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I would like to thank my advisor, Prof. Forsberg, for his guidance and support throughout this study. His creative ideas and initiatives have inspired my learning at MIT. I feel very fortunate to have had a chance to work with Prof. Driscoll who has greatly supported and helped this study. From him, I learned attitudes for lifelong learning. I appreciate Dr. Bernard of the MIT Reactor for giving me valuable advice and help for my graduate studies in general. I am also grateful for my undergraduate advisor Prof. Hee Cheon No, Prof. Jeong Ik Lee and Dr. Sung Joong Kim for giving me advice to successfully make a transition from my undergraduate studies in Korea to graduate studies in the USA. Although I cannot list the names of everyone who helped my studies in this limited space, I do remember and appreciate every single help and advice I have received. I would like to thank my parents for their unconditional love and support throughout my life. No amount of success in my life would be possible if not for them. Finally, I thank God for becoming the meaning of my academic endeavor.