Irradiation growth in Zr-2.5Nb

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 X 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