An overview of nuclear criticality safety performed to support TMI-2 [Three Mile Island Unit 2] defueling

In October 1985 GPU Nuclear Corporation began the fuel removal phase of the recovery from the March 28, 1979, accident at the Three Mile Island Unit 2 (TMI-2) nuclear power station. Almost 3 yr before the start of defueling, however, the planning activities for dealing with the criticality safety aspects of the core material had begun. Nuclear criticality safety has been one of the key safety issues during the recovery. As a result of the accident, the physical configuration of the core was unknown. The various fuel geometries that could exist during defueling activities and postulated hypothetical accidents needed to be considered. These circumstances, coupled with the recovery approach selected, presented unique and first of a kind criticality safety obstacles. To address these issues and to demonstrate that fuel removal could be accomplished, while at the same time maintaining criticality safety, numerous evaluations were performed by an array of organizations. This paper provides an overview of the criticality evaluations performed to support TMI-2 defueling. Furthermore, once defueling operations started, operational and tool design evolutions presented additional criticality safety concerns. An example of this is the work performed to establish limits on foreign materials (e.g., underborated liquids and solid material). Another example is the work recently completed for the plasma torch system, which will used to dismantle the lower core support assembly