Metallurgical Reactions Involving Ag-ln-Cd Control Rod Assemblies

Three pieces of control rod and Zircaloy guide tube from the Three Mile Island Unit 2 reactor have been examined at Argonne National Laboratory. Microstructure and microchemistry of the materials have been characterized as a function of their elevations from the core bottom. Within a short distance between the 47- and 52-cm elevations, the microstructure varied from molten Ag-ln-Cd without stainless steel cladding to candling and cladding/Zircaloy metallurgical reactions, to an intact control rod in its Zircaloy guide tube that had transformed into a beta-phase structure. These microstructures provided semiquantitative markers of the temperatures experienced during the accident at this location.