While much of the CANDU reactor core can be refurbished for periods of extended operations (i.e., >35 Effective Full Power Years (EFPY)), certain key components at the core periphery, such as the calandria vessel, must remain fit for service for the total reactor lifetime. In the context of CANDU long-term operation (LTO), this could be for as long as 100 EFPY. Albeit relatively low for current reactor lifetimes, in the context of LTO, the calandria vessel is subjected to a significant neutron fluence. The combination of the CANDU-specific flux spectrum and the relatively low temperature of irradiation is a niche issue for CANDU reactors for which some key knowledge
gaps have been identified, including the effect of irradiation on mechanical properties, in particular the fracture toughness of welds, and the risk of irradiation assisted stress corrosion cracking (IASCC). To address the gaps, Type 304 stainless steel exposed to CANDU-relevant irradiation conditions was harvested from the decommissioned National Research Universal (NRU) Reactor. The steel was subjected to a relatively high thermal neutron flux for 60 years of operation at temperatures of <100oC. The mechanical properties and microstructure were characterized, and the results will be used to inform future R&D efforts investigating risks to LTO of the calandria
vessel. The results provide insight into the microstructural and mechanical changes and degradation of the Calandria vessel.