EP0051314A1 - Process for reducing the length of at least one spent nuclear fuel rod, casing for performing this process and device for storing a nuclear fuel rod of reduced length - Google Patents

Abstract

It is known for the purpose of reducing the length of a spent fuel rod firstly to introduce the fuel rod into a straight containment tube (4', 4'', 26) and then to deform the rod in order to reduce its length, by winding up the containment tube with the fuel rod included therein. This length-reducing process leads to difficulties in producing the storage vessels provided for storing the coils. To avoid these difficulties it is proposed to assign to the containment tube (4', 4'', 26) at least one deforming element (9), which covers a fuel rod section of prescribed sublength in a sleeve-like fashion, is connected to the containment tube in a gas-tight fashion and remains gas-tight under deformation, and to bend the fuel rod by essentially 180<o> in the deformation region determined by the deforming element, the deforming element being deformed in accordance with the bend, and the containment tube remaining rectilinear over a substantial part (4', 4'') of its length. In this process, the deforming element can be constructed separately or can be a one-piece component of the containment tube. In the latter case, the cross-sections of the deforming element and containment tube can be equal. (See Figure 7a). <IMAGE>

Description

The invention relates to a method for reducing the length of at least one irradiated fuel rod, in which the spring rod is first introduced into an enclosing tube and then deformed to reduce the rod length of the fuel rod.

For the purpose of the interim storage and / or the final storage of irradiated nuclear fuels, such a method for reducing the length of the irradiated brent rods is known (report KfK 3000, Sept. 1930, pp. 465-466), on the one hand, making it easier to handle irradiated nuclear fuels in the deposit and on the other hand, to be able to use ceramic materials as packaging material which are easier and more economical to produce in smaller geometries. In the known method, the spent fuel rods are introduced individually or in groups into a flattened steel encasing tube and then the encasing tube is welded in a gas-tight manner. Preferably, two fuel rods are placed in each containment tube. To reduce their length, for example 4 m, the fuel rods welded into the metallic containment tubes are wound around a mandrel to form a spiral, the diameter of which depends on the smallest allowable bending radius of the containment tube.

When it is wound up, the nuclear fuel, which is preferably in pellet form, breaks into fragments and the cladding tubes of the fuel rods, which envelop the nuclear fuel, tear open. Since the containment tubes remain gas-tight during the deformation caused by the winding, due to the properties of the material used, they retain the radioactive substances and / or gases which are released when the fuel rod is deformed. In some applications, the gas pressure of the gas in the fuel rod is around 80 bar. The lower pressure which arises after deformation in the sheathing tube depends on the empty volume enclosed by the sheathing tube. In the known method for length reduction, the length of the individual fuel rod is reduced, but the deformation leads to spirals with large diameters and to large dead spaces, which are determined by the cylindrical mandrel volume in the middle of the wound fuel rods. The large outer diameters of the wound fuel rods complicate the use of certain ceramic packaging materials for packaging into which the deformed fuel rods located in the sheathing tube are to be packaged, since these packages are made of the ceramic materials in the dimensions required then only in a technically complex manner or not at all have it made. In addition, the large dead spaces determine a small usable volume of the packaging.

It is an object of the present invention to provide a method for reducing the length which enables the length of the fuel rods to be reduced without the usable volume of the packaging being reduced by undesirably large dead spaces when the deformed fuel rods are introduced into the packaging and without the production of the packaging being made difficult by specifying too large dimensions of the receiving cavities of the packaging.

This object is achieved according to the invention in that at least one deformation element covering the fuel tube in a sleeve-like manner, gas-tightly connected to the containment tube and remaining gas-tight under deformation, is assigned to the containment tube and that the fuel rod is bent by essentially 180 ° in the deformation area determined by the deformation element. wherein the deformation element is deformed in accordance with the kink and the enclosing tube remains essentially rectilinear.

