The Hitachi simplified boiling water reactor (BWR) is a natural-circulation BWR with a rated capacity of 600 MW(electric). It has been designed for the classes of small- and medium-sized light water reactors. The power density is ∼ 70% of that in current BWRs because of natural circulation, and the reactor pressure vessel is larger. The components and systems have been simplified by eliminating pumped recirculation systems and pumped emergency core cooling systems. Consequently, the volume of the reactor building is ∼50% that of current BWRs, with the same rated capacity. The construction period is also shorter. In addition, the lower power density allows continuous operation for a 23-month period. The safety characteristics of this BWR are investigated during transient and accident conditions, and the high standards of its simple safety systems are shown.