1. Kyoto University, Research Reactor Institute, Nuclear Engineering Science Division, Asashiro-nishi, Kumatori-cho, Sennan-gun, Osaka 590-0494, Japan
2. Kyoto University, Graduate School of Energy Science, Department of Fundamental Energy Science, Yoshida-honmachi, Sakyo-ku, Kyoto 606-8501, Japan
3. Japan Atomic Energy Agency, Nuclear Science and Engineering Center, Tokai-mura, Naka-gun, Ibaraki 319-1194, Japan