FEP probabilities are assigned based on observations that may cover long periods of time, such as for geologic processes, or from laboratory testing and the extrapolation of such results to conditions that may exist in the distant future, such as for corrosion processes. In today’s final rule, the FEP probability threshold that must be considered in developing performance
assessments represents a policy judgment about how such events should be addressed in order to meet the regulatory challenge recognized by NAS, supported by technical reasoning about the nature of the site database for identifying and characterizing FEPs.