BWR-4 safety systems serviceability during blackout at the Fukushima-1 NPP (Japan)

During the serious accident at the Fukushima-1 NPP (Japan), the cores of the Nos. 2 and 3 units were cooled for a long period of time by safety systems which were not designed for operation under prolonged power loss. The serviceability of BWR-4 high-head makeup systems and the reasons for their failure are analyzed. It is shown that taking dc losses in the No. 2 unit into account the makeup system remained serviceable in the unanticipated regime. The shutdown of the makeup systems in both units could have been due to the maximum backpressure being reached at the turbine outlets or by cavitation in the turbopumps. It is shown that it is important to make a realistic assessment of the depletion time of the storage batteries in the No. 3 unit.