Pressure tube elongation in CANDU (CANada Deuterium Uranium) reactors occurs mainly as a result of irradiation growth and may limit the useful lifetime of the pressure tubes. The object of this investigation was to examine the effects of microstructural variation on the irradiation growth rate as part of a pressure tube development program. Three modifications of Zr-2.5Nb material have been irradiated at 553 K to high fast fluences of 3 × 10²⁵ n/m² in the Dido reactor at the United Kingdom Atomic Energy Authority Harwell, and the irradiation growth was measured at preselected fluences up to the highest fluence.

The particular microstructural parameters examined in these experiments were dislocation density and grain morphology. The three materials tested were: (1) the usual as-received pressure tube which has fine, elongated grains and a high dislocation density; (2) stress-relieved material with essentially the same grain shape as (1) but with a lower dislocation density; and (3) annealed and cold-worked material with larger, more-equiaxed grains and a high dislocation density.

The growth measurements indicate that both the initial transients and the high fluence behaviors of the three materials are qualitatively different with the high dislocation density materials showing increasing growth rates with increasing fluence and the stress-relieved material showing the opposite effect. Specimen-to-specimen variations in growth behavior are correlated with position of the specimens in the irradiation rig. Variations could be attributable to temperature effects. There was no good indication in any of the materials that a steady-state growth rate had been achieved at the highest fluences.

These results are discussed in terms of current models of irradiation growth in zirconium alloys. They are also compared with irradiation growth rates on other zirconium-base alloys.