Patent Number: 
Section: description

The embodiments of the present invention will be described hereinafter with reference to the accompanying drawings. Regarding the reference numerals used to describe the following embodiments, the same reference numerals denote the same portions as those of FIGS. 1 to 5. (First Embodiment) The first embodiment of the present invention will be described with reference to FIGS. 6 and 7. In a fuel assembly according the first embodiment of the present invention, the structure of a connecting portion for connecting its guide thimbles 2 and bottom nozzle 4 is as shown in FIG. 2B and 4 or 5, and a so-called improved guide thimble is employed. An inner diameter D of the lower large-diameter portion of the guide thimble 2 and a diameter d of a drain hole 15 satisfy the following equation (1): 0.04 D less than d less than 0.08 Dxe2x80x83xe2x80x83(1) The function of the fuel assembly according to this embodiment with the above arrangement will be described. FIG. 6 is a graph showing results obtained by measuring a terminal velocity V of a control rod inserted in the guide thimble 2 by free fall in a fuel assembly 1 formed as shown in FIGS. 4 and 5, by using (d/D), which is the ratio of the diameter d of the drain hole 15 of a thimble screw 14 to the inner diameter D of the lower large-diameter portion of the guide thimble 2, as a parameter. The axis of ordinate indicates V/V0 obtained by dividing the terminal velocity V of the control rod inserted in the guide thimble 2 by free fall by a limited terminal velocity V0 determined from the viewpoint of moderating the fall impact of the control rod. More specifically, the range of (V/V0) less than 1 is a range where the terminal velocity V of the control rod inserted in the guide thimble 2 by free fall can be suppressed to be lower than the limited terminal velocity V0. The range of (V/V0)xe2x89xa71 is a range where the terminal velocity V of the control rod inserted in the guide thimble 2 by free fall becomes equal to or more than the limited terminal velocity V0. As shown in FIG. 6, in the range of (d/D) less than 0.08, (V/V0) less than 1 is established, and the terminal velocity V of the control rod inserted in the guide thimble 2 by free fall does not exceed the limited terminal velocity V0 but satisfies the design standard. In the range of (d/D)xe2x89xa70.08, (V/V0)xe2x89xa71 is established, and the terminal velocity V of the control rod inserted in the guide thimble 2 by free fall exceeds the limited terminal velocity V0 and does not satisfy the design standard. Hence, from the viewpoint of the terminal velocity V of the control rod inserted in the guide thimble 2 by free fall, the diameter d of the drain hole 15 of the thimble screw 14 and the inner diameter D of the lower large-diameter portion of the guide thimble 2 must satisfy d less than 0.08D. As described earlier, the drain hole 15 of the thimble screw 14 serves to guide the coolant into the guide thimble 2 in order to cool the non fuel bearing components. From this viewpoint of assuring the cooling function, the larger the diameter d of the drain hole 15 of the thimble screw 14, the better. FIG. 7 is a graph showing results obtained by measuring the cooling ability of the non fuel bearing components in a fuel assembly 1 formed as shown in FIGS. 4 and 5, by using (d/D), which is the ratio of the diameter d of the drain hole 15 of a thimble screw 14 to the inner diameter D of the lower large-diameter portion of the guide thimble 2, as a parameter. The axis of ordinate indicates C/C0 obtained by dividing a coolant inflow amount C from the thimble screw 14 by a coolant inflow amount C0 necessary for cooling the non fuel bearing components when (d/D) is used as the parameter. More specifically, in the range of (C/C0)xe2x89xa61, the coolant inflow amount C does not exceed the necessary coolant inflow amount C0. In the range of (C/C0) greater than 1, the coolant inflow amount C exceeds the necessary coolant inflow amount C0. As shown in FIG. 7, in the range of (d/D) greater than 0.04, (C/C0) greater than 1 is established, and the coolant inflow amount C becomes larger than the necessary coolant inflow amount C0. In the range of (d/D)xe2x89xa60.04, (C/C0)xe2x89xa61 is established, and the coolant inflow amount C does not exceed the necessary coolant inflow amount C0. Hence, from