Iodine-Induced Stress Corrosion Cracking of Zircaloy-4: Identification of Critical Parameters Involved in Intergranular to Transgranular Crack Propagation

摘要

During power transient conditions in nuclear reactors, uranium oxide pellets expand and crack due to the increase in temperature and their poor thermal conductivity. Moreover, the cladding undergoes creep because of the external pressure, and its diameter shortens. These antagonistic phenomena lead to the establishment of a contact between the pellet and the cladding, called the pellet-cladding interaction. The synergistic effect of the hoop tensile stress and strain imposed on the cladding by fuel thermal expansion and corrosion by iodine released from the UO_2 fuel as a fission product at the same time can lead to Iodine-induced Stress Corrosion Cracking (I-SCC) of the Zircaloy-4 cladding. I-SCC failures of zirconium alloys are usually described in three steps: initiation of cracks, intergranular subcritical propagation, and critical propagation with a brittle transgranular propagation mode [1]. Transgranular propagation occurs as soon as the stress intensity factor overshoots a threshold value K_(I, SCC). It is the critical step and leads to the final ductile failure of the cladding. Transgranular cracks propagate by cleavage-like fracture on basal planes of the hexagonal lattice and fluting; it is the result of a competition between a plastic accommodation of the applied strain and the brittle fracture of basal planes by iodine assisted cleavage. Several environmental, mechanical and microstructural parameters can affect each one of the propagation stages. The specimen texture, describing the variation in preferential orientation of the c-axis, appears to be one of the main controlling parameters for transgranular propagation in unirradiated zirconium alloys [1]. Strain-hardening has also been reported as a factor enhancing transgranular propagation of I-SCC cracks [2]. However, strain-hardening includes several parameters such as local internal stresses, grain reorientation, twinning or activation of different slip systems. Each one of those parameters may enhance transgranular cracking, but synergistic effects and critical parameters have not been identified yet. In that context, the purpose of this study is to progress in the identification of critical factors enhancing transgranular propagation of cracks. A strain-hardening pre-treatment is applied on Zircaloy-4 samples, using various elongations and strain rates. Resulting microstructures are characterized by transmission electron microscopy (TEM) with attention paid to respective activation of twinning and slip modes. Internal stresses are evaluated. Pre-strained or as-received tensile Zircaloy-4 specimens are tested under constant load, in presence of iodine methanol solution at room temperature. Fractographic observations carried by scanning electron microscopy (SEM) and TEM observations of grains passed by cracks (intergranular propagation) or crossed by cracks (transgranular propagation) help discussing results.