MAGAZINE FOR FUEL ASSEMBLIES WITH SPENT NUCLEAR FUEL

Abstract: A magazine (1) for storage of nuclear fuel assemblies with spent nuclear fuel. The magazine (1) comprises a tubular casing which includes a number of channels (3) being formed from elongated intersecting first and second plates (8, 9). The plates (8, 9) are provided with a number of slits (12, 14, 20, 21) extending in the longitudinal directions of the plates (8, 9). The plates (8, 9) are assembled crosswise by inserting the slits of the first plate (8) into the slits of the second plate (9).

Description: Magazine for fuel assemblies with spent nuclear fuel The present invention relates to a magazine for storage of spent nuclear fuel in the fuel pool or interim storage of a nuclear power plant pending ultimate disposal by, for example, reprocessing or ultimate waste disposal in bedrock, when the reactivity has dropped to a level which facilitates the management, reprocessing and ultimate waste disposal. The packing degree in the magazine, which consists of a tubular casing containing a number of channels, is thereby determined by factors such as the amount of absorption material in the assembly-separating walls of the channels and the necessary play between the channels formed by the walls and a fuel assembly taking into account the deformations of an assembly. During use in a fuel core, the normally square tubular casing of the fuel assembly is influenced by the neutron irradiation and by the pressure difference between the inside and the outside of the assembly casing. This may result in the walls of the casing being deformed and the casing being curved in the longitudinal direction. The walls may become bulged outwards, and as a consequence thereof known magazines have been dimensioned in view of the greatest deformation which arise in a minority of assem¬ blies . This means that a small number of greatly deformed fuel assemblies have determined the dimensions of all the channels and hence the size of the magazine. To avoid having to overdimension the magazine, it has been proposed to provide the walls of the magazine, which are formed by a number of crossed plates, in the corners of the channels with longitudinal slits . This means that the walls of the channels will have a certain resilience, which makes it possible to make the channels with a somewhat smaller width. To reduce the risk of the occurrence of gaps during deformation of the channel walls in a magazine, which gaps may entail an unacceptable risk of a reactivity increase, the plates in the perpendicular planes crossing each other are provided with slits of different widths. In a first type of plates, the plate may either be unslitted over the greater part of its length, or have slits which have a width smaller than a second type of plates. The second type of plates is provided with slits, the width of which, with the exception of shorter sections at the ends of the slits, may be greater than twice the thickness of the first plate. In the case of an unslitted first plate, the whole of the first plate will then be bent in the wide slit of the second plate when being influenced by a greatly deformed fuel assembly. When using plates with a thickness of 3-4 mm, the slit in the first type of plates may have a width of 0.5 mm and in the second type a width of about 10 mm. In magazines of the above-mentioned embodiment, the crossed plates have been welded both to each other and to the surrounding casing. For various reasons, this embodiment requires advanced welding fixtures which, in turn, entail high manufacturing costs. The invention relates to a maga¬ zine, the crossing plates of which are fixed in relation to each other by means of a system of suitably arranged slits, which means that welding is only required between the casing of the magazine and the short ends of the plates. This entails considerably lower manufacturing costs. As already described, the magazine comprises a tubular casing including a number of channels which are formed by a first type of elongated plates joined together in a cruci¬ form configuration with a second type of elongated plates. According to the invention, the above-mentioned first plate is made with a continuous first intermediate portion exten¬ ding across the plate, a first end portion between the first intermediate portion and the first short side of the plate, and a second end portion between the intermediate portion and the second short side of the plate. The first end por¬ tion is provided with a number of first slits, and the second end portion is provided with a number of second slits. These first and second slits extend in parallel with the longitudinal direction of the plate from the interme¬ diate portion and up to a short side and have a width which is at least equal to the thickness of the second plate. The second plate is made with a continuous third and fourth end portion, respectively, extending across the plate, as well as a second intermediate portion. In this second inter¬ mediate portion a number of third slits are arranged running in parallel with the longitudinal direction of the plate and with a width which is at least equal to the thickness of the first plate. The sum of the length of either a first or a second slit and a third slit shall be at least so great that, with one of the end portions of a second plate inser¬ ted down to the bottom of a first or a second slot, the opposite end portion of the second plate may be fitted over the opposite second and first short sides, respectively, of the first plate. The crossed plates are joined together by arranging the third end portion of the second plate inserted into a first slit belonging to the first plate and the fourth end portion of the second plate into a second slit, being aligned with the first slit and belonging to the same first plate and, further, by welding the short ends of the plates to the casing. The invention will be most readily understood with reference to the accompanying drawings wherein Figure 1 is a side view of a magazine Figure 2 is a top view of the magazine in Figure 1 Figure 3 shows a first type of the crossed plates forming the channels in the magazine according to Figures 1 and 2 Figure 4 shows a second type of the crossed plates forming the channels Figure 5 is a section of two crossing plates 8, 9 Figure 6 shows an alternative embodiment of the first type of plates according to Figure 3 Figures 7-10 show in different phases how two plates of the first and