Fission Product Partitioning in Core Materials

The partitioning and release of fission products from fuel materials in the Three Mile Island Unit 2 (TMI-2) reactor pressure vessel (RPV) are summarized, and the chemistry that resulted in the observed behavior is discussed. As part of the TMI-2 core examination program, samples were examined from all regions within the RPV, from leadscrews in the upper plenum to previously molten material from the lower plenum of the RPV. The results of these examinations indicate significant variations in fission product behavior that were generally consistent with the volatility and chemical behavior of the expected fission product species. Low-volatility species (e.g., ¹⁴⁴Ce) were retained almost entirely in the fuel material matrix, whereas unoxidized species such as ¹²⁵Sb were found with the metallic structural materials. Most of the high-volatility species (¹³⁷Cs and ¹²⁹I) were released from the previously molten fuel; however, the releases were less than expected. These fission products were retained in previously molten fuel that contained concentrations of structural materials at the grain boundaries.