IL35920A - Fuel assembly for a nuclear reactor - Google Patents

Description

Fuel, assembly for a nuclear reactor ΪΗΕ BABCOCK & WILCOX COMPANY fhe present invention relates in general to the construction of nuclear reactors, d 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 multiplicity of fuel rod© and guide tubes are rigidly secured to both the upper and lower grid structures. Access for replacement 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.

Other typical prior art fuel assembly designs are shown, for example in British Patent Ho. 1,132,919 and Israel Patent Io» 14644. In the British atent, a grid for supporting an array of nuclear fuel rods has a plurality of nodes. Soundhead rivets at these nodes each egage a complementing and mating surface that is ormed in the end of a respective fuel rod. The Israel latent discloses nuclear fuel rods, each mounted within an individual tubular jacket, as distinguished from the grid structure that characterises the British Patent.

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 embodimen of fuel element assembly in this invention there is provided an upper end grid to accommodate the guide tubes and fuel pins, f e grid has a plurality of key slots for receiving a plurality of elongated uel rods each of which has an upper end key part 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 the fuel rods are arranged in parallel rows as are the upper grid key slots, when the fcey parts o the fuel pins are in registry with the ke slots the upper grid can be installed by sliding it along a path generally perpendicular to the axes of the fuel rods while disengagement 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 opera'ting 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 is a top view in perspective of a typical portion of the lower end grid used in the fuel assembly of Figure 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 l respectively and by one or more intermediate spacer grids . ■ ,.· · ' ■ [ ' ■ - In the fabrication of fuel assembly 10, the rods 11 and guide tubes 2 are inserted into the spacer grids 15 before = , installing end grids 13 and Ik. Guide tubes 12 serve as conduits 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 1 is connected to and thereby supported by upper end grid 13 by means of the guide tubes 12, each o 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 ΐβ 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 1 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 lb into assembly 10.

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 part 18 is expediently in the form of a cylindrical stem section 20 connected to a cylindrical disk section 21 such as to give key part 18 a T-shaped longitudinal cross-section that is compatible with the T-shaped configuratio of key slots 19.

Expediently, all key parts 18 are o the same configuration, as" are all key" slots 19, as indicated by Figure 3 · sem y In assembling the fuel element /££), the rods 11 are a uniformly positioned axially in grids 15 such that the key parts l8 of all rods 11 in each row thereof are aligned with ne another. Key slots 19 are arranged in parallel rows and have the same lateral spacing as corresponding key parts l8 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 key parts l8 are seated in key slots 19.

When each key part l8 is received in its associated slot 19* the guide tubes 12 are advanced through respective fittings l6 of grid 13 and nuts 17 are screwed onto the tube ends.

With grid 13 thus installed, the lower end grid l4 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 1 is secured to tubes 12 by nuts 17. The lower grid 1 also receives the lower ends of fuel rods 11 and constrains them against later^al movement relative to one another. For such purpose, there are provided in grid 1 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 10.

As will be appreciated by the artisan, the foregoing described features of the invention result in a fuel assembly 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 1 , 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.

Claims (2)

1. In a a wherein a plurality of elongated fuel rods are held a predetermined parallel relation to one the improvement which comprises a grid having a plurality key and plurality of elongated fue rods each one end key received in corresponding slot of said grid to hold and support the fuel rod therefrom in the predetermined parallel according to Claim 1 including upper grid having a plurality of said key plurality elongated fuel rods eac having an upper end key part received in a corresponding slot of said grid to suppor the fuel a lower receiving the lower ends of said fuel rods constraining against lateral relative to connecting said lower grid upper improvement according to Claim 2 wherein said means connecting th lower grid to th upper grid a plurality of guide tubes receiving control each guide being releaseably connected at corresponding to said upper and lower according to

2. Claim 2 o 5 said fuel rods are arranged in parallel and key parts of all fuel rods in each row similar in configuration disposed in alignment one another for in corresponding said slots being arranged in parallel rows the installation and removal of said grid by movement along a path transverse o the longitudinal axes of said el The improvement according to Claim o 4 wherein said key slots are key parts are longitudinal A assembly a nuclear reactor hereinbefore described wit reference to and as illustrated the accompanying For the Applicants insufficientOCRQuality