Nuclear energy has provided a major source of clean electricity for South Korea over decades. However, the South Korean Government announced an energy transition roadmap aiming to reduce nuclear shares and increase renewable shares. However, given the nation’s high population density, the maximum share of renewable sources for electricity generation in South Korea is constrained. The roadmap was silent on how to fill the gap between a reduced nuclear output and the limited renewable potentials. The tacit alternatives are fossil fuels, and their deployment will become the key determining factor on how South Korea approaches the problem of greenhouse gas emissions reductions. We used scenario analysis to investigate two fossil‐intensive cases, alongside a hypothetical renewable case. On the basis of the comparison of the three scenarios with other countries, we provide an insight into the feasibility and limitations of the nonnuclear options and propose the techno‐economic requirements for avoiding the worst outcomes.