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A mitigation system which can keep core melt stable after a severe accident is necessary to a next generation BWR design. Toshiba has been developing a compact core catcher to be placed at the lower drywell in the containment vessel. The cooling water for the core catcher is supplied from the passive flooder and PCCS drain line. After the core catcher is flooded, the molten core would be cooled by both overflooding water and inclined cooling channels, in which water is boiling and natural circulation is established. So the core catcher can operate in passive manner and has no active component inside the containment. This paper summarizes flow dynamics and heat removal capability in an inclined cooling channel of core catcher when cooling water flows by the natural circulation.

Toshiba has developed a core-catcher system. It is to be installed at the bottom of the lower drywell in order to stabilize a molten core flowing down from a reactor vessel. It consists of a round basin made up of inclined cooling channels arranged axisymmetrically, and the structure including risers, downcomers and a water chamber to get natural circulation of the flooding water. So it can cover entire pedestal floor and can work in passive manner. In order to confirm the heat removal capability of the core catcher with natural circulation, we have conducted full scaled tests in several conditions. Some important dimensionless numbers obtained from fundamental equations of the natural circulation are used for the tests. Using dimensionless number and to compare with several analysis, we can verify that the experiment is adequate to simulate the actual plant.

In Japanese BWR plants, thermal recombiners have been installed as the flammability control system (FCS) to keep hydrogen and oxygen concentrations below the flammability limit during a loss-of-coolant accident (LOCA).In the meantime, the passive autocatalytic recombiner which needs no electric power supply and heating sources and has an outstanding cost performance and reliability consequently, has been developed recently. This type of recombiner is already introduced to many PWR plants in the United States and Europe.In this situation, Japanese BWR utilities planned to introduce the passive autocatalytic recombiner to new plants to reduce the equipment and maintenance costs and improve system reliability by eliminating dynamic devices and support systems. The performance of catalytic recombination systems, however, has to be evaluated whether they satisfy the regulatory licensing requirements specific to Japan. Accordingly, the Japanese BWR utilities and plant manufacturers carried out a joint study, and experimental tests and analyses were conducted on the catalytic FCS technology. Based on the data from the tests, necessary number of the recombiners and lay-out planning for the ABWR containment vessel was estimated as a reference to ensure their applicability to ABWR plants.