1. Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, 2-12-1-N1-34, O-okayama, Meguro-ku, Tokyo 152-8550, Japan, Japan Atomic Energy Agency, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan, and Mitsubishi Materials Corporation, 1002-14, Mukohyama, Naka-city, Ibaraki 311-0102, Japan