Precipitate evolution in low-nickel austenitic stainless steels during neutron irradiation at very low dose rates

Neutron-induced microstructural evolution in response to long term irradiation at very low dose rates was studied for a Russian low-nickel austenitic stainless steel designated Х18Н9 that is analogous to AISI 304. The irradiated samples were obtained from an out-of-core support column for the pressure vessel of the BN-600 fast reactor with doses ranging from 1.7 to 20.5 dpa generated at 3.8 × 10−9 to 4.3 × 10−8 dpa/s. The irradiation temperatures were in a very narrow range of 370–375 °С. Microstructural observation showed that in addition to voids and dislocations, an unexpectedly high density of small G-phase precipitates was formed that are not usually observed at higher dpa rates in this temperature range. A similar behavior was observed in a Western stainless steel, namely AISI 304 stainless steel, irradiated at similar temperatures and somewhat higher dpa rates in the EBR-II fast reactor, indicating that irradiation at low dpa rates for many years can lead to a different precipitate microstructure and therefore different associated changes in matrix composition than are generated at higher dpa rates. The contribution of such radiation-induced precipitation to changes in electrical resistivity was measured in the Х18Н9 specimens and was shown to cause significant deviation from predictions based only on void swelling.