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Annealing studies of irradiated and unirradiated pressure vessel steel have led to the conclusion that an active defect complex (ADC) is responsible for the enhanced embrittlement associated with steels containing significant levels of copper (>0.1%). These complexes, which already exist in the unirradiated steel, accumulate vacancies, produced either thermally or by neutron bombardment, and grow to such a size that the dislocation mobility is impaired and the yield stress increased. The number density of the sites appears to be a function of the copper level in the steel. Thus in low-copper steels the site spacing is too great to affect the yield stress.
The stable size of the complex seems to be an inverse function of temperature. Thus after preservice stress relief heat treatment the sites are too small to influence the mechanical properties of the vessel steel. At lower temperatures, however, exposure to a vacancy flux causes the sites to grow to their equilibrium size at a rate determined by both the availability of vacancies and the number density of the sites. Postirradiation annealing will cause the ADCs to shrink to a size that is stable at the annealing temperature. The ADCs also act as traps to positions and cause the line shape of the 511-keV annihilation peak to narrow. This effect appears to be related to the site size. Herein lies a prospect for remote interrogation of a reactor pressure vessel to assess the state of embrittlement resulting from this mechanism, both through life and after annealing.
pressurized water reactors, pressure vessels, low-alloy steels, radiation damage, embrittlement, annealing, recovery, nondestructive tests, hardness tests, positrons, Doppler broadening
Senior Lecturer, Royal Naval College, Greenwich,