<p>Nuclear power generation is a stable basic power source that does not emit CO2 on the premise of ensuring safety, and has recently been re-evaluated as an attractive option from the viewpoint of energy security and environmental protection. Factors such as the recent sluggish power demand, power grid capacity limits, and initial investment limits to avoid risks do not favor large-scale plant output. In order to globalize nuclear power generation to mitigate the greenhouse effect, we need a small modular reactor (SMR) that can be easily adopted in any country and can be modularized and manufactured in factories with short construction periods. The concept of the reactor introduced in this section has a simplified BWR (LSBWR) configuration with a low output, long operating cycle, and comprehensive safety features, which was presented in 1999 at the annual meeting of the JSME and ICONE11 by Narabayashi et, al. To be economically competitive, the LSBWR design includes system and structural simplifications, modularity for short construction times, and increased availability. Comprehensive safety features are not intended to be evacuated by reliable equipment or systems such as lower core layout, IVR features, and hybrid ECCS including passive system. The concept proposed here is to provide flexibility for different site conditions and power demands, reduce investment risk and promote public acceptance. Finally, the author also introduces a new SMR named GX-BWR, which uses a reactor internal recirculation pump (RIP) for the purpose of load follow with fluctuating renewable energy and enhance facilitates for stable grid control.</p>