<p>Provided is a transient alleviation system of a reactor which is capable of reliably cooling a reactor core with respect to a transient phenomenon of the reactor while maintaining a safe water level in a reactor pressure vessel. The transient alleviation system (1) of the reactor is provided with a reactor core isolation cooling system (32) and a high pressure core flooder system (28). The reactor core isolation cooling system (32) is activated when the water level of a reactor pressure vessel (4) lowers to a first predetermined water level, and replenishes cooling water into the reactor pressure vessel (4). The high pressure core flooder system (28) has a high pressure core flooder system pump (27) that is activated when the water level in the reactor pressure vessel (4) lowers to the first predetermined water level and a water injection valve (45) that is opened when the water level in the reactor pressure vessel (4) lowers to a second predetermined water level, which is lower than the first predetermined water level, and injects cooling water into the reactor pressure vessel (4).</p>
