During the accident at Three Mile Island Unit 2 (TMI-2), small quantities of fuel material were transported to plant piping systems outside the reactor coolant system (RCS). The likelihood of a criticality outside of the RCS during the cleanup of the plant systems was very small; however, given the consequence of any possible critical event in the TMI-2 systems, it was always necessary to ensure that all steps were taken to prevent criticality. Therefore, engineered controls were developed to ensure that decontamination of plant systems containing fuel material could be conducted in a manner that precluded criticality.