UPGRADING FUEL ELEMENTS OF NUCLEAR POWER PLANTS

Description

[0001] The present invention relates to a method for upgrading fuel elements for a nuclear reactor, in particular for a boiling water reactor.

[0002] In pressurised water and boiling water reactors, the fuel elements are usually constructed as bundles of individual fuel rods containing the fissile material. It is important that the position of the individual fuel rods within a fuel element is precisely defined in order to prevent individual fuel rods from coming close to neighbouring fuel rods, which would otherwise have a changing criticality. The fuel elements are housed in the reactor in specific receptacles, so-called fuel element canisters. The layout of these receptacles is described, for example, in WO 2020 / 120648 A1. During reactor operation, particularly in boiling water reactors, oxidation of the structural material and the fuel rods occurs, leading to material growth. The fuel rods are connected to one another via spacers, which ensure that the necessary distances between neighbouring fuel rods are maintained.On the other hand, the fuel rods are inserted into a support structure to enable terminal positioning of the fuel rods. In principle, the fuel rods are not connected to this support structure in a materially bonded, form-fitting, or friction-locking manner, but are connected to the spacers. This means that the fuel elements and thus the fuel rods should be easy to remove from the fuel assembly channels and the support structure. However, oxidation and irradiation during operation cause the fuel rods, spacers, and fuel assembly channels to grow. Partially uneven growth, particularly in a plane transverse to the direction of insertion and removal into the fuel assembly channel, can mean that after a certain period of operation, the fuel elements cannot be removed without causing damage. Individual or all of the fuel elements may become jammed in the fuel assembly channel, and the spacers may therefore be damaged during removal.

[0003] This means that after the fuel elements are removed from the fuel assembly canister, the spacers may be damaged or displaced, and they no longer perform their function reliably. If the fuel elements must then be placed in a transport container, such as a CASTOR container, and transported in this container to a reprocessing plant, interim storage facility, or final storage facility, it cannot be assumed that the fuel elements are accident-proof, as it must be ensured that the distance between the fuel rods is maintained even in the event of an accident, for example, if the CASTOR container falls.

[0004] Previously, a so-called re-skeletonization process was used, in which the existing spacers were replaced. However, this represents a considerable effort and requires a significant amount of human labor, as the original spacers or support structures must be mechanically removed and replaced with new ones, taking into account the relevant regulations for working with radioactive materials. Furthermore, this also leads to an increased radiation dose (collective dose) to which the relevant workers are exposed.

[0005] DE102009046668A1 is another relevant document.

[0006] Based on this, the present invention is based on the object of at least partially overcoming the disadvantages known from the prior art.

[0007] The problem is solved with the features of the independent claim. The dependent claims are directed to advantageous further training.

[0008] The method according to the invention for upgrading an existing fuel assembly for nuclear reactors, in particular boiling water reactors, for transport and / or storage, wherein the fuel assembly is formed from a plurality of fuel rods extending in a longitudinal direction, wherein the fuel rods are arranged in rows and columns in a plane perpendicular to the longitudinal direction, wherein the columns are arranged in a first transverse direction extending perpendicular to the longitudinal direction and the rows are arranged in a second transverse direction extending perpendicular to the longitudinal direction and perpendicular to the first transverse direction, wherein the fuel assembly is provided with a first upgrading structure segment and a second upgrading structure segment for storing the fuel rods, wherein the first upgrading structure segment is formed from a first base body and a plurality of first support structures,which extend parallel to each other at right angles from the first base body, wherein the second strengthening structure segment is formed from a second base body and a plurality of second support structures which extend parallel to each other at right angles from the second base body, comprises the following steps: , a) Determining a first position in the longitudinal direction for storing the fuel rods; b) Inserting a first strengthening structure segment at the first position, wherein the first support structures are pushed between the fuel rods in the first transverse direction, and inserting a further first strengthening structure segment at the first position, wherein the first support structures are pushed between the fuel rods counter to the first transverse direction; c) Inserting a second strengthening structure segment at the first position, wherein the second support structures are pushed between the fuel rods in the second transverse direction, and inserting a further second strengthening structure segment at the first position, wherein the second support structures are pushed between the fuel rods counter to the second transverse direction.

[0009] The base bodies and support structures are preferably formed from a metal, in particular from a nickel-chromium-iron-molybdenum alloy, for example an alloy sold under the name Alloy 718, or a zirconium alloy. The base bodies and support structures are preferably formed from sheet metal. Alternatively, the base bodies and support structures can be formed using an additive manufacturing process.

