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.