PURPOSE: To sufficiently cool a steam layer in a reactor container, and to rapidly cool an upper end plate portion, flange portions, etc. in said container without directly contacting cooling water with said end plate portion, said flange portions, etc.

CONSTITUTION: In a reactor that an upper portion in a reactor container is provided with a reactor pressure container and a cooling water spray nozzle 2 for cooling the steam of the inside, an upper end plate portion 1 is disposed and a shielding plate 5 is arranged in parallel with a flange 3 of the upper end plate portion 1 and a flange 4 of a drum portion 10. A number of holes 6 are shaped to the shielding plate 5, the stoppers 7 of cooling water sprayed are mounted to the rears of the holes 6 and small holes 8 for removing drain are further formed to a lower sealed end portion. When working, cooling water jetted from the spray nozzle 2 cools a steam layer in the container, collides with the shielding plate 5, and most portions fall as they are while one portion enters the back of the shielding plate 5 from the holes 6 and collides with the stoppers 7 and falls.