KR910004194B1 - Recontituting a nuclear reactor fuel assembly - Google Patents

Abstract

No content.

Description

Reassembly of Reactor Fuel Assemblies

1 is an elevation showing the design of a fuel assembly incorporating an upper nozzle of the present invention in a control rod guide thimble connection system.

2 is a plan view of the fuel assembly shown in FIG.

3 is an enlarged cross sectional view of the prior art in which a guide thimble and an upper nozzle are welded to a connection system to which the fuel assembly reassembly method of the present invention may be applied.

4 is a partially enlarged cross-sectional view of FIG. 1 showing a connection system embodiment of the present invention in more detail. 4 also shows the results of the fuel assembly reassembly method of the present invention carried out in the connection system of FIG.

5 shows an alternative embodiment of the connection system of FIG. FIG. 5 also shows the results of the fuel assembly reassembly method of the present invention carried out in the connection system of FIG. 4 (as in the second reassembly of the fuel assembly).

* Explanation of symbols for main parts of the drawings

10 reactor fuel assembly 16 fuel pellets

18: spring 20: plug

24: upper nozzle 26: lower nozzle

32: upper nozzle connecting plate 50: guide thimble

52: sleeve 62: connecting plate

72: control rod passage

The present invention relates generally to fuel assemblies in nuclear reactors, and in particular to methods of reassembling fuel assemblies by removal and connection of fuel assemblies and upper nozzles that have been reassembled with an improved top nozzle connection device.

The customary design of the fuel assembly includes a plurality of fuel rods and control rod guide thimbles maintained in an organized arrangement by a regularly spaced grid along the fuel assembly length. The grid is attached to the control rod guide thimble. The upper and lower nozzles located at opposite ends thereof are firmly fixed to the control rod guide thimbles which slightly extend the fuel rod ends upward and downward. At the upper end of the assembly, the guide thimble is connected to a hole provided in the upper nozzle. The guide thimble may be surrounded by a sleeve to connect to the top nozzle and top grid of each.

During the operation of such aggregates in a reactor, cracks initially caused by internal stresses can sometimes develop along their length, so that radioactive fission products can leak or flow into the reactor's primary cooling water. . Such fission products may also be released to flooded reactor cavities during fuel change operations and to coolant circulating through pools where spent fuel assemblies are stored.

Under these circumstances, it is difficult to detect and remove a dead fuel rod. For they are part of a complete collection of guide tubes welded to the upper and lower nozzles. As a result, it is necessary to remove the waste aggregate from the reactor core and to break the welds that secure the upper nozzle to the control rod guide thimble in order to make an upper approach to each fuel rod. By doing so, destructive behavior often makes the fuel assembly unsuitable for further use. This is because of damage to the upper nozzle and guide thimbles that prohibit re-welding.

In view of the high cost of replacing fuel assemblies, interest has been shown in fuel assemblies that can be reassembled in order to minimize the operation and maintenance costs of reactor installations at home and abroad.

Customary reassembleable fuel assemblies are loaded with the design features arranged at the beginning to allow for the rapid removal of each spent fuel rod, the storage of the spent fuel assembly, and the normal handling and additional use of the reactor. Has the option to replace. Reassembly may be enabled by providing a fuel assembly with a removable top nozzle. The upper nozzle is fixed mechanically by an arrangement fitted at the upper end of each control rod guide thimble assembly. And the upper nozzle can be remotely removed from the irradiated fuel assembly while submerged in the neutron absorbing solution. After the top nozzle with rod removal / displacement is re-installed in the control rod guide thimble tubing, the reassembled assembly can be reinserted into the reactor and used to the end of its life, spent fuel pools and other locations in a safe and normal manner. Are stored in.

The customary design principle of existing reassembleable fuel assemblies has been an expensive reassembly characteristic of all fuel assemblies at the beginning of manufacture. Various inserts or other arrangements for releasably connecting each control rod guide thimble to the upper nozzle have only a small fraction of the fuel assembly having fuel rods that tend to leak so that during additional manufacturing operations when reassembly is needed Indicate a high price for time and money.

