Nuclear power occupies about 50% of the total power generation and it will still play a crucial role in the future in Korea. The purpose of this study is to confirm importance of the role of nuclear power generation in energy self-reliance by suggesting an optimal plant mix through consistent and systematic derivation of data on electricity power plant. This study explores the effect of standardization of nuclear power plants in Korea and its influence on long-term electric system expansion planning(ESEP). Efforts are made to improve the applications of methodologies and to clarify some important concepts currently using in ESEP. And it is suggested to lower 8% of real discount rate, which is used currently, in order to reflect risk-free investment of electricity business. (Author).