Current CANDU² reactors use Zr-2.5Nb pressure tubes that are extruded at 1088 K, cold-drawn 27%, and autoclaved at 673 K for 24 h. This results in a metastable, two-phase microstructure consisting of elongated α-Zr grains surrounded by a network of β-Zr filaments. To develop a mathematical model of corrosion and deuterium ingress in pressure tubes, we have considered the impact of variables including: fast neutron flux, temperature, and the asfabricated microstructure and its evolution during irradiation.

Small specimens of Zr-2.5Nb are being exposed under CANDU water chemistry conditions in the Halden Boiling Water Reactor.³ The experiments involve fast neutron fluxes (E ≥ 1.05 MeV) of 0, 1.7, and 4.5 × 10¹⁷ n ∙ m^-2 ∙ s^-1, and temperatures of 523 and 598 K. Specimens have been prepared from pressure tube materials representative of all current CANDU reactors, materials subject to thermal decomposition of the β-Zr phase, and tubes extruded over a range of conditions.

Results from the first three years of the Halden test program are summarized. At both 523 and 598 K, tubes made of β-quenched material exhibit lower oxidation rates than those made from non-β-quenched materials. In short-term out-of-flux exposures at 523 K, three non-β quenched tubes appear to show linear oxidation kinetics. Similar behavior is not observed in tests conducted out-of-flux at 598 K, or in-flux at either temperature. At 598 K, β-quenched tubes exhibit significantly lower deuterium pickup rates than non-β-quenched tubes. When tested at 598 K, thermally aged specimens show declining oxidation and deuterium pickup rates with increasing β-Zr phase decomposition. At 523 K, the impact of thermal aging was less significant. Preliminary results from an “extrusion variable test” suggest that tubes fabricated according to the current CANDU specification show the best corrosion resistance.