By the one or more gas-tightly connected to said enclosure tube deformation elements, it is possible to fold the fuel rod is always at one or more locations substantially 180 ⁰ extent that the rectilinearly remaining when using a deformation element portions of the confining tube pairs are placed substantially in parallel position, and almost come into contact with each other.

With this buckling deformation, no large dead space is built up; the receiving cavities of the packaging can be realized with small diameters.

The fuel rod is either split up when kinking or it is at least partially broken up before kinking. A preferred embodiment of the partial division can be seen in the flattening of the deformation element and the fuel rod before the buckling in the deformation area. This pre-deformation the deformation element is preferably carried out by rolling or pressing.

In a particularly preferred method, the enclosing tube is designed with a constant cross-sectional configuration over substantially its entire length and ensures a gas-tight buckling deformation, and the fuel rod and the wrapping tube are bent into a predetermined length section serving as a deformation element. In this procedure is indeed used as in the known method, a sheath tube which is substantially over its entire length has a constant cross-sectional configuration, but is performed at least one buckling by substantially 180 ⁰ such that a straight line remaining Umschließungsrohrabschnitte come together in abutment. Austenitic steel is preferably used as the material for such a cladding tube with integrated bend deformability.

On the basis of the known casing for at least one irradiated fuel rod with an encasing tube surrounding the fuel rod, the invention is also directed to a casing for carrying out the method according to the invention.

According to the invention, the sheathing is characterized in that at least one deformation element is assigned to the encasing tube, which covers a fuel rod section of a predetermined partial length in a sleeve-like manner, is connected in a gastight manner to the encasing tube and remains gas-tight when the fuel rod is bent. The deformation element can be realized in different ways. The deformation element is preferably pushed onto the enclosing tube and connected to it in a gastight manner. The deformation element can with the Enclosure tube, for example, welded, soldered or glued. The connection can be established over the entire axial length of the deformation element or can be formed only at its end regions.

However, it can also be expedient for the deformation element to be arranged between sections of the enclosing tube and to be connected in a gastight manner to the facing ends of the enclosing tube. The connection techniques mentioned above are also suitable here. Of course, it must be ensured that the connections remain gas-tight when buckling. Finally, there is the possibility of integrally forming the deformation element with the sheathing tube.

The deformation element can be designed in such a way that it is deformed plastically or elastically or plastically-elastically during mechanical bending by the forces applied from the outside.

If the deformation element is formed in one piece with the sheathing tube - but also if the deformation element is manufactured separately - the deformation element can have a greater wall thickness in the area of greater material stretching than in the area of smaller material stretching.

If the deformation element is formed in one piece with the sheathing tube, the sheathing is particularly expedient and easy to manufacture if the deformation element has a cross-sectional configuration that is constant over its length and ensures a gas-tight buckling deformation, and the cross-sectional configuration of the sheathing tube is equal to that of the deformation element, i.e. the cross-sectional configuration of the Cladding tube corresponds substantially to the entire length of the sheath tube of a gas-tight buckling strain 180 ^0-locking cross-sectional configuration.

Further subclaims directed to the casing relate to advantageous embodiments of the casing according to the invention.

The method according to the invention and various embodiments of the casing will now be described in more detail with reference to the attached figures.

• Figur 1) Die erste Ausführungsform der Umhüllung mit einem auf das Umhüllungsrohr aufgeschobenen und plastisch verformbaren Verformungselement,
• Figur 2) Die zweite Ausführungsform mit einem in das Umhüllungsrohr eingeschalteten plastisch verformbaren Verformungselement, das vor Knickung vorgeformt wird,
• Figur 3) Die dritte Ausführungsform in Form eines in das Umschließungsrohr eingeschalteten Verformungselement mit einem mindest teilweise elastisch verformbaren Kompensator,
• Figur 4) Eine weitere Ausführungsform mit elastischem Kompensator,
• Figur 5) Eine Ausführungsform mit elastischem Kompensator und einem Trennkeil zum mindestens teilweisen Zerteilen des Brennstabs vor Durchführung des Knickvorgangs,
• Figur 6) Drei jeweils in einer Knickstelle geknickte Umhüllungsrohre gemäß Figur 4) in Lageranordnung,
• Figur 7) Vier jeweils in einer Knickstelle geknickte Umhüllungsrohre gemäß Figur 2 in Lageranordnung,
• Figur 8) Ein zweifach geknicktes Umhüllungsrohr gemäß Figur 2 und,
• Figur 9) Ein in einer gegenüber Figur 8 abweichenden Weise geknicktes Umhüllungsrohr gemäß Figur 2 das sich insbesondere zur kreisringartigen Lageranordnung der Brennstäbe eignet.

