A nuclear reactor system for generating electricity includes a nuclear reactor that uses water as a coolant and a moderator, and generates thermal energy through nuclear fission. The reactor includes a reactor vessel and a reactor core. The reactor core comprises a plurality of fuel assemblies and one or more core baffles. In an embodiment the reactor core is square-shaped with a hollow center portion, and is concentric with the reactor vessel. The reactor permits decay-heat removal from the fuel assemblies in the absence of the coolant. A power conversion system is arranged to indirectly receive thermal energy generated by the reactor core and to generate electricity. A refueling water storage tank is useable when the reactor core is refueled with nuclear fuel. A containment surrounds the reactor vessel and the refueling water storage tank.