The main objective is to prepare a method for reassembling a normal fuel assembly and preparing a reassembled fuel for use in a reactor.

In view of this object, at present the present invention has a control rod guide thimble connected to the upper nozzle connecting plate and disposed axially in the control rod passage of the upper nozzle connecting plate, in which the guide thimble wall separates the sleeve from the connecting plate. The lower part of the connection surface is cut in the circumferential direction, the upper nozzle including the connecting plate is removed from the cut guide thimble, and the cut guide thimble is inserted into the second upper nozzle to insert a plurality of grooved control rod passages. The second upper nozzle having a connecting plate is disposed axially in the subsurface corresponding to the first upper nozzle connecting plate with the sleeve separated, and the sleeve is connected to the second upper nozzle at an axial distance above the groove. A method for reassembling a reactor fuel assembly inserted into a plate, wherein the inserted cutting guide thimble is bulged in the groove in the circumferential direction. Related.

The invention is more readily apparent from the following description of the preferred embodiment, which is illustrated by way of example only in the accompanying drawings, with the benefit of being best understood by reference to the following description taken in conjunction with the accompanying drawings.

Referring now to the drawings, in which reference figures refer to the same or corresponding parts throughout the several views, a reactor fuel assembly 10 comprising an array of fuel rods 12 in FIGS. It is thus shown that the grids 14, 15, 17 are supported in a uniformly spaced relationship with each other. Each fuel rod includes the end of the rod sealed by the plug 20 at the end of the spring 18 located at each fuel rod plenum of the fuel pellet 16 and everything in customary manner.

To control the cleavage process, a plurality of control rods 21 can be moved to each other in a guide thimble or control rod guide tube located at a predetermined position within each selected fuel assembly in the reactor. The guide thimble is fixed to the grips 14, 15, 17. The reactor has an upper nozzle 24 and a lower nozzle 26 at opposite ends of the control rod guide thimbles that are stuck to form a complete assembly that is likely to be handled customarily without damaging the assembly components.

Typically the guide thimble has a sleeve for weldability of the upper 14 and lower 17 grids with the upper 24 and lower 26 nozzles. In this case the sleeve 53 is used to couple the guide thimble 50 to the lower grid 17 and the lower nozzle 26 as shown in FIGS. 1 and 4. Each guide thimble 50 extends over the entire length of the fuel assembly 10 between the upper nozzle 24 and the lower nozzle 26 and the sleeve 54 extends only from the lower nozzle 26 to the lower grid 17. Guide symbol 50 is secured to sleeve 53 by a bulge fit. The bulge pit is typically three axially spaced sets (two sets between the top nozzle connecting plate 32 and the top grid 14 and the top) with each set consisting of four symmetrically spaced bulges. A bulge 55 arranged in a grid 14 or less). Guide thimble 50 may be secured to sleeve 54 in a similar manner.

As shown in FIGS. 1 and 2, the upper nozzle consists of a housing 28 having a top plate 30 that is square (bottom) in cross section and spaced apart from the connecting plate 32. The assembly papermaking spring 34 fixed to the opposite side of the top plate 30 is supported by bolts 36 to be held in place and squeezed when the reactor top core (not shown) is in place. The upper nozzle further has a rod cluster control assembly 37 consisting of a cylindrical element 38 inserted therein with radially extending fluke or arms 40. The interconnector 42 is an arm that interconnects each control rod 21 with its arrangement such that the rod cluster assembly moves the control rod vertically within the control rod guide thimble, thereby controlling the cleavage process within the assembly. do.

A preferred embodiment of the present invention, which is a method of reassembling the reactor thermal mass assembly 10 by removal and displacement of the upper nozzle, is shown in FIG. 3 as a "before" drawing and in FIG. 4 as a "after" drawing. The control rod guide symbol (only one shown in FIG. 3) has a top sleeve 52 axially disposed in the control rod passage 60 of the upper nozzle connecting plate 62. The sleeve 52 is connected in good condition to the upper nozzle connecting plate 62 by welding (or other metallurgical connection) on the upper surface 64 of the connecting plate. This type of conventional connection system requires less time and money compared to reassembleable fuel assemblies that use connection systems inserted into all assemblies at initial manufacturing.

