Development of Kinetic Models for the Long-Term Corrosion Behavior of Candidate Alloys for the Canadian SCWR

Abstract

Corrosion behavior of Inconel 625 and Incoloy 800H, two of the candidate fuel cladding materials for Canadian supercritical water-cooled reactor (SCWR) designs, was evaluated by exposing the metals to supercritical water (SCW) in the University of New Brunswick’s flow loop. A series of experiments were conducted over a range of temperatures between 370 °C and 600 °C, and the corrosion rates were evaluated as the weight change of the materials over the exposure time (typical experiments measured the weight change at intervals of 100, 250, and 500 h, with some longer-term exposures included). Scanning electron microscopy (SEM) was used to examine and quantify the oxide films formed during exposure and the corrosion mechanisms occurring on the candidate metals. Data from in-house experiments were used to create an empirical kinetic equation for each material that was then compared to literature values of weight change. Dissolved oxygen concentrations varied between experimental sets, but for simplicity were ignored since the effect of dissolved oxygen has been demonstrated to be a minor secondary effect. Activation energies for the alloys were determined with Inconel 625 and Incoloy 800 H showing a distinct difference between the low-temperature electrochemical corrosion (EC) mechanism and direct high-temperature chemical oxidation (CO). The results were modeled using these separate effects showing dependence on the bulk density and dielectric constant of the supercritical water through the hydrogen ion concentration.

On-line monitoring of oxide formation and dissolution on alloy 800H in supercritical water

“…Alloy 800H has been identified as a candidate alloy for the fuel cladding and wire-wrapped spacers in the Canadian SCWR concept [16][17][18]. In Fe-Ni-Cr alloys, outward diffusion of iron and nickel and reaction with oxygen leads to the formation of oxides such as Fe 3 O 4 , Fe 2 O 3 , FeO, and NiO, depending on the oxygen concentration, temperature and exposure time of SCW [19].…”