Experimental and simulation results of Expansion-Due-to-Compression tests with different strain biaxiality ratios on Zircaloy-4 cladding for RIA situation
Résumé
This paper presents an assessment of the mechanical behavior of nuclear fuel cladding in a Reactivity-Initiated Accident (RIA). Experimental data from past experiments in research reactor programs simulating RIA's show that the cladding undergoes a multiaxial loading state characterized by both hoop strain (et949;et952;et952;) and axial strain (et949;zz). The strain biaxiality ratio et949;_zz/et949;_et952;et952; extends between plane-strain (no axial strain in the cladding tube) and equal-biaxial tension (equal tensile strains in the hoop and axial directions). In this study, Expansion Due to Compression (EDC) tests were conducted in order to reproduce the mechanical conditions during the low temperature phase of the power transient in the reactor, also called the Pellet Cladding Mechanical Interaction (PCMI) phase. The tests are intended to reproduce, as closely as possible, the loading mode and macroscopic failure aspects that have been observed during integral RIA tests performed on fuel rods. Different kinds of EDC tests with varying stress biaxiality conditions have been performed in order to study the evolution of the circumferential strain at failure as a function of the strain biaxiality. Three EDC configurations are tested with different values of biaxiality. The first configuration consists of axially compressing, between two pistons, a polymer pellet inserted into a cladding sample with free ends (free-end EDC test). The diametrical expansion of the pellet is imposed on the cladding. In this configuration, the sample contracts in the axial direction. In the second configuration, the ends of the sample are fixed to prevent axial contraction of the cladding tube (fixed-end EDC test), thereby producing plane strain conditions in the sample. The final configuration will be to impose a tensile load on the cladding while simultaneously compressing the pellet with the pistons, thereby producing both axial and circumferential tensile strains in the sample. For each method, the strain field in the sample is measured using a stereo-correlation image analysis technique. The results of the experiments are discussed for each biaxiality ratio and compared with Finite Element Method (FEM) modeling using CAST3M.