AU427997B2 - Fuel assembly fora nuclear reactor - Google Patents

Description

23. 993171 The present invention relates in general to the construction of nuclear reactors, and more particularly to the assembly of a fuel element as used in the core of a nuclear reactor. In certain fuel assemblies of the prior art the multi- plicity of fuel rods and guide tubes are rigidly secured to both the upper and lower grid structures. Access for replace- ment of guide tubes and replacement or the salvaging of fuel pins thus becomes a major time consuming operation since it is necessary to disconnect each and every fuel rod and guide tube in order to remove the grid. With the improved fuel assembly of the invention, either end grid structure can be removed for access to the fuel pins and guide tubes simply by releasing the guide tube connections, which are much fewer in number than the fuel rods. According to a preferred embodiment of fuel element assebmly in this invention there is provided an upper end grid to accommodate the guide tubes and fuel pins. The grid has a plurality of key slots for receiving a plurality of elongated fuel rods each of which has an upper end key received in a corresponding slot of the upper end grid. Additionally there is a lower end grid that receives the lower ends of the guide tubes and the fuel rods and constrains them against lateral movement relative to one another. The fuel rods are dependently secured from and supported by the upper end grid, while the lower end grid is supported from the upper end grid by the guide tubes. Because in this preferred embodiment, the fuel rods are arranged in parallel rows as are the upper grid key slots; when the keys of the fuel pins are in registry with the key slots the upper grid can be installed by sliding it along a path generally perpendicular to 427.997 the axes of the fuel rods while isengagement can be effected by movement of the grid in .the opposite direction. One of the. advantages afforded by the invention is that when the fuel assembly is subjected to normal handling, in which it is lifted- by its upper grid, the weight of the fuel rods is borne directly by the upper grid, thus minimizing the strain on the guide tubes, and enhancing the safety of the entire assembly.

For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawing and descriptive matter in which there is illustrated and described a preferred embodiment of the invention.'.

In the Drawing: Figure 1 is an elevation view, partly in section, of an improved fuel element assembly according to a preferred

embodiment of the invention.

Figure 2 is a plan view of the assembly shown in Figure- 1. Figure 3 is a bottom view in perspective of a typical

portion of the upper end grid used in the fuel assembly of Figures 1 and 2, as shown apart therefrom. Figure 4 is a top view in perspective of a typical portion of the lower end grid used in the fuel assembly of Figures 1 and 2 as shown apart therefrom.

In Figures 1 and 2 there is shown a fuel element assembly 10 for a nuclear reactor (not shown), and having a plurality of elongated upright fuel rods 11 and control rod guide tubes 12 held together by upper and lower end grids 13 and 14 respectively and by one or more intermediate spacer grids

In the fabrication of fuel assembly 10, the rods 11

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23 993 171 .and guide tubes 12 are inserted into the spacer grids 15 before

installing end grids 13 and 14. Guide tubes 12 serve as con- duits for the insertion and withdrawal of control rods (not shown)

and being.much fewer in number than fuel rods 11, the individual

tubes 12 are disposed among the rods 11 at preassigned locations.

Grids 15 hold the rods 11 and tubes 12 in parallel relation to

one another and in plural parallel rows.

'According to the concept of the invention, the fuel

rods 11 are dependently supported by the upper end grid 13.

The lower end grid 14 is connected to and thereby supported by

upper end. grid 13 by means of the guide tubes 12, each of

which is releaseably connected at its upper end and at its

lower end to grids 13 and 14 respectively.

:To accommodate connection of the guide tubes 12,

there are provided as an integral part of end grids 13 and 14

appropriately located fittings 16 through which respective

tubes 12 extend in the completed assembly 10. The details of these fittings 16 are better seen in Figs. 3 and 4. Connection of tubes 12 to grids 13 and 14 is completed by threaded nuts

17 that screw onto threaded end pieces of tubes 12 which project through and beyond fittings 16 to thereby lock the grids

13 and 14 into assembly

For suspending the fuel rods 11 from grid 13, at

the upper end of each rod li, there is provided a key part 18

that is received in a corresponding key slot 19 of grid 13.

