Irradiation embrittlement of austenitic stainless steels in PWR vessel's internals

International audienceIn PWR's, internals made of austenitic stainless steels (Cold Worked (CW) 316 austenitic steels for bolts, 308 for welds and Solution Annealed (SA) 304 for baffle plates, former and core barrel) are subjected to irradiation embrittlement with doses that will reach up to 80 dpa locally after 40 years of operation at temperatures between 280DC and 380DC. The irradiation exposure alters the nanostructure and so consequently do the mechanical properties of stainless steels it increases the yield strength, decreases ductility and promotes plastic instabilities. Moreover, neutron irradiation at low temperature generally causes a reduction in fracture toughness of stainless steels. In addition, these changes seem to be the basis of an increased sensitivity to stress corrosion cracking said to be assisted by irradiation (IASCC). The tensile properties evolutions are commonly ascribed to the formation of a high density of nano-sized irradiation defects clusters mainly fine dislocation loops in stainless steels. Others defects (e.g. precipitates, bubbles and voids) may develop at higher doses. Homogeneous deformation that is observed for virgin material or at low doses is shifted to heterogeneous deformation at higher doses. Ductility loss is mainly ascribed to increased plastic strain localization. So far, much effort has been made to identify radiation effects on material microstructure and to determine experimentally the evolution of mechanical properties. Security and reliability of nuclear power plants have become a main issue and a better understanding of the physical mechanisms leading to the change in mechanical properties is now required. In addition it is also necessary to develop micromechanical constitutive equations to be able to carry out numerical simulations of core internals structures to improve safety and better study the possibility of extending the lifetime of nuclear power reactors. This talk will present some recent works developed to assess the irradiation embrittlement of austenitic stainless steels in PWR's environment.In the first part of the paper, recently proposed crystal plasticity physically-based constitutive equations for neutron-irradiated austenitic stainless steel that have been used in this study are described. Tensile curve predictions are presented and compare to experimental data.Irradiation leads to voids formation in grains of austenitic stainless steel. When these austenitic stainless steels are subjected to loadings, ductile fracture happens as the results of void growth and coalescence. The second part of this talk will be devoted to the description of a multiscale modelling of growth and coalescence of pre-existing voids leading to transgranular ductile fracture of irradiated austenitic stainless steels. Last part of the paper will be devoted to IASCC