1. Kyoto University, Department of Nuclear Engineering, Yoshida, Sakyo-ku, Kyoto 606, Japan
2. University of Tokyo, Institute for Nuclear Study, Midori-cho, Tanashi, Tokyo 188, Japan
3. University of Tokyo, Department of Physics, Hongo, Bunkyo-ku, Tokyo 113, Japan
4. Shimizu Corporation, Institute of Technology, Echujima, Koto-ku, Tokyo 135, Japan