PURPOSE: To intend temperature lowering of a clad tube by arranging a protruding wall at periphery of a spacer which is placed close to an orifice, and from an outlet provided inside a large diameter water channel in order to make the channel of a double tubes structure, and by providing another outlet orifice below the spacer.
CONSTITUTION: At positions of each the sixth and the seventh spacer 6 part above a large diameter water channel 1, an outlet orifice 8 to 8' is provided, and a protruding wall 11 is also arranged at an inside of the channel is order to form a double tubes structure 12. With this constitution, during normal operation, a coolant flowing in from a lower part of the water channel, flows out from an upper orifice 8 as shown by an arrow mark, and, on the other hand, a part of the coolant flows out from an outlet orifice 14 at a lower part of the spacer 6. In this way, it becomes not possible that the characteristics of heat transfer is deteriorated and therewith any concern about break-down of a clad tube is not necessary.