For handling of nuclear fuel during reprocessing or for design of spent-fuel storage and transportation, one needs to know the scale of maximum credible criticality accidents, i.e., the total fission number so as to know the radiological exposure of working personnel as well as the risk to the public in the event of an accident. Some simplified evaluation models for conservatively predicting the number of total fissions during an accident are derived theoretically using the one-point adiabatic reactivity balance model for the homogeneous and heterogeneous systems, respectively, which are frequently seen in nuclear fuel facilities. These simplified evaluation models are subsequently validated with the transient experiment data and actual accident data published to date from the world nuclear community. Some conventionally used simplified evaluation models of this kind are quoted and compared with the results to show the convenience of the current models, having almost no restrictions in the application for any kind of nuclear fuel, material composition, geometry, and dimension, and thus, ensuring adequate margins for predicting the total fission number at the time of a critsssicality accident.