the viewpoint of the cooling ability, the diameter d of the drain hole 15 of the thimble screw 14 and the inner diameter D of the lower large-diameter portion of the guide thimble 2 must satisfy d greater than 0.04 D. In the fuel assembly according to this embodiment, an improved guide thimble is employed, and the inner diameter D of the lower large-diameter portion of the guide thimble 2 and the diameter d of the drain hole 15 of the thimble screw 14 are adjusted to satisfy 0.04 D less than d less than 0.08 D. Hence, the coolant can be sufficiently supplied also from the viewpoint of assuring the cooling function of the non fuel bearing components. From the viewpoint of moderating the fall impact of the control rod as well, the terminal velocity V of the control rod can be suppressed to be equal to or less than the fall velocity with which the fall impact of the control rod can be moderated. Therefore, flexural deformation of a dashpot 20 can be prevented. (Second Embodiment) The second embodiment of the present invention will be described with reference to FIG. 8 and FIGS. 9A to 9C. FIG. 8 is a view showing a state wherein a rotation preventive pin 17 for a thimble screw 14 in a fuel assembly according to the present invention is built into the thimble screw 14. FIGS. 9A, 9B, and 9C are views each showing a rotation preventive pin for a thimble screw in the fuel assembly according to this embodiment. As shown in FIG. 8, a shaft 23 of the thimble screw 14 has a guide hole 24 as a hole extending from a spot facing hole 18 of a seat 16 to a drain hole 15 on the distal end side in the longitudinal direction of the thimble screw 14. With the rotation preventive pin 17 being mounted in the spot facing hole 18, during operation of the nuclear reactor, a coolant enters from the spot facing hole 18 of the seat 16 as shown in the direction indicated by an arrow A and is drained from a drain hole distal end 25. If the control rod is dropped in the scram mode, the coolant enters the drain hole 15 from the drain hole distal end 25 as shown in the direction of an arrow F and is drained from the spot facing hole 18 of the seat 16. In the thimble screw 14 of the fuel assembly according to this embodiment, the rotation preventive pin 17 has, at its upper side namely the guide hole 24 side, a water receiving machined portion 26 formed of a recess with an arcuate section, so it receives the flow of the coolant passing through the guide hole 24 in the direction indicated by the arrow F in FIG. 8. The water receiving machined portion 26 increases the pressure drop of the coolant flowing in the direction of the arrow F. FIGS. 8 and 9C show an arcuate machined portion 30 formed of a recess with an arcuate section as a typical example of the rotation preventive pin 17 with the water receiving machined portion 26. Alternatively, the water receiving machined portion 26 may be a V-shaped machined portion 28 with a V-shaped section, as shown in FIG. 9A, or a flat machined portion 29, as shown in FIG. 9B. The water receiving machined portion 26 can have any shape as far as it can increase the pressure drop in the direction of the arrow F against the flow of the coolant passing through the guide hole 24 in the direction of the arrow F, when compared to a conventional case wherein a rotation preventive pin without a water receiving machined portion 26 is used. With the structure of the conventional rotation preventive pin, the ratio of the pressure drop coefficient for the flow of the coolant entering from the spot facing hole 18 of the seat 16 and draining from the drain hole distal end 25 as shown in the direction of the arrow A, to the pressure drop coefficient of the flow of the coolant entering from the drain hole distal end 25 and draining from the spot facing hole 18 of the seat 16 as shown in the direction of the arrow F, in the opposite manner, is almost 1:1. The thimble screw 14 of the fuel assembly according to this embodiment has the rotation preventive pin 17 with the above arrangement. Thus, the ratio of the pressure drop coefficient of the flow of the coolant entering from the spot facing hole 18 of the seat 16 and draining from the drain hole distal end 25 as shown in the direction of the arrow A, to the pressure drop coefficient of the flow of the coolant entering from the drain hole distal end 25 and draining