Study of the influence of zirconium hydrides on fatigue crack initiation at volumetric flaws on CANDU 600 reactor pressure tubes
Authors:   RATEN Institute for Nuclear Research Pitești 2 Romanian Academy of Technical Sciences, National University of Science and Technology POLITEHNICA Bucharest, University Center Piteșt, JINGA ALEXANDRA , ABRUDEAN MARIOARA ,   RATEN Institute for Nuclear Research Pitești 2 Romanian Academy of Technical Sciences, National University of Science and Technology POLITEHNICA Bucharest, University Center Piteșt, RADU VASILE ,   RATEN Institute for Nuclear Research Pitești 2 Romanian Academy of Technical Sciences, National University of Science and Technology POLITEHNICA Bucharest, University Center Piteșt, NITU ALEXANDRU ,   RATEN Institute for Nuclear Research Pitești 2 Romanian Academy of Technical Sciences, National University of Science and Technology POLITEHNICA Bucharest, University Center Piteșt, STOICA LIVIA ,   RATEN Institute for Nuclear Research Pitești 2 Romanian Academy of Technical Sciences, National University of Science and Technology POLITEHNICA Bucharest, University Center Piteșt
Published: January 2025

The Zr-2.5%Nb alloy, from which the pressure tubes of the CANDU 600 power plant at the Cernavodă NPP are made, is permanently subjected to a corrosion process during the normal operation of the fuel channels in the reactor. At the same time, the formation of specific flaws on the inner surface of the pressure tube under the conditions of prolonged irradiation and absorption of deuterium from the cooling agent, heavy water, leads to the initiation of precursor centers for cracking under mechanical load, called in the specialized literature type cracking Delayed Hydride Cracking (DHC). Among the phenomena that can contribute to the initiation and triggering of the DHC mechanism is the phenomenon of fatigue under mechanical load under the conditions of the formation of defects such as "bearing pad fretting flaw" or "debris fretting flaw". These defects are the most important defects revealed by periodic inspections of CANDU fuel channels, as they are the precursors to DHC propagating cracks. Therefore, an important objective in the studies carried out internationally is to analyze and model the consequences of the fatigue phenomenon of the pressure tube, which may contain this type of flaw, under the conditions of hydrogen accumulation, the formation of zirconium hydride platelets and their reorientation in the thermo-mechanical field. The paper presents part of the experimental studies regarding the performance of thermo mechanical treatments for the reorientation of zirconium hydride platelets under high mechanical stresses in the Zr-2.5%Nb alloy, the micro-structural analysis and the definition of the HCC metric for characterizing the morphology of the hydrides located in the tube wall of CANDU pressure, carrying out analyzes by the finite element method of the mechanical stress field developed at the top of the defects of the "bearing pad fretting flaw" or the complex defect "bearing pad fretting flaw with debris fretting flaw". Finally, based Correspondence address: alexandra.jinga@nuclear.ro 266 Jinga A. at al. / Study of the influence of zirconium hydrides on fatigue crack… on the data obtained from the mechanical fatigue tests on samples with the mentioned type defects, the fatigue lifetime was determined in the format of the corresponding Basquin type equation, „the amplitude of the alternating stress versus the number of cycles to failure". The results of the work are useful in the structural integrity analysis of the CANDU fuel channels.