Irradiation at PWR-relevant temperatures leads tomultiple material degradation modes, including secondphaseformation. It is known that formation ofirradiation-induced bcc-ferrite can occur within austenitegrains due to the tendency of nickel to segregate near thegrain boundaries or void surfaces. In addition to affectingmechanical properties, ferrite may also affect crackingand corrosion behavior in water coolant, especially if theferrite forms on grain boundaries. This work presents newhigh-fluence data derived from the BN-350 fast reactor toexamine the potential for ferrite formation on grainboundaries in PWRs. The very low (<300°C) inlettemperature of BN-350 allows examination at PWRrelevanttemperatures directly. When combined withseveral recent data sets at lower PWR-relevant dpa ratesfrom BOR-60, which has a slightly higher (320-330°C)inlet temperature, it appears that very large amounts offerrite can form on grain boundaries at temperatures anddose rates characteristic of PWR internals, allowing thepossibility that a new degradation mechanism may assertitself at beyond-life-extension conditions, moving frompreviously second-order importance to a first-orderconcern at higher exposure levels.
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