Abstract: A transient mitigation system for a reactor includes: a reactor core isolation cooling system; and a high pressure flooder system including a high pressure flooder pump and a flooder valve. When the water level of the reactor water in the reactor decreases to a first predetermined water level, the reactor core isolation cooling system is activated to supply the cooling water into the reactor. When the water level in the reactor decreases to the first predetermined water level, the high pressure flooder pump is activated. When the water level in the reactor decreases to a second predetermined water level lower than the first predetermined water level, the flooder valve is opened to inject the cooling water into the reactor.