated upper tie plate, and visualization results of developed simulation code are reported.