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International audienceThis paper presents a unified phenomenological model to describe the anisotropic viscoplastic mechanical behavior of cold-worked stress relieved (CWSR) Zircaloy-4 fuel claddings submitted to reactivity initiated accident (RIA) loading conditions. The model relies on a multiplicative viscoplastic formulation and reproduces strain hardening, strain rate sensitivity and plastic anisotropy of the material. It includes temperature, fluence and irradiation conditions dependences within RIA typical ranges. Model parameters have been tuned using axial tensile, hoop tensile and closed-end internal pressurization tests results essentially obtained from the PROMETRA program, dedicated to the study of zirconium alloys under RIA loading conditions. Once calibrated, the model provides a reliable description of the mechanical behavior of the fresh and irradiated (fluence up to View the MathML source or burnup up to 64 GWd/tU) material within large temperature (from 20 °C up to 1100 °C) and strain rate ranges (from View the MathML source up to View the MathML source), representative of the RIA spectrum. Finally, the model is used for the finite element analysis of the hoop tensile tests performed within the PROMETRA program

: Tensile tests on axisymmetrically notched specimens were performed at various temperatures in the ductile-brittle transition region to study the initiation of fracture. Fracture toughness tests on CT specimens were also carried out in the same region to investigate the critical values of stable crack growth occurring before unstable cleavage fracture. Probabilistic modelling is made by coupling the Beremin model for cleavage crack initiation and a mode1 derived from the Gurson potential. In an elementary volume the probability for fracture depends on the probability of both modes of failure. In a larger volume the probability to fracture is expressed in terms of elementary probabilities of fracture through the weakest link theoly. Modelling is applied to axisymmetrically notched bars and to CT specimens. The analysis of results shows that the ductile-brittle transition of femtic steels can be interpreted quantitatively as a competition between both types of damage.