1. University of Tokyo, Faculty of Engineering Nuclear Engineering Research Laboratory, 2-22, Shirane Shirakata, Tokai, Ibaraki, 319-11, Japan
2. Mitsubishi Electric Company, Energy Science and Technology Division Central Research Laboratory, Nuclear Plant Control and Instrumentation Group 8-1-1, Tsukaguchi Honmachi, Amagasaki, Hyougo, 661, Japan