Irradiation enhanced precipitation in stainless steel

Abstract

Experimental findings of an irradiation enhancement of precipitation in stainless steel under observation in the High Voltage Electron Microscope (HVEM) are presented. The results for the case of electron irradiation are compared with previous studies of precipitation under neutron irradiation in thermal and fast reactor conditions. Under low neutron fluxes precipitation is influenced by the production of vacancy clusters which act as nuclei. However, growth of these nuclei is limited unless the temperature is sufficiently high for thermal diffusion to be significant. Under fast reactor conditions the high neutron fluxes permit growth to continue at irradiation temperatures ≳450°c, under the irradiation enhanced diffusion. The observations made in the HVEM show that the production of vacancy clusters is not necessary for precipitation to occur under irradiation. Growth takes place rapidly under the defect supersaturations provided by the electron beam. Extension of the theory of irradiation enhanced diffusion to the case of future fast reactor systems, where neutron fluxes are substantially higher than in the Dounreay Fast Reactor (DFR), indicates that in the former the cladding will be exposed to higher effective temperatures with regard to precipitation but for shorter periods. This may not necessarily be detrimental to the mechanical properties of the clad.