Swelling and in-pile creep of neutron irradiated 15Cr15NiTi austenitic steels in the temperature range of 400 to 600 deg. C

A pressurized tube experiment was carried out in the Prototype Fast Reactor (PFR) ad Dounreay in order to determine swelling, stress-induced swelling and in-pile creep of different austenitic steels. The tubes were made out of different heats of the commercial German austenitic steel DIN 1.4970 and a number of model plain Fe-15Cr-15Ni stainless steels. Special attention was paid on the influence of minor alloying elements like Si, Ti, degree of Ti/C relation and others. The maximum doses achieved are 106 dpaNRT at 420 deg. C, 81 dpaNRT 500 deg. C and 61 dpaNRT at 600 deg. C. The hoop stresses of the pressurized tubes were 0, 60 and 120 MPa at all irradiation temperatures. The length and diameter changes of the pressurized capsules have been determined at up to four intermediate stages and after irradiation. Post irradiation examinations by immersion density measurements and transmission electron microscopy (TEM) are partially done. All alloys exhibited the highest swelling values at 420 deg. C and nearly no swelling at 600 deg.C. The measurements show the large effect of the minor alloying elements upon swelling and in-pile creep. The maximum swelling suppression is achieved for DIN 1.4970 through a high Si-content and an under stoichiometric Ti/C relation (under stabilization). This yields linear swelling of 1.9% after 106 dpaNRT at 420 deg. C. The formerly observed inter correlation between swelling and in-pile creep is confirmed up to 106 dpaNRT. It can be described by an equation consisting of a SIPA term (stress induced preferential absorption) and an inter correlation term similar to the I-creep proposed by Gittus. The estimates of the stress-induced swelling using the Soderberg theorem and the length measurements are compared with the immersion density measurements and results by TEM. The immersion density measurements agree rather good with length measurements. The stress-induced linear swelling can reach values of 0.8% at 100 dpaNRT and 120 MPa hoop stress. The measurements show an effect of the minor alloying elements on stress-induced swelling similar to the effect on stress-free swelling. (author)