A new subsystem called the passive safety and shutdown system (PSSS) is proposed to improve nuclear reactor safety. The subsystem can be added to the existing reactor coolant system (RCS) by a slight modification.To exemplify how the subsystem improves pressurized water reactor safety, anticipated transient without scram under loss of normal ac power is analyzed. The result indicates that the proposed subsystem is very effective in putting an end to the accident.The PSSS not only improves passive reactor safety, but also simplifies RCS and its control and surveillance systems. Items needing further