VAPOR GENERATOR

PURPOSE:To make the circumferential distribution of the flow volume of cooling material uniform by forming an entrance window into a tube-bundled part at the upper end part of an upper member in such a manner that the flow path sectional area of the entrance window becomes maximum on the entrance nozzle side, while it is gradually reduced so as to become minimum on the side opposite thereto. CONSTITUTION:A cooling material entrance plenum 5 is eccentrically provided relative to an upper member 3 so as to form a cooling material entrance window in such a manner as to be allowed to become maximum on the cooing material entrance nozzle 7 side, while it is made gradually smaller so as to become minimum on the side opposite thereto. This allows the flow volume of cooling material into the tube-bundled part of a heat transfer tube to become maximum on the nozzle 7 side, while it is gradually reduced so as to become minimum on the side opposite thereto. Since the opening area of the cooling material entrance window 3, i.e. the flow path sectional area, is gradually reduced from the cooling material entrance nozzle 7 side towards the side opposite thereto, the flow path sectional area of a part where cooling material is allowed to easily flow into the cooling material entrance window due to the inertia force of cooling material flowing in through the nozzle is drawn. Since the flow resistance at the drawn part is increased due to this construction, the circumferential distribution of the flow volume can be made uniform.