Measurement of the Deformability of Austenitic Stainless Steels and Nickel-Base Alloys in Light Water Reactor Cores

Abstract

The deformability of austenitic stainless steels and nickel-base alloys was studied in an experimental program combining the influence of irradiation in a light water reactor (LWR) core environment and high stresses and strains. Tubular specimens filled with ceramic mandrels were inserted into fuel elements of both a BWR and a PWR where they were exposed to the neutron flux and coolant water of the core. The ceramic mandrels swelled under irradiation and applied high stresses to the cladding tube specimens; hence, in case of sensitive material, this led to intergranular stress corrosion cracking. The strain was varied by choosing different ceramic materials and by the axial position of the specimens in the reactor core. During the refueling shutdown of the reactor, the specimens were examined by integrity testing and diameter measurements. The experimental setup, irradiation conditions, examination methods, and deduced stresses will be described and the status of the experiments outlined. Many materials failed by brittle cracking under the conditions applied. In the first program phase only a low phosphorus and silicon stainless steel and an Inconel alloy 718 with a special heat treatment were found to be resistant. In the second phase, when other material charges were used to verify the first results, the good performance of alloy 718 was confirmed. High purity austenitic stainless steels, however, failed during Phase 2 at the same low strain level as commercial purity material. In the case of alloy X-750, it was found that the material surface condition had a significant influence on the resistance to stress corrosion cracking.