STRUCTURE OF NUCLEAR REACTOR

PURPOSE:To prevent occurrence of a temperature layering phenomenon produced in an upper plenum and to eliminate the decay heat produced when a nuclear reactor scrams by providing a small-size pump in the upper plenum of a reactor vessel containing high-temperature Na. CONSTITUTION:At the time of normal rated operations, the fluid spouting out from a reactor core 1 flows in the oblique direction after colliding with the upper mechanism 6 of the reactor and is sucked through the window section of an intermediate heat exchanger 3. The fluid then reaches a cold plenum through the exchanger 3. When the reactor scrams, the heat production of the core 1 is stopped by the insertion of control rods and the fluid spouting out from the core 1 enters the plenum 2 as a cold fluid, since the temperature of the fluid does not rise. Since the cold fluid stays on the bottom of the plenum 2, a temperature layering phenomenon gradually takes place. When a small-size pump for forced circulation provided in the plenum 2 is actuated simultaneously with a reactor scraming signal, the cold liquid is sucked from the bottom section of the plenum 2 and discharged from an outlet nozzle section 10. As a result, a flow is produced in the plenum 2 and the temperature layering phenomenon can be prevented.