Irradiation creep and density changes observed in MA957 pressurized tubes irradiated to doses of 40–110 dpa at 400–750 °C in FFTF

An irradiation creep and swelling study was performed on tubing constructed from the yttrium/titanium oxide dispersion strengthened (ODS) ferritic steel MA957. As a result of the reduction operations during manufacture, the grains in the tubing were highly elongated in the direction of the tubing longitudinal axis. Pressurized creep tubes were irradiated in the Fast Flux Test Facility (FFTF) to doses ranging from 40 dpa to 110 dpa at target temperatures ranging from 400 to 750 °C. ametral strains produced during irradiation exhibit primary (transient) creep strains that are dependent on stress and increase with irradiation temperature and are followed by a temperature-independent steady-state creep rate of ∼0.75 × 10−6 (MPa dpa)−1, a value similar to that of traditional tempered ferritic/martensitic steels. Contributions to primary creep strains may arise not only from classical thermal creep or irradiation creep considerations, but also may result from an irradiation-stimulated growth process whereby the highly elongated grain structure shrinks somewhat in the elongated direction, reducing the tubing aspect ratio to produce slightly fatter grains and thereby increasing the tube diameter. One manifestation of this process is a change in tube diameter that is not accompanied by a density change characteristic of either void swelling or precipitation-induced changes in lattice parameter. results provide the first demonstration that resistance to irradiation creep can be extended to higher temperatures by dispersoid addition, and most importantly, this resistance is maintained to high radiation damage levels at least for temperatures of 600 °C or less.