Stress Corrosion Cracking (SCC) has been detected in Boiling Water Reactors (BWRs) on core shrouds and primary water re-circulation piping made of low carbon stainless steels. Material hardening strongly affects SCC propagation behavior, and SCC growth rates increase with increasing hardness of austenitic stainless steels caused by cold work or neutron irradiation.

Research work has been conducted in the authors’ laboratories with the aim of improving SCC resistance using chemical composition control of stainless steels. It has been previously reported that high stacking fault energy (SFE) materials showed better SCC resistance than low SFE materials due to hardening being suppressed in high SFE materials. In the present study, SCC growth rate (CGR) tests were performed using 15% cold worked Types 316L and 25Cr-20Ni stainless steels in a simulated BWR water environment. The 25Cr-20Ni stainless steel used has high SFE value due to chemical composition control and measured SCC growth rates were lower than those of low SFE stainless steels.

However, oxidation behavior is one of the more important factors influencing SCC of austenitic stainless steels in addition to material hardening behavior, and the influence of the chemical composition control necessary to increase SFE on oxidation behavior in BWR primary coolants is still unclear. In this study, therefore, immersion tests using Types 316L and 25Cr-20Ni stainless steel specimens were also conducted in the simulated BWR water environment. The surface oxide films on the specimens were then analyzed with micro-Raman spectroscopy and glow discharge optical emission spectroscopy in order to help clarify the oxidation behavior.

The results of these tests and analyses showed that the NiFe2O4 content of the outer oxide layers on the high SFE stainless steels was higher than that on the low SFE stainless steels. The inner oxide film on the 25Cr-20Ni stainless steel also had a high chromium content.

Based on the above results, SCC resistance and oxidation behavior of high SFE austenitic stainless steels in a simulated BWR water environment will be discussed.

Under a creep condition of relatively intense stress, dispersive precipitation of TiC nanoparticles could be promptly brought out within the austenite grain when the iron-nickel-base superalloy was fabricated through the specific production of the ingenious alloy design, applicable electro-slag remelting and high-temperature solution-anneal developed in this study. Both the TiC and coexistent M23C6 precipitates are cubo-octahedral in shape, remaining the cube-to-cube orientation relationship and coherent {111}carbide/γ and {100}carbide/γ interphase interface with the austenite, which bring about the dispersion-strengthening effect so that the time to rupture for the present alloy could be increased by 1.8 times of magnitude relative to the commercial creep-resistant product of the same grade. The improved creep property can be attributed to the mechanism that the nanometer-scale intragranular TiC and submicron intergranular M23C6 can act as pinning points for individual dislocations and grain boundaries, respectively, resulting in the raised Orowan strengthening and suppressed grain boundary sliding. Since the strengthening media found in this study are thermally stable and able to emerge readily prior to the formation of γ’ phase at the temperature range studied, this work suggested an improved grade of heat resistant alloy for applications in a relatively harsh environment that the creep lifetime might be shorter than the incubation period of the common strengthening phase for superalloys.

A chemical decomposition and related phase transformation have been observed in 2.2 GeV 197Au irradiated SnO2 nanopowder. X-ray diffraction (XRD), Raman spectroscopy, and transmission electron microscopy (TEM) were used to characterize the transformation from tetragonal SnO2 (P42/mnm) into tetragonal SnO (P4/nmm). Rietveld refinement of the XRD data determined the structures and proportion of these phases up to a fluence of 2.4×1013 ions/cm2. The initially intense diffraction maxima corresponding to SnO2 gradually decrease in intensity with an increase in fluence. At a fluence of approximately 3.9×1012 ions/cm2, diffraction maxima corresponding to SnO become clearly evident and increase in intensity as fluence increases. Both Raman and TEM analyses confirm the transformation from tetragonal SnO2 to SnO. The XRD refinement results are consistent with a multiple-impact model of transformation, confirmed by TEM as no single tracks were observed. Previous swift heavy ion irradiations of SnO2 have led only to changes in grain size, degrees of crystallinity, and the formation of “holes”. The inconsistency in results is discussed in depth. The proposed mechanism for the currently observed transformation is the interrelation of defect accumulation and thermal-spike mechanisms. The formation of SnO, apparent O loss from the transformation regions, and associated Sn reduction are discussed in terms of thermodynamic, kinetic, and thermal-spike model considerations.