In-reactor stress relaxation of 3-point bending CW-316 samples: Experiments and modelling

3-point bending in-pile stress relaxation tests of cold-worked 316 stainless steel have been characterised in the mixed-spectrum OSIRIS reactor up to 7 dpa at 330 °C. Relaxation kinetics were determined based on out of pile interphase measurements. The experimental design allows to study initial prescribed stress levels from 100 MPa up to 600 MPa and to investigate transient (primary) and steady-state (secondary) stress relaxation. Relaxation kinetics were found to be unaffected by initial prescribed stress but to depend on the cold work level of the samples. No incubation dose was observed, even for samples loaded at low initial stresses. Experimental results were interpreted using FE modelling to take into account the non-uniform stress distribution existing in 3-point bending samples loaded in the elasto-plastic domain. The linearly stress-dependent creep law, consisting of two terms representing transient and steady-state creep regimes, allows for a very good description of experimental relaxation kinetics all over the investigated stress range. The primary creep amplitude parameter was found to be dependent on the cold work level, which is consistent with literature. A steady state creep coefficient of 0.96 10⁻⁶ MPa⁻¹.dpa⁻¹ consistent with the values reported from fast reactors was obtained independently on the cold work level. Also it was shown that the stress relaxation ratio can be directly determined based on deflection relaxation ratio despite the non-uniform stress state and residual stresses into the samples.