Power controlling method and device for ABWR

In an ABWR having a plurality of internal pumps, when a steam controlling valve is suddenly closed due to breaking of power generator load, a predetermined number of internal pumps are tripped at once, and selective control rods are immediately inserted, and after a predetermined period of time, remaining internal pumps are run back. Alternatively, first selected number of internal pumps are tripped at once and selective control rods are rapidly inserted. In a case where a turbine bypass valve is not opened rapidly just after the insertion, a second selected number of pumps among remaining internal pumps are tripped to conduct scram. A reactor core flow rate is reduced, a void ratio is increased and insertion of negative reactivity is increased to suppress increase of neutron fluxes. Unnecessary tripping of recycling pumps can be avoided and the time for restart-up of the recycling pump can be shortened by selecting optimum number of pumps to be tripped in accordance with the reactor power and the reactor core flow rate. (N.H.)