[0010] The respective support structures are preferably arranged equidistantly on the respective base body, with the respective spacing corresponding to the spacing between adjacent fuel rods. The support structures can have different lengths, in particular to accommodate, for example, water rods, which typically have a larger diameter than fuel rods.

[0011] Preferably, recesses can be formed in the first support structures through which the second support structures can be inserted. In such an example, step b) is therefore necessarily performed before step c). By inserting reinforcement structure segments, it is possible to easily reinforce fuel assemblies with destroyed, torn-off, or displaced spacers without having to completely re-skeletonize the fuel assemblies.

[0012] The reinforcement structure segments can be installed easily and with minimal friction, without the need to mechanically remove existing spacers, for example, if they have been displaced. These can remain in the fuel elements. The reinforcement structure segments ensure safe storage of the fuel rods, regardless of the condition of the original spacers.

[0013] According to an advantageous embodiment, at least one second position different from the first position is defined in the longitudinal direction for storing the fuel rods and steps b) and c) are carried out at the respective second positions.

[0014] In this way, several destroyed, torn off or displaced spacers can be easily reinforced.

[0015] According to an advantageous embodiment, the existing fuel assembly has target positions for spacers in the longitudinal direction, and the first position is defined at a target position or directly adjacent to a target position in the longitudinal direction. Furthermore, the existing fuel assembly advantageously has target positions for spacers in the longitudinal direction, and the second position is defined at a target position or directly adjacent to a target position in the longitudinal direction.

[0016] The target positions of the spacers are usually determined during the design of the fuel assembly. The target positions of the spacers are determined in such a way that the fuel assembly is accident-proof, i.e., if the fuel assembly falls, for example, there is no relative movement between individual fuel rods. If at least one spacer is damaged, destroyed, torn off, or displaced when the fuel assembly is pulled out of the fuel assembly canister, storage of the fuel rods in at least one of the corresponding target positions is no longer guaranteed, meaning that accident-proof safety can no longer be ensured.

[0017] In this case, if spacers are torn off or displaced, reinforcement structural segments are inserted at the corresponding target positions to restore the appropriate support and thus ensure accident safety. The first and / or second position then corresponds to a target position.

[0018] If at least one spacer is damaged or destroyed, the first and / or second position is selected such that it is immediately adjacent to a desired position in the longitudinal direction. This means that the first and / or second position is placed as close as possible to a desired position, in particular such that the reinforcement structure segments are in longitudinal contact with the corresponding spacer or that there is a maximum distance of 25 mm between the spacer and the reinforcement structure segments in the longitudinal direction. This ensures the safe storage of the fuel rods and thus accident safety.

[0019] According to an advantageous embodiment, the first support structures have recesses through which the second support structures penetrate during insertion. This enables stabilization of the first and second reinforcement structure segments and thus stabilization of the fuel assembly in this area. This allows for better absorption of forces introduced into the fuel assembly, for example, due to lateral support or a fall.

[0020] According to an advantageous embodiment, the first strengthening structure segments and / or the second strengthening structure segments have deflector tabs formed on the respective base body, in particular on the upper and / or lower side, each viewed in the longitudinal direction. These enable easy recasting, in which the fuel element is reinserted into a fuel assembly canister after strengthening. The deflector tabs are preferably formed in the form of metal tongues on the base body. Upon insertion into the fuel assembly canister, the deflector tabs rest against the inside of the fuel assembly canister and center the fuel element during insertion, thus reliably preventing jamming during insertion.

[0021] According to an advantageous embodiment, the first base body has first snap-in means that form a snap connection with the fuel rods upon insertion of the first reinforcement structure segment. This allows for simple fixation of the first reinforcement structure segment to the fuel element. The snap connection is preferably made with the respective outer fuel rods.

[0022] According to an advantageous embodiment, the second base body has second snap-in means that form a snap connection with the fuel rods upon insertion of the second reinforcement structure segment. This allows for simple fixation of the second reinforcement structure segment to the fuel element. The snap connection is preferably made with the respective outer fuel rods.

[0023] According to an advantageous embodiment, spacer plates are fixed to the fuel rods to prevent longitudinal displacement of the reinforcement structure segments, particularly during recasing. Preferably, two spacer plates are placed between reinforcement structure segments and spacers fixed in the support structure to prevent a reduction in the distance between reinforcement structures and, if applicable, spacers. This allows the fuel rods of the fuel assembly to be stored in a manner that reliably prevents fuel rods from approaching neighboring fuel rods, even in the event of a malfunction.

