TMI-2 Vessel Investigation Project (VIP) Metallurgical Program

This report summarizes the work performed by Argonne National Laboratory (ANL) on the Three Mile Island Unit 2 (TMI-2) Vessel Investigation Project (VIP) Metallurgical Program during the nine-month period from October 1989 through June 1990. During the reporting period, a series of heat treatment experiments on archive material from the lower head of the Midland nuclear reactor has been completed, the resulting microstructures have been examined, and hardness values have been determined. The results have been compared with the predictions of published continuous-cooling transformation diagrams for A533 Grade B steel. Round-robin microstructural characterizations and mechanical tests on the archive material have also been completed by the participating Organization for Economic Co-operation and Development (OECD) partner laboratories. Decontamination of samples from the TMI-2 lower head is underway at ANL; the procedure being utilized is described in this report. Detailed microstructural and scanning electron microscope examinations of Specimen E-6 have been carried out in an attempt to determine the extent and cause of cracking observed in the clad surface. The results of microstructural observations, hardness measurements, and tensile tests indicate that the base metal in the vicinity of the crack attained a maximum temperature of ∼1000 to 1100 degree C during the accident. The molten fuel apparently did not penetrate into the cracks and interact with the base metal. It was tentatively concluded that the cracking of the cladding in Sample E-6 probably occurred during the early stages of cool-down after the accident, when the still-hot cladding layer was placed into tension because of thermal contraction. 7 refs., 52 figs., 17 tabs