It shows:

• FIG. 1) The first embodiment of the sheathing with a plastically deformable deformation element pushed onto the sheathing tube,
• FIG. 2) The second embodiment with a plastically deformable deformation element which is switched into the sheathing tube and which is preformed before kinking,
• FIG. 3) The third embodiment in the form of a deformation element switched into the enclosing tube with an at least partially elastically deformable compensator,
• Figure 4) Another embodiment with an elastic compensator,
• FIG. 5) An embodiment with an elastic compensator and a separating wedge for at least partially cutting up the fuel rod before carrying out the buckling process,
• FIG. 6) three sheathing tubes according to FIG. 4), each bent in a kink, in a bearing arrangement,
• FIG. 7) Four cladding tubes according to FIG. 2, each bent in a kink, in a bearing arrangement,
• FIG. 8) A double-folded cladding tube according to FIG. 2 and,
• FIG. 9) A cladding tube according to FIG. 2, which is bent in a way that differs from FIG. 8 which is particularly suitable for the annular bearing arrangement of the fuel rods.

FIG. 1 a shows a section of a fuel rod 1, which usually consists of a cladding tube 2 and fuel 3 introduced into the cladding tube in the form of fuel pellets, as indicated in FIG. 1 a below. In the following FIGS. 2-9, the fuel rod 1 is shown as a solid rod in order to facilitate understanding of the actual invention. The fuel rod 1 is inserted into an enclosing tube 4, the ends of which are sealed gas-tight.

A deformation element 5 is pushed onto the enclosing tube 4 and connected to the surface of the enclosing tube in a gas-tight manner by welds 7 applied to the ends of the deforming element. Although in some cases a configuration of the deformation element as a pipe section is possible, in the preferred embodiment according to FIG. 1 the deformation element is provided with a thicker outer wall in the area of the larger material stretching during buckling than in the area of smaller material stretching, so that after buckling a minimum wall thickness in the area greater material stretch is present. In addition, the deformation element 5 is curved in the manner shown in Figure la, so that a cavity 6 is formed between the deformation element 5 and the enclosing tube 4, whereby the necessary elongation of the material is reduced.

To carry out the deformation, the encircling tube 4 according to FIG. 1 a is placed approximately in the middle of the deformation element 5 on a mandrel 8 shown in section in FIG. 1 c and bent over this mandrel of small diameter to such an extent that the two legs 4 'and. 4 '' of the enclosing tube lie parallel to each other. The one after pulling out The dead space remaining in the deformation mandrel is negligibly small in comparison to the volume of the enclosing tube. The shape of the deformation element 5 shown in FIG. 1b in the plane perpendicular to the axis of the inserted fuel rods 1 and 1 ', which deviates from a circular cross-section, ensures that the plastic material deformation is minimized in the highly stressed buckling plane and cracks such as them eg when kinking round pipe cross-sections, can be avoided.

In the embodiment shown in FIG. 2, the deformation element 9 is arranged between pieces 4'u.4 "of the enclosing tube 4 and is gas-tightly connected by welding 7 to the facing ends of the two sections of the enclosing tube 2c in order to carry out the mechanical buckling, the deformation element 9 is deformed by means of two rollers lo and lo 'in the manner shown in FIG. 2a in the region of the kink with the enclosed fuel rod, the cladding tube and the structure of the ceramic fuel The change in cross-section by the two rollers lo and lo 'takes place in such a way that the fiber to be stretched for bending is shifted towards the neutral fiber in such a way that the elongation of the fiber is minimal during the bending process.

