Fatigue Behavior of Type 316 Stainless Steel Irradiated in a Mixed Spectrum Fission Reactor Forming Helium

Abstract

In a tokamak reactor that operates in a cyclic mode, thermal stresses will result in fatigue in structural components, especially in the first wall and blanket. There has been limited work on fatigue in irradiated alloys, but none on irradiated materials containing irradiation-induced helium, which will be characteristic of fusion service.

Specimens of 20% cold-worked Type 316 stainless steel were irradiated in the High Flux Isotope Reactor, which produces atomic displacement damage as well as helium through a two-step neutron absorption reaction with nickel. The specimens were irradiated at 430°C to up to 15 dpa and 900 at. ppm helium. Following irradiation, specimens were tested in a vacuum at the irradiation temperature with total strain ranges from 0.30 to 2.0%.

The irradiated specimens exhibited a reduction in fatigue life of a factor of 3 to 10 compared to unirradiated material. An endurance limit was observed at a total strain range of 0.3%) for irradiated material. A fracture mechanism with surface morphology similar to cleavage, believed to be related to precipitation along slip bands, was observed in the irradiated specimens.

The endurance limit occurs at a sufficiently high strain that fusion machines built to existing designs would be able to operate with Type 316 stainless steel first walls at 430°C. Since the specimens have been irradiated to damage levels only as high as 15 dpa, the equivalent of ~1.5 MWyr/m², it can be safely inferred that wall exposure can be at least this high without fatigue failure.