1. Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan
2. Center for Mathematical Sciences, University of Aizu, Aizuwakamatsu, Fukushima 965-8580, Japan
3. Radioisotope Center, Niigata University, Niigata 951-8580, Japan
4. Research Institute of Nuclear Engineering, University of Fukui, Fukui 910-8507, Japan