Patent Number: 
Section: claims

1. A fuel assembly charged in a reactor core of a nuclear reactor using a liquid metal as a coolant, the fuel assembly comprising:a vertically disposed wrapper tube including an entrance nozzle at a lower end that introduces the coolant and an operation handling head at an upper end, the wrapper tube stores a plurality of fuel pins;grids that support the plurality of fuel pins in the wrapper tube in a radial direction of the wrapper tube; andperipheral flow suppressing members, formed by a plurality of blocks, each block being a unitary structure extending around all fuel pins disposed on the periphery and controlling a flow of the coolant, the peripheral flow suppressing members being disposed in a peripheral flow passage between fuel pins disposed on the periphery of the fuel assembly and the wrapper tube, and extending over a length corresponding to a heat generation length, wherein the heat generation length corresponds to a length in the axial direction of the fuel pins storing a radioactive fuel material, and the blocks having an inner circumferential surface formed as a plurality of convexities, each of the convexities having a same focus and a larger radius as the respective fuel pin disposed on the periphery of the fuel assembly,wherein the radii of the convexities is determined such that a lateral cross sectional area of a first flow passage is substantially equal to a lateral cross sectional area of a second flow passage, the first flow passage being a flow passage disposed outside of a boundary line formed by a line drawn through the foci of the fuel pins disposed on the periphery of the fuel assembly, the lateral cross sectional area of the first flow passage being an area located outside of the boundary line and between outer circumferential portions of each of the fuel pins disposed on the periphery of the fuel assembly and the convexities formed on the inner circumferential surface of the blocks, and the lateral cross sectional area of the second flow passage being determined as an area located inside the boundary line and not occupied by the fuel pins. 2. The fuel assembly according to claim 1, wherein each of the blocks is disposed in the peripheral flow passage to reduce the lateral cross sectional area of the first flow passage. 3. The fuel assembly according to claim 2, wherein the blocks are stacked along an axial direction of the grids, and groups of the stacked blocks keep relative positions in the axial direction of the grids. 4. The fuel assembly according to claim 2, wherein the blocks are disposed in a ring shape at an inner circumferential surface of the wrapper tube and a coolant circulation hole is formed through each of the blocks to form a coolant flow passage that passes from the inner circumferential surface of the block to an outer circumferential side of the block. 5. The fuel assembly according to claim 1, further comprising sleeves disposed to cover the outer circumferential portions of each of the fuel pins disposed on the periphery of the fuel assembly within a range of a gas plenum of each fuel pin and to keep relative positions between the grids. 6. The fuel assembly according to claim 5, wherein a lower portion of the operation handling head is provided with a spring for pressing down one of the sleeves, a downward pressing force of the spring presses and holds the grids and the peripheral flow suppressing members from an upper side via the sleeve with an elastic force.