FUEL ASSEMBLY AND CORE

PURPOSE:To obtain a fuel assembly and a core capable of spectrum shift operation accordingly to the void fraction change in water rod or water cross freely in high power near the steady power regardless of the core flow and the power distribution in the fuel assembly. CONSTITUTION:A water cross 50 is provided in between fuel rods. A part of the water cross has a coolant upflow path 53 having a water coolant inlet 42a opening at above a fuel rod support part, and a coolant downflow path 54 which is connected with the coolant upflow path, has a coolant outlet 43a opening in a control element guide thimble at the axial position near the fuel rod support part and guides the coolant downward opposite to the coolant flow in the coolant upflow path. In the vicinity of a control element guide thimble 24, the downflow path 54 is provided together with the upflow path 53 arranged next to it. The upper end of the control element thimble 24 has an opening 32 and its lower end is opened for forming the insertion mouth for the control element 7.