Fuel assembly and nuclear reactor

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 spacers disposed in an axial direction of the fuel assembly and maintaining a spacing among the fuel rods;
• a first coolant passage formed in the at least one water rod;
• a second coolant passage formed outside of the at least one water rod, the second coolant passage 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 including
• means 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 and introducing all of the coolant supplied into the coolant ascending 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, and
• the 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 the 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;
• and
• at 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 a reactor having the 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;
• and
• wherein an average enrichment in the upper portion of the fuel assembly is set to be greater than an average enrichment of the lower portion thereof.

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 first coolant passage formed in the at least one water rod;
• a second coolant passage formed 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 second coolant passage being supplied with coolant from a plurality of third coolant passages in the fuel rod holding portion, 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 first coolant passage having a coolant inlet port located at a position in a region of the fuel assembly lower than an upper surface of the fuel rod holding portion;
• the first coolant passage having a coolant delivery port connected with the second coolant passage at a position located higher than the position of the fuel rod holding portion, 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;
• the first coolant passage including a coolant ascending path for coolant ascending from the coolant inlet port and a coolant descending path for coolant descending toward the coolant delivery port, the coolant descending path being connected to 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;
• and
• at 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, so as to enable generation of a state wherein a void appears in the first coolant passage in accordance with the first coolant flow rate of the coolant being supplied into the fuel assembly.