Modelling cladding response to changing conditions

The cladding of the nuclear fuel is subjected to varying conditions during fuel reactor life. Conventionally the effect of the changing conditions to cladding creep are modelled using strain hardening law: using creep curves obtained from single stress experiments and assuming the strain is invariant when translating between curves representing different external conditions. While even the original works showed that the strain hardening law applies to zirconium alloys in only a limited set of conditions, it has been applied quite universally in fuel behaviour codes. The situations where strain hardening law fails are the load drop and the reversal of load direction; as a consequence modelling both creep and stress relaxation with the same model has proven to be problematic. In our paper we show that the load drops and reversals can be modelled by taking cladding viscoelastic behaviour into account. Viscoelastic contribution to the deformation of metals is usually considered small enough to be ignored, and in many applications it merely contributes to the primary part of the creep curve. With nuclear fuel cladding the high temperature and irradiation as well as the need to analyse the variable load all emphasise the need to also inspect the viscoelasticity of the cladding. A simple model is constructed using tools of linear viscoelasticity. We show it can explain the observed creep behaviour from variable stress tests and improve the description of stress relaxation tests without resorting to dedicated models for the two different situations. The model development and the investigation of individual cases have been originally published in References [1-3].