PURPOSE: To contrive the improvement of economy while securing the sodium- water reaction by making a steam generator in dual tube structure, providing a gate valve in an outlet tubing and an inlet tubing and preparing a guard vessel on the outer periphery of a reactor vessel.

CONSTITUTION: A steam generator 9 which transfers the heat of high temperature sodium generated in a reactor vessel 1 to water is made dual tube structure between sodium and water, an outlet tubing and an inlet tubing are set at position higher than the water level of emergency liquid and gate valves 7, 12 preparing radiation shields among the reactor vessel 1 and are provided respectively. A guard vessel 4 having the upper end part which is higher than the water level of emergency liquid is prepared with the outsides of the reactor vessel 1 and the gate valves 7, 12 being enclosed. Therefore, the overflow of reactor coolant can be prevented by closing the gate valves 7, 12 provided at the outlet tubing and the inlet tubing even if burning reaction occurs with reducing the possibility of burning reaction in which a tubing is broken and sodium and water come in contact with each other.