second types, crossing each other, are joined together Figure 11 is a perspective view of the top portion of the magazine in section XI-XI in Figure 1, and Figure 12 is a perspective view of the bottom portion of the magazine seen in section XII-XII. In Figure 1, 1 designates the whole magazine and 2 its tubu¬ lar casing containing the channels 3 for fuel assemblies 4. The magazine 1 is provided with a top portion 5 with means for lifting the magazine 1 and a bottom portion 6 with a supporting grid 7 for the fuel assemblies 4. The channels 3 are formed from the first type of plates 8 which are arranged in cruciform configuration with the second type of plates 9 (Figure 2) . In Figure 2, the same designations as in Figure 1 are used. Figure 3 shows a type of elongated first plates 8 which are provided with a continuous first intermediate portion 10 of a length B, extending across the first plate 8, a first end portion 11 of the length A provided with first slits 12 also with the length A and with a width which is at least equal to the thickness of the second plate 9 and preferably per¬ mits the insertion of the plate 9 into the slits 12 and 14 with a certain small play. The second end portion 13 of the plate 8 of the length C is also provided with second slits 14 of the same width as the slits 12. These second slits 14 have a length C. The plate 8 has a first short side 15 and a second short side 16. Figure 4 shows a second plate 9 with a continuous third end portion 17 and a fourth end portion 18 of the length A, extending across the second plate 9. The plate 9 has a second intermediate portion 19 between these end portions 17, 18. This second intermediate portion 19 is provided with a type of third slits 20 of the length of the intermediate portion and with a width which is at least equal to the thickness of the first plate 8. In the embodiment shown, the third slit 20, along an intermediate section with the length F, is made with a larger width which is at least equal to twice the thickness of the first plate 8. The sections of the third slit 20 with the lengths D and E, respectively, have a width which is somewhat larger than the thickness of the first plate 8 and thus allows the insertion thereof into the slit 20 with a certain small play. Figure 5 shows a section of two crossing plates 8, 9 in a section where the third slit 20 is widest and the plate 8 is made according to the alternative of Figure 6, that is, the first intermediate portion 10 of the plate is provided with a fourth slit 21. This slit 21 should have a width which is considerably smaller than the thickness of the plate 9. As mentioned, Figure 6 shows an alternative embodiment of the first plate 8 according to Figure 3. The intermediate portion 10 of the plate 8 is here provided with a type of fourth slits 21 of the with mentioned above and the length F. Thus, the intermediate portion 10 will have two con¬ tinuous unslitted ends or parts 22, 23 of a length D. As in Figure 3, the slits 12 and 14, respectively, have the length A and C, respectively. The lengths indicated in the diffe¬ rent figures are only to be seen as examples of an embodi¬ ment. However, it is suitable that, for example, the length A of the third end portion_ 17 be equal to the length of the first slit 12 so that, when joining the plates 8 and 9 together, the end portion 17 can be fitted in full into the first slit 12. The same applies concerning the length D of the part 22 and the narrower section of the third slit 20 with the length E. This, therefore, means that A+E should be equal to C+D. When joining together the plates 8, 9 in a cruciform con¬ figuration, as shown in an alternative with reference to Figures 7-10, it is clear from Figure 7 that the plate 9 is bent allowing the plate 8 to be fitted into one of the third slits 20 of the plate 9. Thereafter, according to Figure 8, the plate 9 is pushed upwards such that the fourth end por¬ tion 18 of the plate 9 slides to the bottom of a second slit 14 on the first plate 8. In this mutual position of plates 8, 9, the third slit 20 in the plate 9 shall now have such a length that the third end portion 17 on the plate 9 may be fitted in over the first short side 15 of the plate 8 and be straightened out as shown in Figure 9. Thereafter, the plate 9 is pushed downwards until the third end portion 17 of the plate 9 reaches the bottom in a first slit 12 as shown in Figure 10. Further, it is necessary for all the plates 8 in the magazine to be connected together by each plate 9. For that reason, all the plates 8 are arranged in parallel at a channel width's distance from each other and with the slits 12, 14 aligned. The example shows four plates 8, that is, the same number as the number of slits 20 in the plate 9. When the plates are connected, a plate 8 is now inserted into each slit 20 on a plate 9, whereupon the plates 9 are fitted over all of these plates 8 at the same time and connect these together as shown in Figures 7-10. In this way, the plates 8 and 9 are successively picked together to form a lattice-work of crossing plates which is then inser¬ ted into the casing 2. The plates now need not be welded to each other but only to the surrounding casing 2 at the short sides of the plates. These welds 24 are clear from Figures 11 and 12, which are otherwise furnished with the same designations as used above. The plates 8, 9 are suitably made of absorber material, for example boron-alloyed steel whereas the casing 2 may consist of stainless sheet metal. Even if the plates 8 are formed with an unslitted intermediate portion, a satisfactory resilience in the walls of the channels 3 is obtained by the wide slits 20. The channel walls formed by the plate 9 may be displaced individually whereas the channel walls formed■ by the plate 8 are displaced simultaneously upon pressure on a channel wall. If it is desired that also these walls should be displaced individually, the alternative according to Figure 6 with the fourth slits 21 must be used. It should be noted that when connecting the plates 8, 9 together, this connection procedure can be the reverse to that described with reference to Figures 7-10. However, the slits 12 have to be made longer and the unslitted part 22 be moved downwards such that the third end portion 17 of the second plate 9 can be inserted so far into a slit 12 that the opposite fourth end portion 18 thereof can be fitted over the second short side 16 of the first plate 8 and then be inserted into a second slit 14.

