Study of Microstructure and Property Changes in Irradiated SS316 Wrapper of Fast Breeder Test Reactor

This paper presents the results of a study on mechanical properties and microstructure in a SS316 wrapper irradiated in a 40MWt/13MWe fast breeder test reactor at Kalpakkam, India. Transmission electron microscopy (TEM) examination and mechanical property evaluation were carried out on the hexagonal wrapper subjected to different displacement damages up to a maximum of 83 dpa at an operating temperature of about 673 K. The steel irradiated to 83 dpa showed an increase in the yield strength and ultimate tensile strength, with a reduction in uniform elongation to about 8 %–10 % from about 20 % in the unirradiated material. Density measurements on the specimen from different portions of the irradiated wrapper showed a peak volumetric swelling of about 3.5 % at a damage of 83 dpa. TEM studies showed extensive void formation at 40 dpa and beyond in addition to extensive precipitation and formation of dislocation loops. The void density and size showed a progressive increase with displacement damage. The precipitates were identified to be of nickel and silicon enriched M₆C type of η phase, while radiation induced G phase was also observed at 83 dpa. The increase in strength and reduction in ductility with increase in dpa is attributed to irradiation hardening, which is supported by the increase in defect density as observed from the dislocation substructure and increase in the void density and size. The paper would discuss the degradation in mechanical property in terms of the microstructural changes.