In-core structure of reactor vessel

Within an upper plenum of a reactor vessel, a portion of a heated coolant flows radially outward from a central portion of a core barrel towards outlet nozzles in a region of an upper core plate extending outside of an outer periphery of the core along an inner wall of a core barrel. Portions of the coolant flows beneath the outlet nozzles. Thus, streams of heated coolant flowing in opposite directions may collide with each other. After collision, the flow directions of the heated coolant are changed to flow upward. Due to the collision, the coolant flow behavior becomes complicated and unstable, making it difficult to measure the temperature of the heated coolant with an outlet pipe connected to the outlet nozzle. The solution of this disclosed invention is that this in-core structure is partitioned in the upper part of fuel region (33) where heated fluid flows, and a flow stabilizer (1) shorter than the outlet nozzle (12) is placed in the upper plenum (40) which communicates with fluid to a plurality of outlet nozzles (12) provided at the side of the reactor vessel (10), in the outside region of the fuel region (33) and in the vicinity of core barrel (30).