Pressure tubes in CANDU reactors are manufactured by extrusion and cold work from the Zr-2.5Nb alloy. Historically, the details of the pre-extrusion production of the tubes have changed since the first tubes were produced (1969) for Ontario Hydro's Pickering A station. The pressure tubes exhibit variations in elongation rate during service that may partially result from these changes. Irradiation growth is a component of the elongation. This paper describes the longitudinal irradiation growth of specimens prepared from pressure tubes with a range of iron contents and in which billets were extruded either in the as-forged condition or after β-quenching.

The irradiations are performed in the Osiris reactor at Saclay, France, in two inserts with nominal operating temperatures of 280°C (Trillium 2) and 310°C (Trillium 3). Results are reported for fast fluences up to 4.3 × 10²⁵ n/m², E > 1 MeV. Throughout the irradiation, the temperature of each tier of six specimens is recorded and this shows that the actual range of temperatures is 262 to 309°C. In agreement with previous studies, the temperature dependence of growth is negative, that is, an increase in temperature decreases the growth rate.

When this experimental variable is taken into account, there is still considerable variation among specimens from different pressure tubes. There is no effect of β-quenching. However, an increase in iron in the range 367 to 1060 ppm (by weight used exclusively in this paper) reduces the growth rate by about 2 × 10^-29 m²/n, E > 1 MeV at both 280 and 310°C. If this difference extends to high fluences, it accounts for about half of the variability in elongation rates observed in pressure tubes in-service.