AbstractFission product release tests and control tests recently conducted at Oak Ridge National Laboratory have provided new experimental data that help characterize the mechanism of fission product tellurium release behavior under severe light water reactor accident conditions. Release of tellurium from the fuel rod segments has been found to be dependent on the rate and extent of cladding oxidation. Tellurium was observed to be significantly retained by metallic Zircaloy cladding at test temperatures up to 2000°C. The results indicate that the tellurium was bound by the Zircaloy cladding as zirconium telluride, but once the available metallic zirconium was oxidized by the steam, tellurium was released in favor of continued zirconium oxide formation. The collection behavior of the released tellurium indicated that it was probably released from the fuel rods as tellurides of tin, cesium, and rubidium rather than as elemental tellurium.