[0024] As a precaution, it should be noted that the numerals used here ("first", "second", ...) primarily serve (only) to distinguish between several similar objects, quantities, or processes, and therefore do not necessarily dictate any interdependence and / or sequence of these objects, quantities, or processes. Should a dependence and / or sequence be required, this is explicitly stated here or will be obvious to the person skilled in the art upon studying the specifically described embodiment.

[0025] The invention and the technical environment are explained in more detail below with reference to the figures. It should be noted that the invention is not intended to be limited by the exemplary embodiments shown. In particular, unless explicitly stated otherwise, it is also possible to extract partial aspects of the facts explained in the figures and combine them with other components and findings from the present description and / or figures. In particular, it should be noted that the figures and in particular the illustrated proportions are only schematic. The same reference numerals denote the same objects, so that explanations from other figures can be used as a supplement if necessary. They show: Fig. 1 a perspective view of an example of a first strengthening structure segment; Fig. 2 a perspective view of an example of a second strengthening structure segment; Fig. 3 a cross-section of an example of a fuel assembly in the original spacer; Fig. 4 a perspective view of the examples of the first and second strengthening structure segments; and Fig. 5 a perspective view of a fuel assembly with mounted reinforcement structure segments.

[0026] Fig. 1 shows schematically an example of a first strengthening structure segment 1. This comprises a first base body 2 with first deflector lugs 3 and first snap means 4. A plurality of first support structures 5, which in this example are designed as metal sheets, extend from a first side 29 of the first base body 2. Each first support structure 5 has a right angle with the first base body 2, so that the first support structures 5 are parallel to one another. Recesses 6 are formed in each first support structure 5, which are aligned so that an element such as a metal sheet can be pushed parallel to the first base body 2 through the recesses 6 of the first support structures 5. Furthermore, the first support structures 5 have knobs 30, which clamp the fuel rods of the fuel assembly in the installed state, as in particular Fig. 3 For the sake of clarity, not all studs 30 are provided with reference numerals. The first deflector lugs 3 initially extend away from the first base body 2 and are then bent toward the first support structures 5. This facilitates the centering of the fuel element when it is placed in a fuel element canister.

[0027] Fig. 2 shows a schematic example of a second reinforcement structure segment 7. This has a second base body 8 with second deflector tabs 9 and second snap means 10. A plurality of second support structures 11 extend away from the second base body 8. Each second support structure 11 has a right angle with the second base body 8, so that the second support structures 11 extend parallel to one another. The second support structures 11 have a height 12 that is smaller than the corresponding extension 13 of the recesses 6 of the first support structures 5, so that the second support structures 11 can be pushed through the recesses 6 of the first support structures 5. The second support structures 11 also have corresponding studs 30. For the sake of clarity, not all studs 30 are provided with reference numerals.The second deflector lugs 9 initially extend away from the second base body 10 and are then bent toward the second support structures 11. This facilitates the centering of the fuel element when it is placed in a fuel assembly canister.

[0028] Fig. 3 shows a schematic cross-section of a fuel element 14 for a boiling water reactor with a plurality of fuel rods 15 arranged in a grid with a length and width of ten fuel rods 15 each. The fuel element 14 also has two water rods 29. For ease of understanding, the following directional definitions are used in this document. The direction in which the fuel rods 15 extend is referred to as the longitudinal direction 16. All fuel rods 15 are aligned parallel in the longitudinal direction 16. Furthermore, a second transverse direction 17 and a first transverse direction 18 are defined. Each transverse direction 17, 18 is aligned perpendicular to the longitudinal direction 16 and perpendicular to the other transverse direction 18, 17, so that a three-dimensional Cartesian coordinate system is created.

[0029] The fuel element 14 has a grid of columns 19 and rows 20 of fuel rods 15. Each row 20, which is provided with a reference symbol only as an example, extends in the second transverse direction 17 and each column 19, which is also only partially provided with reference symbols for the sake of clarity, extends in the first transverse direction 18. The fuel element 14 therefore has ten rows 20 and ten columns 19 of fuel elements, which are interrupted by the water rods 29.

[0030] In order to strengthen the fuel element 14, two first strengthening structure segments 1 and two second strengthening structure segments 7 with their support structures 5, 11 are each inserted between adjacent fuel rods 15 in the region of the cross section, so that the first support structures 5 extend in the first transverse direction 18 and the second support structures 11 extend in the second transverse direction 19.

