This International Standard provides a methodology to estimate a reasonably maximal value of the number of fissions of a postulated criticality accident.The fission number estimate, associated with its postulated criticality accident, impacts the accident emergency planning and response because it is used for the estimation of radiation doses and of radioactive materials release.This International Standard does not provide a methodology and guidance to determine bounding accident scenarios.This International Standard does not cover criticality accident detection which is dealt with by ISO 7753.This International Standard does apply to nuclear facilities, plants, laboratories, storage, and transportation of fissile material (but not to nuclear power reactor cores) where a credible criticality accident may occur.