Abstract
Supercritical water-cooled reactor (SCWR) is one of the various fourth-generation nuclear reactors. The temperature of its operating environment must exceed 374 ℃ and the pressure is higher than 22.1 MPa which was above the critical point of water. Compared with the light-water reactor, SCWR has some advantages such like its design for safe improvement, reduce costs and so on. In addition, supercritical water reactor operates at very high temperature, according to this, it can provide a very high thermal efficiency, about 45% (nowadays, the light water reactor thermal efficiency is about 33%). Inconel 625 Ni-base superalloy was set in the environment of supercritical water which dissolved oxygen 8.3ppm and 600℃. According to experimental results, suggesting that pitting was related to the original (Nb, Ti)C precipitates phase. And the change in mass per unit area of 300,600 and 1000 hours showed a trend line, w2.21 = 1.4 × 10-5 t, consistent with the parabolic law. The result of oxide layer analysis experiment result showed a continuous and compact oxide layer structure which was composed of spinel Ni (Cr, Fe)2O4 in the outer layer and spinel Ni(Cr, Fe)2O4 with Cr2O3 in the inner layer. SEM measures the average oxide layer thickness, we obtained a trend line, log (x) = 0.432log (t) -1.2334. Comparing our result with xn = kt, Reaction order n will be 2.32. experiment showed γ "phase would not exist at 400 and 500℃, further evidence revealed great agreement for pitting with the original (Nb, Ti) C precipitates phase. Oxide layer analysis of the parameter of 400 ℃ experiment, the oxide layer can be divided into two layers. Outer layer was composed of NiO nano-size particles, its average size d fitted the following formula: (d) = 0.3378t +1.4873. The equation compares with wn = kt, and obtained n = 2.96. Inner layer was composed of a continuous spinel Ni (Cr, Fe)2O4, which that average size L fitted following formula: (L) = 0.5647t 0.0673 compared with wn = kt. We also obtained n = 1.77 (when n = 2, the above equation was a parabolic line). In the experiment of oxide layer analysis under 500 ℃, the oxide layer also can be divided into two layers. However, there were chromium oxide particles on its surface, which average size was about 0.5 μm. Because the thickness of the oxide layer at 600 ℃ is thicker than its at 500 ℃, nickel and chromium can’t diffuse to the surface and form single oxide layer. The single oxide layer is spinel Ni(Cr,Fe)2O4, and it forms a continuous compact oxide layer structure that composed of spinel Ni(Cr, Fe)2O4 and Cr2O3, due to the oxygen diffusion.