Metallurgical Examination of Bore Samples from the Three Mile Island Unit 2 Reactor Core

Samples of the bores obtained from the melted core of the Three Mile Island Unit 2 (TMI-2) reactor were investigated as part of the TMI-2 accident evaluation program. The samples included fuel rod segments, control rod cladding, melted core rocks, and powder debris from various bores into the reactor core. The microstructure of the specimens was determined by examination and analysis of surface and cross section by means of electro-optical techniques (including energy-dispersive X-ray scanning electron microscopy and microprobe analysis). Gamma spectroscopy and a fission product release study were also performed on the specimens. The melted core rock specimens obtained from the G12 bores were mostly porous ceramic mixtures of uranium and zirconium oxides in the form of a fine eutectic of UO₂- and ZrO₂-rich phases and oxidized ferrous material derived from the stainless steel components. The rock specimens showed variations in porosity and ferrous content but a similar UO₂-ZrO₂ eutectic structure. Fission product analysis of the fuel segments and rocks indicated relatively low levels of activity due to fission products such as ^I37Cs, ¹⁰⁶Ru, ^I54Eu, and fuel irradiation products. No volatile ¹²⁹I was detected, but most other products displayed some retention in the melted core samples. According to whether eutectics were formed between Zr(O) and UO₂ or ZrO₂ and UO₂ (i.e., the oxygen potential in the core), reference to the phase diagrams suggests temperatures of 2173 to 2873 K (1900 to 2600°C) for substantial periods and even up to 3073 K (2800°C) (U0₂ melting point) for the completely melted core specimens. Agglomerate specimens indicate lower temperatures [∼1673 K (1400°C), the melting point of stainless steel] and shorter excursion times at the edge of the melt zone. The remaining fuel rod segments showed very little change, indicating that the severe overheating is localized to the central part of the reactor pile.