A Type 348 stainless steel in-pile tube irradiated to a fluence of 3 × 10²² neutrons/cm² (E > 1 MeV), 57 displacements per atom, was destructively examined. The service had resulted in a maximum total creep of 1.8 percent at the high fluence. The metal temperature ranged between 623 and 652 K, hence the thermal creep portion of the total was negligible. Total creep was greater than had been anticipated from creep data for austenitic stainless steeis irradiated in other reactors.

The objectives of the destructive examination were to determine the service-induced changes of mechanical and physical properties and to assess the possibility of adverse effects of both these changes and the greater total creep on prospective service life of other tubes.

Measured bowing (0.51 mm) was correlated with a structural model. Postirradiation measurements included immersion density, fracture toughness, tensile strength and ductility, and creep-rupture strength. A reduction in fracture toughness owing to irradiation creep was apparent.