A FP chemistry database named ECUME including cesium and iodine was developed to accurately estimate the chemistry of FPs under an SA based on chemical reaction kinetics. The amount of cesium vapors during transportation in a reactor under an SA were estimated by applying ECUME to chemical reaction calculation. Large discrepancies were seen between the amounts of cesium vapors obtained using ECUME and using chemical equilibrium calculations applied to the existing SA analysis codes. Thus, the release amount of cesium may be over- or under-estimated using the existing SA analysis codes; chemical reaction kinetics should rather be used to more accurately evaluate public exposure to FPs.