When a decision is made to reassemble the fuel assembly 10 (which occurs only at a small percentage of the fuel assembly for the lifetime of the use), the connecting plate (circularly cut) is used to circumferentially cut the sleeve wall below the connection surface using the method of the present invention. Remove sleeve 52 from 62. For example, such cutting can be accomplished in the control rod passage 60 using a custom intubation cutter thereby cutting the sleeve wall under the weld. This cutting can likewise proceed from the upper surface 64 of the connecting plate 62 to a depth and diameter sufficient to cut the weld using a conventional hole saw type cutter. The upper nozzle with connecting plate 62 is removed from the grid sleeve by conventional means. Cutting and removal operations are performed remotely to the irradiated fuel assembly 10 while immersed in a neutron absorbing solution such as water and provide a passage at the upper end of the fuel assembly for detection and removal of the spent fuel rods.

The modified upper nozzle 24 is obtained in the same form as is common for the upper nozzle removed with a connecting plate 32 having a number of such passages 72 with channels or grooves 74. The groove 74 is located axially below the corresponding surface on the upper nozzle connecting plate 62 removed from where the sleeve 52 has been separated. The grooves 74 may be concave in various dimensions everywhere in the passageway 72. Preferably the modified top nozzle 24 is not irradiated, but the removed top nozzle can be retrofitted on demand. The formation of grooves can be accomplished by customary cutting methods known to those skilled in the art.

A cutting sleeve 53 with a control rod guide thimble 50 is inserted into the control rod passageway 72 of the water plate nozzle 24 connecting plate modified at an axial distance above the groove 74. Preferably, the cut sleeves 53 are inserted from the bottom of the passageway 72 in the same axial position generally as where they were previously cut. A corresponding number of insert sleeves 53 are bulged into the grooves 74 in the circumferential direction. By “corresponding majority” herein is meant a sleeve 53 inserted into a plurality of passageways 72 with grooves 74. Circumferential bulging is carried out by customary methods known to those skilled in the art. For example, bulging is caused by the hydraulic pressure of the fluid entering the control rod passage through a hole in the fluid supply tubing having a seal on the top and bottom of the groove for containing the fluid in the passage to bulge the sleeve into the groove. Is achieved. Insertion and bulging can be performed remotely.

For example, the method described above also allows fuel rods access within the assembly to account for the transfer of partially used fuel rods from one damaged skeleton to another, allowing the fuel rods to achieve the intended combustion level. Do it. The present invention may also take an approach for relocating and removing fuel rods to obtain better uranium utilization in the reactor.

The method of the present invention can also be applied to reassemble a fuel assembly having control rod guide thimbles (such as welding) directly attached to the upper nozzle connecting plate without any intervening sleeves. In this case cutting and bulging will be performed on the guide thimble instead of the sleeve. The present invention is also applicable to reassembly of a fuel assembly having a guide thimble (or sleeve) circumferentially bulged into a groove in a connecting plate control rod passage (initially or at the time of primary manufacture). It will be appreciated by those skilled in the art. This allows for additional reassembly of previously reassembled fuel assemblies. For example, FIG. 4 may represent a "previous" drawing and FIG. 5 may represent a "after" drawing of two reassembled fuel assemblies. In FIG. 5 the lower groove 90 of the control rod passage 92 in which the control rod guide thimbles 50 themselves, as well as the sleeve 80 in which the control rod guide thimbles 50 are cut twice, are in the connecting plate 94 of the other upper nozzle. It is circumferentially circulated within. The groove 90 is situated below the corresponding surface of the upper nozzle connecting plate 32 in FIG. 4 with the sleeve 53 separated. The circumferential bulging of the sleeve 80 due to the low position of the groove also requires the circumferential bulging of the overlapping control rod thimble 50.

