Microstructure degradation of austenitic stainless steels after 45 years of operation as VVER-440 reactor internals

Abstract

Specimens of reactor vessel internals (RVI) made of 18Cr–10Ni–Ti steel from the decommissioned VVER-440 unit after 45 years of operation were investigated. The aim was to assess the structure degradation degree responsible for the properties changes of RVI components. Radiation-induced structural elements of RVI baffle, former, and flux thimble specimens with the damaging dose 7.9–43.0 dpa and irradiation temperature 280–315 °C were studied using TEM, SEM, and APT methods. TEM-studies showed the negligible swelling under these conditions with a slight increase of radiation swelling due to the formation of the low-density large void population in case of higher temperature irradiation. APT-studies revealed the formation of the high-density Ni–Si-rich clusters (presumably G- and γ′- phase precursors) and an increase of its volume fraction with the temperature increase. The Orowan equation was used to assess the hardening contribution of different radiation-induced structural elements. The observed yield strength increase is insignificant under the studied conditions.