Mossbauer spectroscopy of tin in unirradiated and neutron irradiated Zircaloys

Mosbauer spectroscopy of the 23.9 keV gamma-rays in Sn-119 nuclei was appli ed to study Zircaloy-2, Zircaloy-4, and other tin-bearing zirconium-based a lloys of interest to nuclear power technology. Zircaloys are extensively us ed in nuclear reactors as fuel cladding. In CANDU reactors, Zircaloys are a lso used as major structural components such as calandria tubes, and were u sed until the late 1970's as pressure tubes (now replaced by Zr-2.5Nb alloy ). Unirradiated specimens of these alloys, as well as radioactive specimens , both neutron-irradiated in high-flux test reactors and extracted from nuc lear power-reactor components after many years of service, were examined. T he obtained spectra consistently showed tin in substitutional solid solutio n in alpha-Zr, whereas no evidence was found of metallic Sn or intermetalli c Zr4Sn precipitates. In oxide scrapes removed from Zircaloy-2 pressure tub e of one of CANDU reactors, where the alloy was exposed for about 10 years to pressurized heavy water coolant at temperatures of similar to 280 degree s C, a considerable fraction of tin was found in the Sn(IV) state, in the f orm that coincides with the state of tin in stannic oxide, SnO2. The same f orm of tin was identified in filterable deposits in the primary heavy water coolant of CANDU reactors. For comparison, in Zircaloy heated in air, SnO2 was formed only at temperatures above 500 degrees C. (C) 1999 Elsevier Sci ence B.V. All rights reserved.