Results of examinations of pressure vessel samples and instrument nozzles from the TMI-2 lower head

Fifteen prism-shaped steel samples were removed from the lower head of the damaged TMI-2 reactor pressure vessel to assess the effects of approximately 19 metric tons of molten core debris that had relocated there during the 1979 loss-of-coolant accident. Metallographic examinations of the samples revealed that inside surface temperatures of 800 to 1,100 degree C were attained during the accident in an elliptical shaped ''hot spot'' ∼1 x 0.7 m. Tensile, creep, and Charpy V-notch specimens were also cut from the samples to assess the mechanical properties of the lower head material at temperatures up to the peak accident temperature. These properties were used in a margin to failure analysis of the lower head. Examinations of instrument nozzles removed from the lower head region assisted in defining the relocation scenario of the molten core debris and showed that the lower head was largely protected from catastrophic failure by a solidified layer around the molten core debris that acted as a partial thermal insulator