AUSTENITIC STAINLESS STEELS FOR FUTURE NUCLEAR FUEL CLADDINGS

Abstract

Nuclear power systems have been under continuous development since the first nuclear power plant started operation in 1954. They are categorized into different generations, with each new generation having significant technological advances over the previous one. The worldwide effort to develop the next generation of nuclear reactors was defined at the Generation IV International Forum (GIF) in 2000. Six types of design were proposed, including supercritical water cooled reactor (SCWR). Materials in this reactor will be exposed to more severe environments than the current generation of reactors to assure higher efficiency in energy production and the current materials used for fuel cladding need to be improved or new materials should be developed. In this thesis, the behavior of two existing nuclear materials, stainless steels 310S and 316L was investigated, under conditions approximating the nuclear reactor environment. An environment with dynamic loop of supercritical water (SCW) was used to test the performance of the alloys and the oxides formed were analyzed. Oxidation of the alloys in air was also performed for comparison. It was found that although both alloys showed good oxidation resistance in air at 600ºC, stainless steel 310S has better resistance in SCW environment compared to stainless steel 316L. A thin protective oxide layer of Mn2CrO4 spinel delays oxidation in alloy 310S. In order to improve the oxidation resistance of 310S and 316L stainless steels, thermo-mechanical processing (TMP) was applied to modify their microstructures. The deformation and annealing texture of the as-received and processed samples were investigated by means of X-ray diffraction (XRD) and orientation imaging microscopy (OIM). Different rolling paths and different deformation levels before annealing were used to produce samples of different grain size with similar texture and samples of similar grain size with different textures. Subsequently, the oxidation resistance of thermo-mechanically processed 316L and 310S samples in SCW was studied. It was found that the oxidation resistance of stainless steels 316L and 310S can be improved up to four and five times, respectively, by decreasing the grain size below a critical value of 3 µm. It was demonstrated that samples with smaller grain size provided higher fraction of grain boundaries for fast diffusion of chromium to reach the surface and compensate losses due to dissolution of chromium in the oxidation media. External oxide layers formed on as-received and thermo-mechanically processed stainless steel 316L samples was characterized to establish possible correlation between orientation of the substrate and oxide grains. Micro and macro textures of the substrate and the oxide layers were examined and the results showed that the texture of substrate did not affect the texture of magnetite (Fe3O4) in the upper oxide layer. In addition, the texture of magnetite did not affect the texture of hematite (Fe2O3) on samples where hematite was an additional oxide phase. The strong texture of both oxides was explained with surface free energy minimization and strain energy minimization theory. This means that the texture of both oxides is dictated by a competition between their surface and strain energies.