[0031] When inserting the upgrading structure segments 1 and 7, a first upgrading structure segment 1 with the corresponding first support structures 5 is first pushed from the outside in the first transverse direction 18 between the fuel rods 15 and then a further first upgrading structure segment 1 with the corresponding first support structures 5 is pushed from the outside between the fuel rods 15 against the first transverse direction 18.Subsequently, first a second strengthening structure segment 7 is inserted in the second transverse direction 17, so that the second support structures 11 are inserted between the fuel rods 15 and in the process penetrate the recesses 6 of the first support structures 5, and then a further second strengthening structure segment 7 is inserted counter to the second transverse direction 17, so that the second support structures 11 are inserted between the fuel rods 15 and in the process penetrate the recesses 6 of the first support structures 5. The number of support structures 5, 11 is preferably selected such that it is one less than the corresponding number of columns 19 or rows 20 of the fuel assembly 14, so that it is ensured that support structures 5, 11 can be positioned between two fuel rods 15 of the fuel assembly 14.A first distance 21 between two adjacent first support structures 5 (cf. . Fig. 1 ) corresponds to a first intermediate space 22 (cf. Fig. 3 ) between two adjacent fuel rods 15 in the second transverse direction 17. A second distance 23 between two adjacent second support structures 11 (cf. Fig. 2 ) corresponds to a second intermediate space 24 in the first transverse direction 17 (cf. Fig. 3 ). The definition of the spaces 22, 24 refers to areas of the grid in which fuel rods 15 are directly adjacent to one another, i.e. not the areas of the grid in which the water rods 29 are formed. The lengths of the support structures 5, 11 are therefore selected such that they leave no space for the water rods 29 when installed. As already explained above, the support structures 5, 11 have studs 30 that clamp and fix the fuel rods 15. The studs 30 are formed on both sides of the respective support structures 5, 11 and, for the sake of clarity, are only partially provided with reference symbols.

[0032] The respective base bodies 2, 8 are located on the outside of the fuel assembly 14 and thus on the outer fuel rods 15 of the fuel assembly 14. Via the first snap means 4 and second snap means 10, as in Fig. 3 shown, a fixation to the fuel rods 15, which are formed in the corners of the grid. The first snap means 4 and the second snap means 10 (cf. Fig. 1 und 2 ) are designed in such a way that in other positions in the longitudinal direction 16 they form a force and form fit with the fuel rods 15.

[0033] Fig. 4 shows schematically the assembled first strengthening structure segments 1 and second strengthening structure segments 7. To avoid repetition, reference is made to the description of Fig. 3 referred to.

[0034] Fig. 5schematically shows a fuel assembly 14 with fuel rods 15. These have reinforcement structure segments 1, 7 at a first position 25 in the longitudinal direction 16 and at a second position 26 in the longitudinal direction 16, which are spaced apart in the longitudinal direction 16 by a distance 27. The first position 25 and the second position 26 correspond to target positions 31 at which spacers (not shown) were formed on the fuel assembly 14 before the box removal, which spacers were torn off during the box removal and are no longer present.By forming the upgrading structure segments 1, 7 at the first position 25 and the second position 26, the storage of the fuel rods 15 in the longitudinal direction 16 is carried out as designed, so that even with the upgraded fuel assembly 14, accident safety is ensured. Regardless of a change in the position of the fuel assembly 14 or an event such as the fuel assembly 14 falling, the distances between the fuel rods 14 remain constant in order to prevent an increase in the local concentration of fissile material. In order to fix the first position 25 and the second position 26, spacer plates 28 are formed which prevent displacement of the upgrading structure segments 1, 7 at the first position 25 and the second position 26 and thus prevent them from leaving the target positions 31. The spacer plates 28 are preferably also fixed to the fuel rods 15 via a snap connection in order to enable simple assembly.

[0035] Base bodies 2, 8 and respective support structures 5, 11 are preferably formed from a sheet metal or by an additive manufacturing process (3D printing). Base bodies 2, 8 and respective support structures 5, 11 are preferably formed from a nickel-chromium-iron-molybdenum alloy, for example an alloy sold under the name Alloy 718, or a zirconium alloy.

