Deformation behaviour of zirconium alloys under and after irradiation

Zirconium alloys used as fuel cladding or structural material in PWRs undergo permanent deformations due to the stresses applied and the irradiation conditions. These deformations can be partitioned in three additive terms, radiation induced and thermally activated creep and growth. The understanding of the behavior of these components under irradiation, and the ability to predict it accurately are of crucial importance for the design of reactors as well as their safe operation. In this study we will focus on the deformation sustained by the fuel cladding alloys under normal operating conditions, and also during power ramps, when the thermal expansion of the fuel pellet exerts high stresses on the cladding (Pellet Clad Interaction (PCI)). To obtain data under normal operating conditions, prepressurized tubular samples were introduced in the CEA experimental reactor SDLOE. However, the stress range explored is limited to 120 MPa due to safety regulations. In order to obtain data under PCI conditions, creep and relaxation tests were performed on irradiated material and the assumption was made that at the temperatures (≥ 350 C) and stresses considered, the radiation induced part of the creep deformation was negligible with respect to the thermally activated creep deformation. All these tests were then used for the development of semi empirical laws used for design codes and safety evaluations. All these tests were performed for the AFA-2G low tin stress relieved (SRA) Zircaloy-4, which was the reference alloy for fuel cladding application. We also performed creep tests at stresses representative of normal operating conditions on Zircaloy-4 in the recrystallized state (RXA) as well as on two new alloys industrialized by Framatome, M4, a quaternary material (Zr, Sn, Fe, V) and M5 a ternary material (Zr, Nb, O), both in the recrystallized state. These alloys have been developed within the scope of an ongoing program to develop superior cladding material, in order to increase the performance of PWR fuel to burnups beyond 65 GWd/tU and in more demanding environments (author) (ml)