1. Fuel Cycle and Environment Division, Research Reactor Institute, Kyoto University, Noda, Kumatori, Sennan, Osaka 590-0494, Japan, Department of Environmental Sciences, Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195, Japan, Department of Nuclear Engineering, Graduate School of Engineering, Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto 606-8501, Japan, and Department of Nuclear Engineering, Graduate School of Engineering, Osaka University, Yamadaoka, Suita, Osaka 565-0871, Japan