TMI-2 instrument nozzle examinations at Argonne National Laboratory, February 1991--June 1993

Abstract

The accident at the Three Mile Island Unit 2 (TMI-2) reactor in March 1979 resulted in the relocation of approximately 19,000 kg of molten core material to the lower head of the reactor vessel. This material caused extensive damage to the instrument guide tubes and nozzles and was suspected of having caused significant metallurgical changes in the condition of the lower head itself. These changes and their effect on the margin-to-failure of the lower head became the focal point of an investigation co-sponsored by the United States Nuclear Regulatory Commission (NRC) and the Organization for Economic Co-operation and Development (OECD). The TMI-2 Vessel Investigation Project (VIP) was formed to determine the metallurgical state of the vessel at the lower head and to assess the margin-to-failure of the vessel under the conditions existing during the accident. This report was prepared under the auspices of the OECD/NEA Three Mile Island Vessel Investigation Project. Under the auspices of the VIP, specimens of the reactor vessel were removed in February 1990 by MPR Associates, Inc. In addition to these specimens, fourteen instrument nozzle segments and two segments of instrument guide tubes were retrieved for metallurgical evaluation. The purpose of this evaluation was to provide additional information on the thermal conditions on the lower head that would influence the margin-to-failure, and to provide insight into the progression of the accident scenario, specifically the movement of the molten fuel across the lower head.