PURPOSE: To simplify the structure and improve the maintainability and the approachability of an operator in maintenance by circulating a primary cooling material and a secondary cooling material without using cooling material tubing.

CONSTITUTION: The primary cooling material which is heated by a reactor core flows in an intermediate heat exchanger 32 through a primary high temperature pool 29 and exchanges heat with the secondary cooling material in the exchanger 32 and is sent to a primary low temperature pool 30. The primary cooling material in the pool 30 is sucked by a primary cooling material electromagnetic pump 3 and sent to the reactor core 27. The secondary cooling material heated in the exchanger 32, on the other hand, is discharged from the upper end of the exchanger 32 to a secondary high temperature pool 39 to flow in a steam generator 41 and exchange heat with water from a feed water pipe 4. Superheated steam produced by the heat exchanging is sent to a steam turbine through a superheated steam tube 43 and the secondary cooling material which drops in temperature by the heat exchanging is sent out to a secondary low temperature pool 40. The secondary cooling material in the pool 40 is sucked by a secondary cooling material electromagnetic pump 36 through a through holes 37 and sent in the exchanger 32.