A cross-sectional change of the deformation element can e.g. also done by pressing. The only thing that matters is to shift the fibers to be stretched towards the neutral fibers.

In order to protect the deformation element from fragments of the broken ceramic fuel possibly emerging from the cladding tube 2, protective plates 11 and 12 can be provided on the inner surface of the deformation element 9 be provided in the arrangement shown in FIG. 2e, which prevent the contact of fuel bodies under the deformation pressure with the deformation element when the cross section is changed by external forces.

The pre-deformation described can of course also be carried out with a deformation element formed in one piece with the sheathing tube; but also with a sheathing tube, the cross-sectional configuration of which is adapted over its entire length to the buckling process or processes.

In the two previously described embodiments of the casing according to the invention, plastic deformation occurs essentially during the buckling process. In the embodiment shown in FIG. 3, a partial elastic deformation of the deformation element also occurs. The deformation element 13 consists of a welded 7 with two sections 4 'u. 4 "of the sheath tube connected with an approximately central predetermined breaking point 14a and a compensator 15 connected to the sleeve in a gas-tight manner and enclosing the predetermined breaking point, which has a depression or other marking 15a for identifying the predetermined breaking point 14a In one embodiment, the ends of the compensator 15 are connected to the predetermined breaking sleeve 14 by means of welds 16. Of course, other gas-tight connection techniques can also be used here The compensator consists of a section 15b which is subjected to a plastic deformation during the bending process and a section which is essentially subjected to an elastic deformation during the bending process Section 15c In the embodiment shown, the two sections 15b and 15c of the compensator 15 are formed in one piece with one another.

When buckling, the deformation element 13 is placed in the region of the depression 15a on a buckled edge 8 'shown in dashed lines and buckled around it. The fuel rod is divided by the reinforced portions of the predetermined breaking sleeve 14 at the predetermined breaking point 14a. In the case of plastic-elastic deformation from the position according to FIG. 3a to the position according to FIG. 3c, the compensator 15 ensures that the kink is secured against gas leakage. The approximately larger dead space that occurs as a result of the larger diameter of the two sections 4'u.4 '' of the enclosing tube is still negligibly small and can be reduced by selecting the appropriate material for the predetermined breaking sleeve by reducing the wall thickness.

In the embodiment of the casing shown in FIG. 4, the deformation element 17 has two sleeves 18 and 19, which are located coaxially opposite one another. Shear rings 2o and 21, which determine shear edges 2oa, 21a, are introduced into recesses at the opposite ends. An elastically deformable compensator 22 is connected to the two mutually opposite sleeves 18 and 19 in a manner comparable to that in FIG.

To initiate the length reduction, the two shear sleeves are first shifted against each other in such a way that the fuel rod is divided (see FIG. 4b), the two sections are separated from one another (see FIG. 4c) and bent until they assume the position shown in FIG. 4d . From the above description of the embodiment according to FIG. 4 it can be seen that in the claims and in the description the term “buckling” is also understood to mean such a process in which when the 180 ° pivoting movement is carried out the two Fuel rod sections are no longer connected.

In the embodiment according to FIG. 5, a deformation element 23 is used, which is also connected to sections 4 ′ or 4 ″ of the sheathing tube 4 via welds 7. The deformation element 23 here has the shape of a compensator, which is comparable to the compensator according to FIG. 3 and thus also consists of two sections 23a and 23b, the section 23b being essentially plastically deformable, as the transition from FIG. 5a to 5c clearly shows. As in the embodiment according to FIG. 3, the two compensator sections can be formed in one piece with one another or separately from one another, in which case both sections must of course be connected to one another in a suitable manner (e.g. welding).

