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CANDU Zr-2.5 Nb pressure tubes are susceptible to a cracking mechanism known as Delayed Hydride Cracking (DHC), which is a repetitive process that involves hydrogen diffusion, hydride precipitation and fracture at a crack tip. As defense-in-depth, when DHC is postulated to have initiated from a flaw, it is required to demonstrate that the crack can be detected by the leak monitoring system and the reactor safely shut down before the crack reaches the critical length for pressure tube rupture. DHC growth rates (DHCR) in the axial direction of the tube are required for such leak-before-break assessment. In this test program, the effect of hydrogen isotope and its concentration on DHCR in an unirradiated Zr-2.5 Nb pressure tube is studied. Pressure tube sections were hydrided or deuterided to different concentrations (nominal concentrations of 60, 100 and 190 ppm by weight). For the deuterided tube sections, they contained about 10 ppm of hydrogen from the manufacturing process. The DHC growth rate tests were performed on fatigue pre-cracked curved compact tension specimens, machined from the hydrided or deuterided tube sections, in several stepper-motor controlled load frames with cracking being monitored by direct current potential drop and acoustic emission techniques. DHCR at three test temperatures (270°C, 240°C and 200°C) were obtained from each specimen with the test temperatures approached from a peak temperature of 330°C. Some specimens were tested with a peak temperature of either 370°C or 300°C. The two main conclusions from the study are: (1) DHCR are affected by the hydrogen in solution at the test temperature and not by the amount of bulk hydrides present. The hydrogen in solution at a given test temperature depends on the hydrogen concentration of the specimen, as well as the thermal history (peak temperature in the initial thermal cycle and the test temperature) as a result of the hysteresis of Terminal Solid Solubility between hydride dissolution during heating and precipitation during cooling. (2) The DHC growth rates of the hydrided material are higher than those of the deuterided material because of the higher diffusion rate of hydrogen than deuterium. The isotope effect of hydrogen on DHC growth rates depends on the test temperature, with no apparent effect at 200°C and about 37% difference at 270°C which is slightly below the factor of √2 expected from the mass law of diffusion. The observed temperature dependence could be due to the presence of about 10 ppm hydrogen in the deuterided specimens, which dominates the DHC process at 200°C but insufficient to have a large effect at 270°C. The implication of the observed isotope and concentration effect of hydrogen on DHC growth rates on leak-fore-break assessment of flaws in pressure tubes is discussed.