Patent ID: 6278757
Filing Date: 2001-08-21
Classification: G21C,Y02E

Abstract:
A fuel assembly comprising:an upper tie plate;a lower tie plate;a plurality of fuel rods having a lower end portion thereof held by a fuel rod holding portion of the lower tie plate;at least one water rod arranged among the fuel rods and held by the fuel rod holding portion;a plurality of fuel spacers disposed in an axial direction of the fuel assembly and maintaining a spacing among the fuel rods;a first coolant passage formed within the at least one water rod;a second coolant passage formed outside of the at least one water rod and being provided among the fuel rods and between the at least one water rod and the fuel rods, between the upper tie plate and the fuel rod holding portion;the fuel rod holding portion having a plurality of third coolant passages for introducing coolant into the second coolant passage from a region lower than the fuel rod holding portion, and each of the third coolant passages having a cross-sectional area smaller than a cross-sectional area of the second coolant passage at least in an area of the second coolant passage immediately above the fuel rod holding portion;the at least one water rod having the first coolant passage includingmeans delimiting a coolant ascending path,means delimiting a coolant descending path connected with the coolant ascending path at a top portion of the first coolant passage so that all of the coolant supplied into the coolant ascending path is introduced into the coolant descending path in a downward direction opposite to the direction of the flow of coolant in the coolant ascending path,the coolant ascending path including a coolant inlet port, andthe coolant descending path including a coolant delivery port arranged in a position higher than the fuel rod holding portion and lower than the top portion of the first coolant passage, the coolant descending path being connected with the second coolant passage, the coolant inlet port and coolant delivery port being positioned such that during flow of coolant under operation of a reactor core containing the fuel assembly pressure differentials are developed between the coolant inlet port and the coolant delivery port, dependent on coolant flow rate; andat least the cross-sectional area of each of the third coolant passages having a relation to that of the second coolant passage, to thereby provide the pressure differentials upon flow of coolant during operation of the reactor core, so as to enable a static coolant head within the coolant ascending path with respect to the coolant descending path at a first coolant flow rate under operation of the reactor core, and so as to enable a rise of a liquid surface of the coolant formed in the coolant ascending path until a state is obtained wherein the liquid surface is not formed in the at least one water rod by increase of a flow rate of the coolant being supplied into the fuel assembly, under operation of the reactor core with the fuel assembly disposed therein, whereby in said state coolant flows through the coolant ascending and descending paths in substantially a single phase stream.