The TOSBWR-900P, a natural-circulation boiling water reactor design with 300-MW(electric) power emphasizing passive safety, is studied. The TOSBWR-900P has steam drums above the pressure vessel that minimize the lower drywell volume associated with a top-mounted gravity-driven control rod device. The flow to the core is controlled by the valves on the downcomer pipes. The emergency core cooling system consists of high- and low-pressure accumulators, a gravity-driven core cooling system, and an automatic depressurization system, which guard against core uncovery in case of a design-basis accident. Long-term decay heat removal is achieved passively by a drywell water wall. The gravity-driven drywell spray water can completely flood the reactor pressure vessel during a severe core damage accident.