from the spot facing hole 18 of the seat 16 as shown in the direction of the arrow F, in the opposite manner, can be raised to the range of 1:2 to 1:3. With the thimble screw 14 of the fuel assembly according to this embodiment, when the rotation preventive pin 17 with the shape as described above is used, the thimble screw 14 can also serve as a diode. Thus, while the non fuel bearing components have the same cooling ability as that of the conventional case, which is caused by the flow of the coolant in the direction of the arrow A, the decelerating effect of the control rod can be improved by the increase in fluid resistance against the flow in the direction of the arrow F. (Third Embodiment) The third embodiment of the present invention will be described with reference to FIGS. 10 and 11. FIGS. 10 and 11 are views each showing a thimble screw in a fuel assembly according to this embodiment. In a thimble screw 14 for the fuel assembly according to this embodiment, a guide hole 24 is formed in the thimble screw 14 on a distal end side 31, and a drain hole 15 is arranged in the thimble screw 14 on a seat 16 side. Referring to FIG. 10, a shaft 23 of the thimble screw 14 has the drain hole 15 extending between the guide hole 24 and a spot facing hole 18 of the seat 16. The opening area of the drain hole 15 is set smaller than the opening area of the guide hole 24 or the opening area of the spot facing hole 18. With a rotation preventive pin 17 being mounted in the spot facing hole 18, during operation of the nuclear reactor, a coolant enters from the spot facing hole 18 of the seat 16 in the direction of an arrow A in FIG. 10 and is drained from a guide hole distal end 32. When a control rod is dropped in the scram mode, the coolant flows in the direction of an arrow F and enters from the guide hole distal end 32 to flow through the spot facing hole 18 of the seat 16. After passing through the drain hole 15, the coolant forms a jet as the flow path area is abruptly increased by the spot facing hole 18 of the seat 16, and jets out toward the rotation preventive pin 17. With this arrangement, when the control rod is dropped in the scram mode and the coolant enters from the distal end side 31 in the direction of the arrow F, the rotation preventive pin 17 functions strongly as the fluid resistance against the jet. Thus, the pressure drop against the flow of the coolant in the direction of the arrow F can be increased, and the decelerating effect of the control rod can be improved. In the thimble screw 14 of the fuel assembly shown in FIG. 11, the rotation preventive pin 17 with the water receiving machined portion 26 in the thimble screw 14 shown in FIG. 8 and FIG. 9A, 9B, or 9C is combined with the thimble screw 14 with the arrangement shown in FIG. 10. This rotation preventive pin 17 has a water receiving machined portion 26, in the same manner as the rotation preventive pin 17 of the second embodiment. Since the thimble screw of the fuel assembly according to this embodiment has the above arrangement, the coolant enters from the guide hole distal end 32 in the direction of the arrow F shown in FIG. 10 and is discharged in the form of a jet from the drain hole 15 toward the rotation preventive pin 17. As the rotation preventive pin 17 strongly functions as a fluid resistance against the jet, the pressure drop for the flow of the coolant in the direction of the arrow F can be increased, and the decelerating effect of the control rod can be improved. Meanwhile, the rotation preventive pin 17 does not influence the flow rate resistance of the coolant in the direction of the arrow A. Thus, the coolant flow rate is assured, and the cooling ability of the non fuel bearing components can maintain the same effect as that of the conventional case. When the rotation preventive pin 17 with the water receiving machined portion 26 is combined with a thimble screw in which the opening area of the drain hole 15 is set smaller than the opening area of the guide hole 24 or the opening area of the spot facing hole 18, as in the thimble screw 14 of the fuel assembly shown in FIG. 11, the difference in pressure drop of the flow in the direction of the arrow A or F further increases. Therefore, a thimble screw for a fuel assembly with a better diode performance can be provided. Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.