A section 23c is assigned to the essentially elastically deformable section 23b in the manner shown in FIG. 3 and carries a straight separating wedge 24 directed inwards (cf. FIG. 5d). Instead of the straight dividing wedge, a crescent-shaped dividing wedge can also be used.

On the surface section 23a of the deformation element 23 opposite the separating wedge, a separating wedge abutment 23d is formed, on which the fuel rod is supported.

To initiate the buckling process, the separating wedge 24 is loaded by a stamp 25 acting on the compensator from the outside, so that it is pressed into the cladding tube 2 of the fuel rod and into the fuel 3 of the fuel rod 1. This will completely split the fuel rod or weakened at the intended kink point so that it is divided during the subsequent kinking. Here, too, the compensator 23 ensures a gas-tight enclosure of the kink during the separation and kinking process.

In the embodiments according to FIGS. 3 and 4, the sleeves 14 or 18 and 19 can also be formed in one piece with the sheathing tube 4.

In FIG. 3, a compensator in the form of a corrugated metal hose can also be used, as the compensator according to FIG. 4 has.

When using a compensator, the compensator as a whole or its elastically deformable sections can be pre-shaped like a bellows to impress the elastic deformability.

Steels are particularly suitable for achieving elastic shape behavior. For the plastically deformable deformation elements or sections thereof, tough metallic materials are used which have a high elongation at break behavior.

FIG. 6a shows three sheathing tubes of the embodiment according to FIG. 4, each deformed by a kink. The free ends of the two sections 4'u.4 '' are each closed by a closure cover 26 which is connected gas-tight to the associated section, for example welded . An attachment for a manipulator is provided on the free end face of the cover. The fuel rods reduced in length by the one kink have been brought into a U-shape. When packing the fuel rods, they can be layered in the layers shown in FIGS. 6b can be packed. The deformation elements 22 of the individual layers each lie on one side of the layer, while the straight-line sections 4'u.4 "of one layer touch the sections of the other layer in the manner shown in FIG. 6, the checkered gussets remaining between the pipe sections, which, however, represent a very small dead space compared to the volume of the cladding tubes.

A bearing arrangement is described in FIGS. 7a and 7b, as can be achieved with simply bent fuel rods according to FIG. 2. As can be seen from the section according to FIG. 7b, a packing density which is hexagonal in section is achieved. The gussets are only triangular. In this arrangement too, the deformation elements of a layer lie on one and the same side of the layer.

FIG. 8a shows the side view of a double-bent jacket tube 4 according to FIG. 2. Two deformation elements 9 are therefore welded into the sheathing tube, so that it consists of the sections 4 ', 4' 'and 4' ''. The deformation elements 9 have each been rolled such that the directions of buckling are not identical, but rather are oriented so that when buckling the configuration shown in FIG .4 '' 'essentially define an equilateral triangle with a plane perpendicular to them. An essentially hexagonal packing configuration can also be achieved with fuel rods bent in this way.

FIG. 9 shows a double-folded envelope tion tube, in which two deformation elements 9 according to FIG. 2 are switched on. The deformation elements 9 are rolled in such a way that the bearing arrangement shown in FIG. 9b can be reached, in which the bent fuel rods can be arranged like a ring. In the cylinder-like free space remaining within the circular rings constructed from such bent fuel elements, rods can be introduced, for example, which are bent according to FIG. 8, so that an essentially hexagonal packing density can also be achieved here.

The bearing arrangements shown in FIGS. 6-9 can also be achieved with the other deformation elements. In configurations with several kinks, the separately or integrally formed deformation elements with a pre-embossed kink direction must of course be aligned in a predetermined manner when they are applied to the encasing tube, when they are switched on in the encasing tube or when they are formed in the encasing tube. When using a correspondingly thick-walled deformation element or section in the cladding tube that has no pre-embossed direction of buckling, such an alignment need not be taken into account. However, the deformation elements preferably have an impressed direction of buckling.

