Modeling of Radiation-induced Dimensional Instability of Metals and Alloys in Nuclear Systems: Improved Understanding of the Stress and Alloying Elements Effects by Rate Theory

Department of Nuclear EngineeringHistorically a rate theory is one of the most favorite modeling methodologies to simulate the radiation effects on structural materials for nuclear reactors. The easy usability of rate theory, caused by various simplification, could help many researchers to grasp fundamental the understanding. However, simplification in rate theory gives not only easy accessibility but also inaccurate information. Various effect such as alloying element, stress, and cascade effect were neglected because lattice distortion could not be directly considered. Hence, in this paper, to overcome the simplification of rate theory, two approaches had been applied to enhance the performance of the rate theory. In the first approach, detail behavior of defect, which is derived by the state of art simulation tools, were adopted in diffusion coefficient under the stress and alloying element condition. In the second, cascade effect on sink annihilation was considered by a fitting method from the experimental observation. The stress effect on interstitial diffusivity also recently studied by MD simulations. C. Kang and M. Banislaman show that migration energy of interstitial and its clusters is reduced by applied stress in iron. Hence in this study, irradiation swelling, and creep is predicted by considering the recent calculation result. In ultra-long fuel cycle condition, hoop stress could be nearly up to 600 MPa. By MD simulation it was observed that diffusivity of interstitial has 8.437??10-5 cm2 /s at 873 K. This value was 11 times higher than non-stress applied condition (7.414??10-6 cm2 /s). In detail, M. Christensen reveals that Nb decreases the interstitial diffusivity nearly 1/5.64 at 500 K and 1/5.88 at 700 K with 0.5 % Nb in zirconium. Also, the same method was applied to account the Sn effect. Sn shows the high dependency with temperature and relatively less effect than Nb. Interstitial diffusivity is decreased about 1/4.05 at 500 K and 1/1.38 at 700 K with 2.5 Sn. This information was adopted in rate theory and then irradiation growth was predicted. By adding 0.5 % Nb in cold-worked zirconium, 0.2 % strain is obtained whilst cold-worked pure zirconium shows nearly 0.3 % strain at 10 dpa. Since there was no experiment about irradiation growth of Zr-0.5Nb, Zr-2.5Nb experimental result was compared with simulation results. In case of cascade annihilation of sinks, there is no study including theoretical method. However, by F. Garner, it was experimentally observed that network dislocation density of iron-based alloy is saturated nearly 1011 cm2 no matter how fabrication process is applied to the specimen. Hence by the fitting method, sink annihilation rate by cascade is adopted in rate theory. As results, irradiation swelling rate is decreased by nearly 30 % compared with non-considering the cascade annihilation of sink case. Until now, three representative types of rate theory were developed in the research area of radiation VI effectthese are SRT, CDM, and PBM. In case of SRT and CDM, there is no big difference except the consideration of cluster number density. However, there is a large gap between PBM and SRT/CDM because 1-D reaction kinetics, caused by mobile interstitial cluster, is accounted in PBM. Since the purpose of this dissertation is an improvement of scientific understanding of the RIDI behavior within rate theory frames, there was attempting to verify the stress and alloy element effect between SRT and PBM in zirconium, and between CDM and PBM in iron. However, unfortunately, in case of zirconium, PBM could not be carried out because many parameters for cluster such as migration, formation, and binding energy are not discovered. Hence, stress and alloy element effect were verified only in SRT. In case of an iron-based alloy, since there was significant progress about study of defect behavior, stress and alloy effect could be examined in between CDM and PBM. There was also a conventional way to account the stress and alloy element effect. To improved scientific understanding, the result of the conventional method and results of this work are compared in the discussion chapter. Therefore, the result of this study could enhance the fundamental understanding of radiation effect on structural materials because various effects were considered within various rate theory frame.ope