Key pew4 18 is expediently in the form of a cylindrical stem

section 20 connected to a cylindrical disk section 21 such as

to give key pAt 18 a T-shaped longitudinal cross-section that

is compatible wit the T-shaped configuration of key slots 19. Expediently, all y p:rt 18 are of the same configuration, as' are all key slots 19, as indicated by Figure 3. 4 427.997

2 .3993 P1, In assembling the fuel element 10, the rods 11 are

uniformly positioned axially in grids 15 such that the keys

18 of all rods 11 in each row thereof are aligne& with

one another. Key slots 19 are arranged in parallel rows

and have the same lateral spacing as corresponding keys 18

such that the grid 13 can be incorporated into the assembled

rods 11 by moving grid 13 along a path transverse to the

longitudinal axes of rods 11 until keys 18 are seated in key

slots 19. When each key 18 is received in its associated

slot 19, the guide tubes 12 are advanced through respective

fittings 16 of grid 13 and nuts 17 are screwed onto the tube ends. With grid 13 thus installed, the lower end grid 14

is positioned in alignment with the lower ends of tubes 12 and advanced axially until such tube ends are received through

respective fittings 16 of grid 14. As in the case of grid 13,

the grid 14 is secured to tubes 12 by nuts 17. The lower grid

14 also receives the lower ends of fuel rods 11 and constrains them against laterial movement relative to one another. For

such purpose, there are provided in grid 14 a plurality of

recesses 22 that are spaced in accordance with the configuration of rods 11, and, as better seen in Figure 4, these recesses 22 receive projecting pins 23 of corresponding rods 11. Recesses

22 are so sized in relation to pins 23 as to constrain same,

and hence constrain the lower ends of rods 11 against lateral

movement, but yet allow the rods 11 to expand freely lengthwise

for a limited distance which, by design, is somewhat greater than the maximum anticipated thermal expansion of the assembly As will be appreciated by the artisan, the foregoing

described features of the invention result in a fuel assembly

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23.99 371 in which the weight of each of the numerous fuel rods 11 is individually carried by the upper grid 13, while the guide tubes 12 carry only the weight of the lower grid 14, and hence are relatively lightly stressed. Since the fuel rods 11 are suspended from upper grid 13 and are free to expand lengthwise in lower grid 14, any differential in thermal expansion between rods 11 and guide tubes 12 will impose no additional strain upon the guide tubes 12.

427.997

Claims (1)

1. Theclimsdeinig21e 99371 The claims defining the invention are as follows 1. In a fuel assembly for a nuclear reactor wherein a

   plurality of elongated fuel rods are held in parallel rows, the

   improvement which comprises a grid structure having a plurality

   of key slots formed therein and a plurality of el6ngated fuel

   rods each havirg, at one end thereof, a key receivable in.a

   corresponding one of said key slots of said grid structure to

   hold and support the fuel rods in said parallel rows; the: keys of all fuel rods in each row being similar in configuration

   and disposed in alignment with one another for reception in

   corresponding grid structure key slots, said key slots being

   arranged in parallel rows acconmodating the installation and

   removal of said grid structure by movement along a path generally

   perpendicular to the longitudinal axes of said fuel rods.

   2. The improvement according to claim 1 including an

   upper grid structure having,a plurality of said key slots* a

   plurality of elongated fuel rods each having an upper end key

   received in a correspondig slot of said upper grid structure,

   to suspendingly support the fuel rod therefrom, a lower grid structure receiving the lower ends of said fuel rods and

   constraining same against lateral movement relative to one another, and control rod guide tubes connecting said lower grid

   structure to said upper grid structure. 3o The improvement according to claim 1 or claim 2 wherein

   said key slots are T-shaped and said keys are T-shaped in longitudinal cross-section. 4, The improvemert according to claim 2 wherein said control rod guide tubes connecting the lower grid structure to the upper grid structure are each releaseably connected at

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   23,99 3I corresponding ends to said upper and lower grid structures.

   DATED this 4th day of September, 1972.

   THE BABCOCK WILCOX CC2HPANY Patent Attorneys for the Applicant: F.B RIC CO1.

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