The Zr-2.5%Nb alloy used to manufacture the pressure tubes for the CANDU 600 power plant experiences continual corrosion when the reactor’s fuel channels are functioning normally. Analysis of the hydrogen accumulation circumstances, the creation of zirconium hydride platelets, evaluation of the platelets’ reorientation in the thermo–mechanical field at volumetric flaws, and determination of the hydrogen morphology are therefore significant goals of the worldwide research being conducted in this field. In this paper, pressure tube alloy samples with artificial V flaws were hydrided by the technique of electrolytic deposition of a hydrogen film layer, followed by a specific thermal treatment. Micrographs of the Zr-2.5%Nb alloy samples in the radial cross-section were taken to examine the hydride reorientation phenomenon that occurred when the samples were heated under mechanical stress. The complex stress field around a tip flaw promoted the formation of some circles from hydride filaments. To characterize the zirconium hydride morphology in the reorientation zone at the volumetric flaw tip, image analysis and processing techniques were applied. To better characterize the reorientation zone from the point of view of embrittlement and mechanical behavior, this paper proposes a novel metric for the morphology of hydrides located in the reorientation zone. This work’s findings are beneficial for the investigation of CANDU fuel channels’ structural integrity.

The precipitation-hardenable nickel-based superalloy Rene 41 exhibits remarkable mechanical characteristics and high corrosion resistance at high temperatures, properties that allow it to be used in high-end applications. This research paper presents findings on the influence of thermal shocks on its microstructure, hardness, and thermal diffusivity at temperatures between 700 and 1000 °C. Solar energy was used for cyclic thermal shock tests. The samples were characterized using microhardness measurements, optical microscopic analysis, scanning electron microscopy coupled with EDS elemental chemical analysis, X-ray diffraction, and flash thermal diffusivity measurements. Structural transformations and the variation of properties were observed with an increase in the number of shocks applied at the same temperature and with temperature variation for the same number of thermal shocks.

The pressure tubes of the CANDU 600 nuclear power plant at CNE Cernavoda Romania are made of Zr-2.5%Nb alloy, which is susceptible to hydrogen accumulation during normal operation. As part of the work, structural integrity analyses will be performed regarding the initiation of the Delayed Hydride Cracking (DHC) phenomenon at the complex flaws in the pressure tubes, which can be detected by the periodic inspections performed on the fuel channels. These flaws are described by the Canadian standard CAN/CSA N285.8 as a combination of a Bearing Pad Fretting Flaw (BPFF) with a Debris Fretting Flaw (DFF). The analysis of the mechanical stresses and strains field is obtained by finite element analysis (FEA) in the process zone of the flaws, that are located on the inner surface of the CANDU pressure tube. The work develops a method based on FEA, regarding the evaluation of the phenomenon of mechanical stress relaxation by creep in the process zone of flaws for the time interval between two periodic inspections of the CANDU fuel channels. This method allows obtaining the relaxation of mechanical stresses, by inserting the explicit function of the radial strain rate of the CANDU pressure tube (Zr-2.5%Nb alloy) into the algorithm for obtaining iterative numerical solutions in creep. The explicit function was obtained by the Multilayer Feedforward Neural Network (MFNN) method under the conditions of irradiation in-service specific to the CANDU fuel channels. The results of the work are used in the assessment of structural integrity by analysing the prevention of DHC initiation in the pressure tubes of a CANDU plant.