A prediction system with a one-region (1R) model was developed to predict water temperature in a spent fuel pit (SFP) after the shutdown of its cooling systems based on three-dimensional (3D) thermal-hydraulic behavior computed by using the computational fluid dynamics (CFD) software, FLUENT 6.3.26. The system was later extended to compute the water level in the SFP during the loss of all AC power supplies. This study aimed at confirming the applicability of the 1R model by the comparison of 3D computation results and 1R calculation results. Some of the effects that influence the SFP water temperature increase are decay heat and its distribution. Also, decay heat decreases with time, so for low decay heat, natural circulation force in the SFP becomes weak and the effect of heat loss to air for the water temperature increase will be relatively bigger than that for high decay heat. Therefore, in this study, the 3D computations with FLUENT 15.0 were done for four typical patterns of decay heat distribution and for three decay heat values (10, 5, and 1-MW). The computational results were compared to each other and evaluated. It was found that the effects of decay heat distribution were small on water temperature calculations, and the 1R model for SFP water was applicable to the prediction of SFP water temperature during the loss of all AC power supplies without consideration of the decay heat distribution.