Patent Number: 
Section: description

The present invention will be described in detail in conjunction with what are presently considered preferred or typical embodiments thereof with reference to the accompanying drawings. In the following description like reference characters designate like or corresponding parts throughout the several views. Now, description will be made of the fuel assembly according to a first embodiment of the present invention with reference to FIGS. 1 to 3. The fuel assembly according to the embodiment of the invention is comprised of a lower nozzle 2 disposed on a lower core plate 1, an upper nozzle 4 having hold-down springs 3 for pressing and holding down the lower nozzle 2 against the lower core plate 1, a plurality of control rod guide thimbles 5 for guiding control rods extending through the upper nozzle 4 toward the lower core plate 1, a plurality of support grids 6 mounted onto the control rod guide thimbles 5, and a number of fuel rods 7 held in parallel with the control rod guide thimbles 5 by the support grids 6. The lower nozzle 2 is constituted by a plate portion 2a formed in a square shape, and having four leg portions 2b formed on the bottom surface at four corners thereof respectively. A number of coolant flow holes are opened in the plate portion 2a of the lower nozzle 2. Additionally a number of thimble mounting holes normally corresponding to the number of the control rod guide thimbles 5 are opened in the plate portion 2a. In each of the thimble mounting holes, a thimble mounting bolt 8 (see FIG. 2) is inserted from the bottom side of the lower nozzle 2. Lower end portions of the control rod guide thimbles 5 are secured onto the top surface of the lower nozzle 2 by means of these thimble mounting bolts 8. The upper nozzle 4 is formed as a box-like structure having a central recess formed in a top cover portion thereof, wherein a plurality of control rod receiving through-holes 9 are formed in the upper nozzle 4 (see FIG. 3). These control rod receiving through-holes 9 are provided in correspondence to the control rod guide thimbles 5, respectively, wherein a connecting pipe 10 is welded to each of the control rod receiving through-holes 9 for connecting the top end portion of the control rod guide thimble 5 to the upper nozzle 4. The connecting pipe 10 has an inner diameter slightly greater than the outer diameter of the control rod guide thimble 5. The control rod guide thimble 5 and the connecting pipe 10 are joined together by a bulging process. Each of the supporting grids 6 comprises a frame with a square shape, and a number of metal plates assembled together inside of the square frame, wherein a plurality of sleeves 11 are secured to the metal plates by welding. The sleeves 11 are provided for mounting each of the supporting grids 6 to the control rod guide thimbles 5, wherein the associated control rod guide thimble 5 and sleeve 11 are joined together by a bulging process. The control rod guide thimbles 5 are formed in the shape of a straight tube, wherein a lower end portion of each control rod guide thimble 5 is provided with a dashpot 12. The dashpot 12 is designed to dampen an impact force applied to the upper nozzle 4 by reducing the falling speed of the control rod upon detachment thereof from the control rod driving unit. With xe2x80x9cLxe2x80x9d representing the length of the control rod guide thimble 5, the dashpot 12 has a length ranging from 0.16 L to 0.18 L. Further, a large diameter section 13a is formed at a lower portion of each dashpot 12, while the upper portion of each dashpot 12 is formed as a small diameter section 13b. The outer diameter of the large diameter section 13a is dimensioned to be approximately equal to that of the control rod guide thimble 5. The length of the dashpot 12, exclusive of the large diameter section 13a, i.e., the effective length S of the small diameter section 13b, is so selected as to fall within a range of from 0.03 L to 0.1 L, preferably within a range of from 0.04 L to 0.06 L, wherein xe2x80x9cLxe2x80x9d represents the entire length of the control rod guide thimble 5. Consequently, the length Sxe2x80x2 of the large diameter section 13a is dimensioned to be within a range of from 0.06 L to 0.15 L and preferably within a range of from 0.10 L to 0.14 L. As can be viewed from FIG. 6, the dashpot 12 includes another small diameter section 13c located at its lower end portion, close to a lower nozzle 2. FIG. 4 is a graph illustrating the results of analysis concerning the relationship between the impact force F applied to the upper nozzle 4 when the control rods are detached from the associated control rod driving unit and the length of the dashpot 12, exclusive of the large diameter section 13a; i.e., the effective length S of the small diameter section 13b. As can be seen from this figure, the effective length S of the small diameter section 13b of the dashpot 12 should preferably be greater than 0.03 L in order to make the impact force F applied to the upper nozzle 4 smaller than the permissible limit value F0. Next, FIG. 5 is a graph illustrating the results of analysis concerning the relationship between the flexural rigidity of the dashpot 12 and the effective length S of the small diameter section 13b of the dashpot. As can be seen from this figure, when the effective length S of the small diameter section 13b of the dashpot is selected to be equal to 0.1 L, the flexural rigidity of the dashpot 12 increases by about 15% compared to the conventional dashpot employed in the fuel assembly known heretofore. Further, when the effective length S of the small diameter section 13b of the dashpot is selected to be S 0.05 L, the flexural rigidity of the dashpot 12 increases by about 30% compared to the conventional dashpot. Thus, it can be understood from the foregoing description that when the effective length S of the small diameter section 13b of the dashpot 12, exclusive of the large diameter section 13a, is selected so as to fall within the range of 0.03 L to 0.1 L and preferably within a range of 0.04 L to 0.06 L, the impact force F applied to the upper nozzle 4 upon detachment of the control rods can be suppressed to be smaller than the permissible limit value F0, and the flexural rigidity of the dashpot 12 can be increased as well. Thus, the dashpot 12 can be protected against flexural deformation under a compression load acting in the axial direction of the control rod guide thimble 5. Furthermore, the present invention is not intended to be limited to the embodiment described above, and numerous modifications may be conceived. By way of example, in the case of the fuel assembly described above, the outer diameter of the lower end portions of the dashpots 12 is formed so as to be approximately equal to that of the control rod guide thimble 5. However, the outer diameter of the lower end portions of the dashpots 12 may be formed smaller than that of the control rod guide thimbles 5 as shown in FIG. 6, which shows the fuel assembly according to a second embodiment of the present invention. As is apparent from the foregoing description, with the arrangement of the present invention, since large diameter sections having approximately the same diameter as that of the control rod guide thimbles are formed in lower portions of the dashpots and the length of the dashpot, exclusive of the large diameter section, i.e., the length of the small diameter section, is selected so as to lie within a range of 0.03 L to 0.1 L (where L represents the entire length of the control rod guide thimble), a fuel assembly can be realized in which the dashpots of the control rod guide thimbles are protected against flexural deformation which may otherwise occur under the compression loads acting in the axial or longitudinal direction of the control rod guide thimbles. Many modifications and variations of the present invention are possible in the light of the above techniques. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced other than as specifically described.