In this study, the SAMPSON code was modified to evaluate severe accidents in a spent fuel pool (SFP). Not only the SFP but also upper spaces of the SFP, walls of the reactor building, and the blowout panel were included. Air oxidation models obtained by the Zircaroy-4 cladding (ANL model) and the Zircaroy-2 cladding (JAEA model) were included in the modified SAMPSON code. Experiments done by Sandia National Laboratory using simulated fuel assemblies equivalent to those of an actual BWR plant were analyzed by the modified SAMPSON code to confirm the function of fuel temperature calculation in the event of loss of fuel cooling in the SFP. The rapid fuel rod temperature rise due to the Zr air oxidation reaction could be reasonably evaluated by the SAMPSON analysis for the radial propagation experiment. The effect of the oxidation reaction model was evaluated by the analysis of the SFP assuming no initial water level. There was almost no difference in the maximum temperature transient of the fuel rod surface between the ANL and JAEA models since the extent of the oxidation reaction was limited by the amount of oxygen supplied in the current analysis conditions. The analysis was conducted with different initial water levels which were no water, water level at bottom of active fuel, and water level at half of active fuel. The present analysis showed that the earliest temperature rise of the fuel rod surface occurred when there was no water in the SFP and natural circulation of air became possible.