Abstract
Supercritical Water-Cooled Reactor (SCWR) is one of the Generation-IV nuclear reactor designs which operates above the thermodynamic critical point of the water (374℃ and 22.1MPa), and exhibits a higher thermal efficiency and a simpler system configuration with respect to a conventional light water reactor (LWR). However, the water radiolysis effect in the core of SCWR is much severer than that in LWR because the supercritical water could dissolve the non-polar gas, such as oxygen, without the solubility limit, much higher temperatures and higher power density. This phenomenon makes the environment in SCWR much corrosive so that evaluating the feasibility of materials is a significant issue for developing SCWR. Ni-based superalloy Inconel 625 was exposed in supercritical water environment with 150 ppb, 1 ppm and 8.3 ppm dissolved oxygen [DO] at 700℃ and 24.8MPa for various periods of time up to 1000h. Before 600h, the mass gains (w) in the samples as a function of test duration (t) could be fitted by an equation of w^2.095 = 2.60 × 10^-5t approximately followed the parabolic law. In addition, oxides with a double-layer structure were observed. The outer layer of the scales was constructed by spinels Ni(Cr, Fe)2O4 and the compact inner layer was Cr2O3. Nevertheless, the thickness of outer layer in 150ppb low [DO] was smaller than in 8.3ppm high [DO] environment but the thickness of inner layer was in the same order, indicating that the Ni-containing oxides, such as NiO and Ni(Cr, Fe)2O4 were stable in high [DO] and Cr-containing oxides were favorable in low [DO]. After oxidation, large amount of γ” precipitates were formed in the matrix of Inconel 625 alloy. Extending the exposure time to 1000 h, the density of γ” precipitates decreased and the γ” phase was replaced by the δ phase precipitate.