[0036] Within the scope of the method presented here, structural upgrade segments 1, 7 are inserted into the fuel assembly 14 at one or more positions 25, 26 to achieve design-compliant storage of the fuel rods 15 of the fuel assembly 14. This ensures that the fuel rods 15 are fixed transversely to a longitudinal direction 16 in which the fuel rods 15 extend, preventing relative movement of the fuel rods 15 against one another, even in the event of a malfunction. It is not absolutely necessary to remove existing defective or displaced spacers. This allows fuel assemblies 14 to be easily upgraded for transport and / or storage. Reference symbol

[0037] 1First strengthening structure segment 2First base body 3First positioning aid 4First snap-in means 5First support structure 6Recess 7Second strengthening structure segment 8Second base body 9Second positioning aid 10Second snap-in means 11Second support structure 12Height 13Extension 14Fuel element 15Fuel rod 16Longitudinal direction 17Second transverse direction 18First transverse direction 19Column 20Row 21First distance 22First gap 23Second distance 24Second gap 25First position 26Second position 27Distance 28Spacer plate 29First side 30Nubs 31Target position

Claims

1. Method for strengthening an existing fuel element (14) for nuclear reactors, in particular boiling-water reactors, for transport and / or storage purposes, wherein the fuel element (14) is formed from a plurality of fuel rods (15) which extend in a longitudinal direction (16), wherein the fuel rods (15) are arranged in rows (20) and columns (19) in a plane perpendicular to the longitudinal direction (16), wherein the columns (19) are arranged in a first transverse direction (18), which extends perpendicularly to the longitudinal direction (16), and the rows (20) are arranged in a second transverse direction (17), which extends perpendicularly to the longitudinal direction (16) and perpendicularly to the first transverse direction (18), wherein the fuel element (14) is provided with a first strengthening structure segment (1) and a second strengthening structure segment (7) for the storing of the fuel rods (15), wherein the first strengthening structure segment (1) is formed from a first main body (2) and multiple first support structures (5), which extend from the first main body (2) at a right angle and parallel to one another, wherein the second strengthening structure segment (7) is formed from a second main body (8) and multiple second support structures (11), which extend from the second main body (8) at a right angle and parallel to one another, comprising the following steps: a) defining a first position (25) in the longitudinal direction (16) for the storing of the fuel rods (15); b) introducing a first strengthening structure segment (1) at the first position (25), wherein the first support structures (5) are pushed between the fuel rods (15) in the first transverse direction (18), and introducing a further first strengthening structure segment (1) at the first position (25), wherein the first support structures (5) are pushed between the fuel rods (15) counter to the first transverse direction (18); c) introducing a second strengthening structure segment (7) at the first position (25), wherein the second support structures (11) are pushed between the fuel rods (15) in the second transverse direction (17), and introducing a further second strengthening structure segment (7) at the first position (25), wherein the second support structures (11) are pushed between the fuel rods counter to the second transverse direction (17);2. Method according to Claim 1, in which at least one second position (26), different from the first position (25), is defined in the longitudinal direction (16) for the storing of the fuel rods (15), and steps b) and c) are carried out at the respective second positions (26).

3. Method according to either of the preceding claims, in which the existing fuel element (14) has in the longitudinal direction (16) desired positions (31) for spacers, and the first position (25) is defined at a desired position (31) or directly adjacent in the longitudinal direction (16) to a desired position (31).

4. Method according to either of Claims 2 and 3, in which the existing fuel element (14) has in the longitudinal direction (16) desired positions (31) for spacers, and the second position (25) is defined at a desired position (31) or directly adjacent in the longitudinal direction (16) to a desired position (31).

5. Method according to one of the preceding claims, in which the first support structures (5) have cutouts (6) through which the second support structures (11) pass during the pushing-in process.

6. Method according to one of the preceding claims, in which the first strengthening structure segments (1) and / or the second strengthening structure segments (7) have deflector tabs (3, 9) which are formed on the respective main body (2, 8).

7. Method according to Claim 6, in which the deflector tabs (3, 9) are in the form of metal tongues on the main body (2, 8).

8. Method according to one of the preceding claims, in which the first main body (2) has first snap-action means (4) which enter into a snap-fit connection with fuel rods (15) during the pushing-in process for the first strengthening structure segments (1).

9. Method according to one of the preceding claims, in which the second main body (8) has second snap-action means (10) which enter into a snap-fit connection with fuel rods (15) during the pushing-in process for the second strengthening structure segments (7).

10. Method according to one of the preceding claims, in which spacer sheets (28) are fixed to fuel rods (15) and prevent displacement of the first strengthening structure segments (1) and / or the second strengthening structure segments (7) in the longitudinal direction (16).