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We performed measurements on the elongation of many different austentic stainless steel alloys irradiating at 100°C in a low flux channel of the High Flux Reactor at Petten varying the applied stress between zero and 300 MPa. We irradiated in total 98 samples in two irradiation rigs. Of these samples only 26 samples could be tested up to a dose of 2.1 dpa, and 13 samples up to a dose of 0.21 dpa. The steels tested are listed in Table I. In the second irradiation rig four TIG-welded samples and one EB-welded sample were irradiated. We found that the length of the samples increased up to an irradiation dose of 0.11 dpa and then either decreased or increased slightly depending on the magnitude of the applied stress. We attributed the increase in length to the volume change due to the formation of carbides and to the accommodation of carbides to the applied stress. The decrease of the length with irradiation time is attributed to the formation of brittle α-ferrite. The amount of α-ferrite formed increases with decreasing irradiation temperature and increases with decreasing applied stress. Eight samples broke during irradiation in 8 columns or stems in two rigs before the first elongation test at 0.11 or 0.21 dpa could be performed. Irradiation of 343 samples of the same materials in the last fifteen years at temperatures between 300 and 500°C did not cause fracture.

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This is a final report on the results of measurements of neutron irradiation creep which have been performed in non-instrumented creep rigs (Trieste) on many different stainless steel alloys during the last fifteen years in high flux positions in the High Flux Reactor (HFR) at Petten. The creep elongations were measured in hot cells during reactor shut down periods. A few investigations of the creep elongation were performed in fully instrumented creep rigs (Crisp). All the materials were irradiated in the as-received state, which was usually a type of solution-annealed state, after annealing at 400, or at 600, or at 800°C, and after 20% cold work. The irradiation temperature ranged from 300 to 500°C and the applied stresses were between 25 and 300 MPa. The variations in length found up to irradiation doses of about 5 dpa are mainly attributed to the formation of radiation-induced microstructural changes, which are connected either with increases or with decreases in the volume of the materials. We are advancing an interpretation of the data which is not without contradictions. We believe that among the many microstructural changes only the formation of carbides, which is connected with an increase in volume, and only the formation of α-ferrite, which is connected with a decrease in volume, are of importance. The α-ferrite phase is very brittle, decreasing the ductility of the stainless steels dramatically. An almost thermal equilibrium state of microstructure while irradiating with high energy particles is obtained after about 5 dpa, depending also on the irradiation temperature. The increase in length obtained for doses larger than 5 dpa is attributed mainly to irradiation creep. The normalized creep rates for all stainless steel materials are almost equal in HFR, ORR, and in EBR II. The creep rates increase linearly with stress and flux, and they are slightly dependent on the irradiation temperature (Qirr= 0.132 eV) for irradiation temperatures below about 450°C. For irradiation temperatures above 450°C a contribution of thermal vacancies to irradiation creep is noticed and the irradiation creep rates can no longer be ascribed to a simple irradiation creep relation.