Microstructural Aspects of Corrosion and Hydrogen Ingress in Zr-2.5Nb

Abstract

Zr-2.5Nb alloy pressure tubes for CANDU® reactors are nominally extruded at 815°C, cold-worked about 27%, and stress-relieved at 400°C for 24 h. The resulting structure consists of elongated α-Zr grains interspersed with a network of thin β-Zr filaments. Corrosion tests on unirradiated and preirradiated material have investigated the effects of microstructure and microchemistry on corrosion and hydrogen ingress. In two-phase (α-Zr+β-Zr) structures, the corrosion and hydrogen pickup increases with increasing volume fraction of β-Zr. Corrosion is highest for single β-phase material although hydrogen pickup reverts to a minimum value. Tests on alloys with low Nb concentration show that the optimum corrosion resistance occurs at a Nb content of about 0.1 wt% Nb. Thermal aging the metastable two-phase structure reduces corrosion and is consistent with a lower β-phase volume fraction and a lower concentration of Nb in the α-phase. Cold working the as-extruded two-phase structure up to about 80%, prior to stress relieving, reduces the out-reactor corrosion by about a factor of two. However, in-reactor, the benefits of cold work are negligible since there is a suppression of corrosion in irradiated Zr-2.5Nb that dominates all other effects. Irradiation results in an increase in dislocation density due to dislocation loop formation and also enhances the progression to an equilibrium α-phase composition manifested by the appearance of Nb-rich precipitates. Both of these effects of irradiation on microstructure are associated with improved corrosion properties based on tests of materials with controlled microstructures and microchemistry. Any thermally induced decomposition of the α-phase, resulting from the stress-relief heat-treatment, is slowed or even reversed by irradiation, depending on flux and temperature, and is therefore unlikely to have a significant effect on corrosion of irradiated materials. One of the most important factors leading to improved corrosion properties in Zr-2.5Nb pressure tubing seems to be the precipitation of β-Nb particles and the concomitant reduction of Nb in the matrix of the α-Zr grains during irradiation. Apart from any direct effects of cold-working or dislocation loop formation, it is likely that increased dislocation densities will also enhance Nb precipitation.

Irradiation Effects on ZR-2.5NB in Power Reactors

“…Typical grain structure of Zr-2.5Nb pressure in CAUDU reactors. The light-coloured α-phase grains are interspersed with dark-coloured β-phase filaments [9]. [8].…”

“…In as-fabricated Zr-2.5Nb pressure tubes, α-Zr is not in an equilibrium condition but is supersaturated with Nb. After irradiation with a fluence of 1 × 10 25 n·m −2 at 573 K, the Nb in the α phase precipitates as fine Nb rich particles 2-5 nm in diameter [9]. The size of the precipitates increases with fluence, increasing from <3 nm in diameter at 1 × 10 25 n·m −2 to between 5 and 10 nm diameter for fluences >5 × 10 25 n·m −2 at 573 K. Apart from transmission electron microscopy (TEM) measurements, lattice parameter measurements are used as a measure of the Nb content in the α.…”

Performance of Pressure Tubes in Candu Reactors

“…There is a transient period of about 2 to 3 years of operation for a CANDU pressure tube (corresponding with a fluence of about 2-3 × 10 25 n·m −2 in the central span of an average power pressure tube) while the material attains this equilibrium condition (Figure 23). The fluence for the transient period of the beta-phase state exceeds that for the dislocation density and is confirmed by test reactor data at the same temperature ( Figure 23) [15,30]. The steady-state condition for the beta-phase is then a function of fast neutron flux at a given temperature (Figure 24).…”

“…The CANDU data are from ex-service pressure tubes. Adapted from Pan et al [9] and Urbanic and Griffiths [30]. …”

“…In addition to the monitoring program, research programs are conducted to understand the factors that influence inreactor corrosion and deuterium ingress [28][29][30][31][32][33][34][35]. In many cases there are synergistic relationships between the variables making it difficult to determine the relative importance of any one variable in terms of its influence on pressure-tube corrosion and deuterium ingress behaviour from reactor data alone.…”