Radiation Induced Microstructural Evolution in Reactor Pressure Vessel Steels

The evolution of the fine scale microstructural features leading to irradiation embrittlement of reactor pressure vessel steels is described. Copper rich phases undergo accelerated precipitation from supersaturated solution due to radiation enhanced diffusion. In steels with significant trace quantities of copper the precipitates, characterized by high concentrations and small sizes, are the dominant embrittling feature. Precipitate concentrations, sizes, volume fractions and compositions are consistent with thermodynamic and kinetic models that rationalize the effects of a number of irradiation and metallurgical variables. Phosphide and carbonitride phases may also develop along with new manganese nickel rich precipitates, promoted by high nickel contents. These features may lead to severe embrittlement at high fluence even in low copper steels. While their detailed identity and characteristics are not known, defect cluster-solute complexes with a range of thermal stability are important both directly and indirectly; for example, in mediating flux and temperature effects. In conjunction with the application of state-of-the-art characterization methods, development of advanced modeling tools will be needed to address a number of outstanding issues.