The kinking of the fuel rods in a sheath with integrated kink deformability enables the wrapped fuel rods to be packaged in containers in such a way that there is essentially no dead space left in the container, since a hexagon-shaped cross section essentially perpendicular to the straight sections of the fuel rods remains. Packing density can be achieved. In this way, ceramic containers with small internal dimensions can be used.

Claims (18)

1) Method for reducing the length of at least one exposed fuel rod, ie the fuel rod is first introduced into a straight sheathing tube and then deformed to reduce the fuel rod length of the fuel rod, characterized in that the sheathing tube covers at least one part-length of a predetermined length of fuel rod sleeve-like, gas-tight with the sheathing tube connected and remains gas-tight under deformation deformation element to ^g eordent and that the fuel rod is bent in the deformation region determined by the deformation element by essentially 180 °, the deformation element being deformed in accordance with the kink and the enclosing tube remains rectilinear for part of its length. 2) Method according to claim 1, characterized in that the fuel rod is at least partially divided before kinking. 3) Method according to claim 1, characterized in that the fuel rod is broken up when buckling. 4) Method according to claim] or 2, characterized in that the deformation element and the fuel rod is flattened before buckling in the deformation area. 5) Method according to one of claims 1-4, characterized in that the enclosing tube is designed over substantially its entire length with a constant and a gas-tight buckling ensuring cross-sectional configuration and the fuel rod and the wrapping tube are bent into a predetermined length section serving as a deformation element . 6) Cladding for at least one irradiated fuel rod with a surrounding tube surrounding the fuel rod for carrying out the method according to one of claims 1 to 5, characterized in that the surrounding tube (4) is assigned at least one deformation element (5; 9; 13; 18; 23) is that covers a section of the fuel rod of a given partial length in a sleeve-like manner, is gas-tight (7, 16) connected to the surrounding tube (4) and remains gas-tight when the fuel rod is bent. 7) Enclosure according to claim 6, characterized in that the deformation element (5) on the enclosing tube (4) is pushed and gas-tightly connected thereto. 8) Enclosure according to claim 6, characterized in that the deformation element (9; 13; 17; 23) between sections (4 '/ 4'') of the enclosing tube (4) and gas-tight with the facing ends of the two sections of the envelope tube is connected. 9) casing according to claim 6, characterized in that the Verlenungsplenent is integrally formed with the original cladding tube. lo) Enclosure according to one of claims 6 to 9, characterized in that the deformation element (5; 9) is made of a plastically deformable, tough material. 11) Covering according to one of claims 6 to 9, characterized in that the cross section of the deformation element (5; 9; 13; 17; 23) deviates from the circular shape substantially along the entire axial length of the sleeve-like deformation element. 12) Covering according to claim 6 to lo, characterized in that at least one support plate (11, 12) is arranged between the deformation element and the fuel rod (1). 13) Covering according to one of claims 6 to 12, characterized in that the deformation element (5) has a greater wall thickness in the region of greater material stretch than in the region of smaller material stretch. 14) A wrapper according to any one of claims 6 to 12, characterized in that the deformation element constant over its length, having a gas-tight buckling deformation securing cross-sectional configuration and the cross-sectional configuration of said sheath tube is equal to that of Verformun ^g s-elements. 15) casing according to one of claims 6 to 13, characterized in that the deformation element (5) is curved outwards in the region of greater material stretching from the axis of the fuel steel to form a cavity (6). 16) Covering according to one of claims 6 to 9, characterized in that the deformation element (13; 17; 23) is at least partially elastically deformable. 17) Covering according to claim 16, characterized in that the deformation element (13) consists of a one-part or multi-part sleeve (14; 18, 19) and a gas-tightly connected to the sleeve and enclosing the predetermined breaking point compensator (15; 22). 18) Arrangement for receiving at least one burned-off fuel rod with a container including a recess for receiving the fuel rod introduced into a casing, characterized in that a fuel rod bent in the casing tube is introduced into the container.

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