Assessment of Delayed Hydride Cracking in Candu Pressure Tube Using the Process Zone With Creep Equation From Artificial Neural Network Modelling

Abstract

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.