Adherent activity on internal surfaces

The adherent radionuclide cesium activity observed on the TMI-2 internal surfaces is consistent with the formation of cesium silicate within the surface duplex oxide on the metal surfaces. Chemical and physical examinations (ES, SEM, XRD, AES, metallography, hardness, particle analyses, etc.) of debris and metallographic sections from internal reactor components removed from the Three Mile Island Unit-2 have been performed. The principal components removed were three leadscrews (H8, B8, E9), a small bottom end section of the H8 Leadscrew Support Tube (LST), and a resistance thermal detector thermowell. Examination of the leadscrews indicate that radionuclides and original core materials are associated with Loosely Adherent surface Deposits (LAD) and Adherent surface Deposits (AD). The LAD generally comprises material associated with or evaporated from the Reactor Coolant System (RCS) fluid whereas the reaction products between accident material and the leadscrew surface, and in-service corrosion film constitute the AD. The AD contains the principal gamma activity (up to 3 mCi Cs-137/cm2) associated with the leadscrew surfaces. An aggressive solution (HNO3-HF) was required to remove the cesium activity from the AD. Hence, surface characterization analyses were performed to elucidate the chemical nature of the AD. Results indicate there is a dense region with a cermet type structure of various elemental compositions at the LAD/AD interface. The surface analyses also indicate that silicon and cesium levels are spatially correlated, a fact pertinent to understanding the adherent cesium activity