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Indentation hardness measurements as a function of annealing were made on broken halves of Charpy impact surveillance samples. The samples had been irradiated in commercial power reactors to a neutron fluence of approximately 1 × 1018 neutrons per cm2, E>1 MeV, at a temperature of about 300°C(570°F). Results are reported for the weld metal, which showed greater radiation hardening than the base plate or heat-affected zone material. Isochronal and isothermal anneals were conducted on the irradiated surveillance samples and on unirradiated control samples. No hardness changes upon annealing occurred for the control samples. The recovery in hardness for the irradiated samples took place mostly between 400 and 500°C. Based on the Meechan-Brinkman method of analysis, the activation energy for annealing was found to be 0.60 ± 0.06 eV. According to computer simulation calculations of Beeler, the activation energy for migration of vacancies in alpha iron is about 0.67 eV. Therefore, the results of this preliminary study appear to be consistent with a mechanism of annealing of radiation damage in pressure vessel steels based on the migration of radiation-produced lattice vacancies.
annealing kinetics, indentation hardness, pressure-vessel steels, radiation embrittlement, radiation hardening, surveillance samples
Nuclear Engineer, Boston Edison Company, Boston, MA
Senior Metallurgist, Ames Laboratory USDOE, and Professor, Iowa State University, Ames, IA