1. Ph.D., Korea Atomic Energy Research Institute, 1045, Daedukdaero, Yuseong-Gu, Daejeon, 305-353, Korea.
2. Professor, Dept. of Energy and Environmental Science, Keimyung Univ., 2800, Dalgubeoldaero, Dalseo-Gu, Daegu, 704-701, Korea (corresponding author).