Patent Number: 
Section: claims

1. A fuel assembly, comprising:a plurality of fuel rods containing a fissile material;a lower tie plate which supports each lower end portion of the fuel rods;an upper tie plate which holds each upper end portion of the fuel rods;a plurality of fuel spacers, each of which bundles the plurality of fuel rods;a channel box attached to the upper tie plate, extending toward the lower tie plate, and surrounding the plurality of fuel rods bundled by the fuel spacers; andflow resistance members, which are disposed in an inner side of an outermost layer region of an array of the plurality of fuel rods in a cross section in a direction perpendicular to a center axis of the channel box, and through which coolant paths are formed, and which the fuel rods penetrate, whereinwhen the number of the fuel rods held by the flow resistance member is R, the number of all the fuel rods in the fuel assembly is A, and a projected area ratio C of projected area Sa of the flow resistance member from an upper tie plate side to projected area Sb of the fuel spacer which holds all the fuel rods in the fuel assembly from the upper tie plate side is defined by the following formula (1), the projected area ratio C is within a range of 1.5 to 5.2,C=(Sa/Sb)×(A/R)  (1),Sa is smaller than Sb,in one of the flow resistance members, ferrules are arranged in a square grid of six rows and six columns in a square-shaped band, and two water rod disposition regions, each of which occupies a region capable of disposing four ferrules, are formed in a central portion,adjacent ferrules are joined to each other by welding, and each of the ferrules disposed in an outermost layer is joined to the square-shaped band by welding,in each of the ferrules, only two protrusions are formed in a circumferential direction, and only a single spring member is provided across adjacent pairs of ferrules and attached thereto,a bridge member is provided only at every two adjacent ferrules facing a water rod disposition region and is attached to a side surface of each ferrule, anda spring member is attached to only one bridge member among the four bridge members facing each of the water rod disposition region. 2. The fuel assembly according to claim 1, whereinwhen a distance from a center of a given ferrule to the center of a ferrule which is immediately adjacent thereto and comes into contact with the given ferrule is taken as 1,the distance from the center of the given ferrule to the center of the ferrule which is immediately adjacent thereto and comes into contact with another ferrule in a direction orthogonal to a straight line connecting the center of the given ferrule to a center of the another ferrule is √2, andthe given ferrule and the ferrule do not come into contact with each other, and a space is formed between the given ferrule and the ferrule, so that a space formed between the given ferrule and the ferrule formed between two ferrules immediately adjacent to each other in a diagonal direction of the flow resistance member,some of the resistance members are round plate-shaped members made of a zirconium alloy, and come into contact with each side surface of immediately adjacent four ferrules and are fixed by welding to the side surfaces of the immediately adjacent four ferrules,other resistance members are round plate-shaped members made of a zirconium alloy, and are disposed in each second space formed by the square-shaped band, anda diameter of the other resistance members is smaller than a diameter of the some of the resistance members.