1. Research Institute for Advanced Science and Technology, Osaka Prefecture University, 1-2, Gakuen-cho, Sakai, Osaka 593, Japan, Department of Applied Materials Science, College of Engineering, Osaka Prefecture University, 1-1, Gakuen-cho, Sakai, Osaka 593, Japan, and Department of Nuclear Engineering, College of Engineering, Osaka University, 2-1, Yamadaoka, Suita, Osaka 565, Japan