DHC BEHAVIOUR OF IRRADIATED ZR-2.5NB PRESSURE TUBES UPTO 365°C

摘要

Under certain conditions, zirconium alloy pressure tubes in CANDU? reactors can be susceptible to amechanism of cracking called Delayed Hydride Cracking (DHC). The rate of initial crack penetration and propagation is typically low, and permits sufficient time for leak detection before an unstable crack length is obtained, termed Leak-Before-Break. Since the DHC velocity is temperature dependent, it is necessary to quantify the DHC velocity over the range of operating temperatures in order to confirm that the margins against unstable fracture are adequate to ensure Leak-Before-Break. The objective of the present work is to establish the DHC temperature dependence in irradiated material to a higher temperature than has been done previously. Theresults from an ex-service (irradiated) cold-worked Zr-2.5Nb pressure tube doped to a hydrogen isotope concentration of 153 wt ppm shows that between 150 and 325°C the DHC velocity follows an Arrhenius relationship. At temperatures higher than 325°C, the velocity deviates from the Arrhenius relationship, proceeding at much lower velocities until an upper temperature limit for DHC is achieved at about 365°C. Thephysical reason for the observed cessation temperature for DHC in this material, containing only 153 wt ppmhydrogen isotope, is thought to be due to insufficient hydrogen, making hydride nucleation at the crack tip impossible. It is expected that the strength-based DHC cessation temperature in irradiated material containing sufficient hydrogen would be higher.