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The influence of irradiation temperature and displacement rate on point defect cluster formation has been investigated using a kinetic model. The model considers both interstitial and vacancy clustering; the former are treated as Frank loops and the latter as microvoids. Clusters can form directly in displacement cascades or by diffusive encounters between free point defects. The calculations indicate that the assumption of steady state point defect concentrations is not valid for temperatures below about 200°C. At lower temperatures, the time required to reach steady state can approach an operating reactor's lifetime. At higher temperatures, the point defect transient could influence the interpretation of irradiation experiments conducted at accelerated damage rates. The hardening due to point defect clusters was calculated using a simple dislocation barrier model. Both cluster types can give rise to significant hardening; their relative importance is a function of irradiation temperature and displacement rate. The results suggest that the apparently accelerated embrittlement reported for the pressure vessel of the High Flux Isotope Reactor is not the result of a low displacement rate.
pressure vessel steels, hardening, embrittlement, defect clusters, displacement rate, irradiation temperature, theoretical models
Research Staff, Oak Ridge National Laboratory, Oak Ridge, TN