PURPOSE: To automatically put a material having negative reactivity into a reactor core with a simple action and improve the safety of the reactor core by utilizing the difference in density between a coolant and a absorber float when the coolant temperature rises higher than a rated value, and inserting a neutron absorber into a reactor core region.

CONSTITUTION: When the coolant temperature rises during operation, a metal plug 19 is melted, the gas in a buoyancy adjustment section 18 flows to the outside, a coolant flows into an absorber float 20, and the volume of the float 20 is decreased. The coolant slightly flows in a stagnant region 15, and the float 20 is pushed up by little force of the flow. The buoyancy of the float 20 is reduced, the float 20 is moved to the lower section of the region 15, and a neutron absorber 17 is inserted into a reactor core. Even when the coolant temperature rises higher than a rated value to a temperature lower than the melting point of the metal plug 19, the neutron absorber 17 can be inserted into the reactor core by the adjustment of the density of the float 20.