National energy planning has become increasingly complex owing to a pressing need to incorporate sustainability considerations. In this context, we applied least-cost and cost-risk optimization models to allocate energy sources for sustainable development in the Korean electric power generation industry. The least-cost model determined an electricity generation mix from 2012 to 2030 that incurs minimum generation cost to meet electricity demand. The cost-risk model determined electricity generation mixes in 2030 considering the risks associated with each energy source in order to lessen external risks. In deriving these optimal electricity generation mixes, we considered both conventional and renewable energy sources in conjunction with physical and policy constraints that realistically reflect Korean circumstances. Moreover, we accounted for CO2 and external costs within the electricity generation costs for each energy source. For sustainable development in Korea, we conclude that a portion of the coal and gas in the electricity generation mix must be substituted with nuclear and renewable energy. Furthermore, we found that least-cost allocation is sub-optimal from cost-risk perspective and that it limits the adoption of renewables. Finally, we also discuss the implications of decisions taken by the Korean government regarding the electricity generation mix for next-generation energy planning to achieve sustainability. (C) 2014 Elsevier Ltd. All rights reserved.