It can be seen from the previous discussion that the present invention includes a reactor fuel assembly top nozzle in a control rod guide thimble connection system. The connection system can be used in the initial manufacture of the fuel assembly and better used in reassembling the fuel assembly. The use of the connection system of the present invention only in reassembly results in less time and money when the initial fuel assembly uses an upper nozzle welded to the control rod guide thimble connection. This is due to the fact that only a small proportion of the fuel assemblies require reassembly.

As shown in FIG. 4, a preferred embodiment of the upper nozzle in the guide thimble connection system comprises a connecting plate 32 having a plurality of (at least two or preferably both) control rod passages 72 modified with grooves 74; Apply the top nozzle together. The connection system also uses a control rod guide thimble 50 with a sleeve 53. The sleeve is disposed axially in the control rod passage. The corresponding number of sleeves is located above the groove in the axial direction and bulged circumferentially in the groove. "Corresponding number" means a sleeve with which the number of control rod passages with grooves is associated.

Another replacement connection system (not shown in the figure) has a guide thimble without a sleeve with a guide thimble circumferentially circumferential in the groove. Another connection system (FIG. 5) has a guide thimble 50 and a sleeve 80 circumferentially bulged in the groove 90.

Optionally, a means for positioning the sleeve (or guide thimble) at a predetermined axial distance in the control rod passage is applied. As shown in Figure 5, this means preferably determines the length of the sleeve that has been previously cut so that the passage of the passage by means of a shodder face axially positioned to maintain a constant height of the upper nozzle in the fuel assembly. An upper diameter reduced diameter portion coupled to the normal diameter portion. Alternatively, the sleeve 80 may fit snugly with a collar adjacent to the lower side of the connecting plate 94.

Tensile strength tests on control rod guide thimble sleeves circumferentially circumferentially circumferentially circumferentially grooved in the control rod passage of the upper nozzle connecting plate show that the connection is stronger than the sleeve itself.

Claims (5)

In the method of reassembling a nuclear reactor fuel assembly having a control rod guide thimble (50) disposed axially in the control rod passage (60) of the upper nozzle connecting plate (32) and secured to the upper nozzle connecting plate (32). The thimble wall is cut circumferentially below the connecting surface to separate the sleeve from the connecting plate 32, the upper nozzle 24 including the connecting plate is removed from the cut guide thimble 50, and the cut guide thimble 50 is inserted into the second upper nozzle 24, and the second upper nozzle 24 is axially mounted under the surface corresponding to the first upper nozzle connecting plate 32 from which the sleeve 50 is separated. A connecting plate with a plurality of control rod passages 72 with grooves 74 arranged in a direction, the sleeve being inserted into the second upper nozzle connecting plate 32 at an axial distance of the upper portion of the groove, Cutting guide thimble 50 is circumferentially formed in the grooves 74 and 90. How the reassembly of a nuclear reactor fuel assembly, characterized in that a bulge. A guide thimble (50) is provided with sleeves (53, 80), and the sleeve is connected to the guide thimble (50) to form part thereof, and the sleeves (53, 80) are grooved. A method for reassembling a nuclear reactor fuel assembly, characterized in that it is bulged within (74, 90). A reactor fuel assembly consisting of an upper nozzle 24 with a filter plate 32 having a control rod passage along with a control rod guide thimble structure 50 disposed axially within the passage 72, the passage 72 being a It has a circumferential groove (74, 90) inside the wall, the thimble structure (50) is spread over the axial distance above the groove (74, 90), characterized in that circumferentially bulging into the groove (74, 90) A method for reassembling a reactor fuel assembly. 4. The guide thimble structure (50) according to claim 3, wherein the guide thimble structure (50) comprises a sleeve fitted in the thimble (50) and the sleeves (53, 80) are unfolded in the passage (72) and bulged in the grooves (74, 90). Method for reassembling the reactor fuel assembly, characterized in that the. 5. The method of claim 4, wherein means (96) are provided for positioning said sleeve (80) at a predetermined axial position within said passageway (92).

Images