Claims: CLAIMS 1. A magazine (1) for storage of fuel assemblies (4) with spent nuclear fuel comprising a tubular casing (2) including a number of channels (3) for said fuel assemblies (4), said channels (3) being formed by a first type of elongated plates (8) joined together in a cruciform configuration with a second type of elongated plates (9) , characterized in that each one of said first plates (8) is formed with at least one continuous first intermediate portion (10) extending across the plate (8) , a first end portion (11) between said first intermediate portion (10) and the first short side (15) of the plate (8), a second end portion (13) between the intermediate portion (10) and the second short side (16) of the plate (8) , said first end portion (11) being provided with a number of first slits (12) , said second end portion (13) with a number of second slits (14), said first (12) and second (14) slits extending in parallel with the longitudi¬ nal direction of the plate (8) from the intermediate portion and up to a short side and having a width at least equal to the thickness of said second plate (9) , said second plate (9) is made with a continuous third (17) and fourth (18) end portion extending across the plate (9), a second intermediate portion (19) between said end portions (17, 18) , a number of third slits (20) extending in said second intermediate portion (19) and being parallel with the longitudinal direction of the plate (9) and having a width at least equal to the thickness of said first plate (8), the sum of the length of either a first (12) or a second (14) slit and a third slit (20) is at least so great that, with one of the end portions of a second plate (9) inserted into the bottom of a first (12) or a second (14) slit, the opposite end portion of said second plate (9) may be fitted over the opposite second (16) or first (15) short side of said first plate (8) . 2. A magazine according to claim 1, characterized in that crossed plates (8, 9) are joined together by inserting the third end portion (17) of the second plate (9) into a first slit (12) belonging to the first plate (8) and inser¬ ting the fourth end portion (18) of the second plate (9) into a second slit (14) which is aligned with said first slit (12) and belongs to the same first plate (8), and welding the short sides of the plates (8, 9) to the casing (2) . 3. A magazine according to claim 1 or 2, characterized in that said third slit (20) at each end has a smaller section with a width just allowing the insertion of said first plate (8) through this part of the third slit (10) and that the third slit (20) between said sections has a width at least equal to twice the thickness of said first plate (8) . 4. A magazine according to claim 1, characterized in that the first intermediate portion (10) along part of its length is provided with a number of fourth slits (21) aligned with said first (12) and second (14) slits such that the ends (22, 23) of the intermediate portion (10) remain unslitted. 5. A method for assembly of a magazine (1) for spent nuclear fuel assemblies (4) comprising a tubular casing (2) containing a number of channels (3) formed by a first type of rectangular plates (8) joined together crosswise with a second type of rectangular plates (9) according to claim 1, characterized in that each one of said first plates (8) is made with a continuous first intermediate portion (10) extending across the plate, a first end portion (11) between said first intermediate portion (10) and the first short side (15) of the plate, a second end portion (13) between the intermediate portion (10) and the second short side (16) of the plate, a number of first slits (12) being provided in said first end portion (11) and a number of second slits (14) being provided in said second end portion (13), said first (12) and second (14) slits being arranged in parallel with the longitudinal direction of the plate (8) from the intermediate portion (10) and up to a first (15) and second (16) short side, respectively, and having a width at least equal to the thickness of said second plate (8) , each one of said second plates (9) is made with a continuous third (13) and fourth (18) end portion extending across the plate (9), and a second intermediate portion (19) between said end portions (17, 18) , a number of third slits (20) which are arranged in said second intermediate portion (19) and are parallel to the longitudinal direction of the plate (9) and have a width at least equal to the thickness of said plate (8), said first plates (8) are arranged with their plane in parallel and with the corresponding slits of the plates (8) in the different planes arranged in a line, a crossing second plate (9) is bent and one of its end portions inserted into the bottom of first (12) or second (14) slits positioned along a line, whereupon the opposite end portion of the plate (9) is fitted over the opposite second (16) and first (15) short sides, respectively, of the first plates (8), and the second plate (9) is straightened and inserted into second (14) and first (12) slits, respec- tively, opening out into said short sides, the plates (8, 9) are displaced in relation to each other so that their short sides substantially coincide, the method is repeated with additional second plates (9) until all the plates (8, 9) have been joined together to form a lattice-work of channels (3) , whereupon said lattice-work is inserted into the casing (2) and is joined to this. 6. A method according to claim 5, characterized in that the slits (12, 14, 20, 21) in the different plates (8, 9) are arranged at a channel width's distance from each other.

Inventors: SUVANTO ANTTI, SUVANTO, ANTTI
Applicants: ASEA ATOM AB, ABB ATOM AB
Application year: 1992
Main classification: G21C19
IPC codes: G21C19