EP0005623B1

EP0005623B1 - Storage container for holding spent nuclear fuel rods at a reactor site

Info

Publication number: EP0005623B1
Application number: EP79300830A
Authority: EP
Inventors: Stanley Kmonk; John M. Shallenberger; Stephen J. Ferlan
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1978-05-15
Filing date: 1979-05-15
Publication date: 1982-08-04
Also published as: GR67749B; ES480582A0; IT1113971B; EP0005623A3; FR2426315A1; EP0005623A2; JPS54150591A; IL57166A; ES8104623A1; KR820000551B1; PH17641A; IT7922645A0; EG14893A; ZA791691B; JPS6017075B2; FR2426315B1; DE2963465D1

Description

The present invention relates to nuclear reactor spent fuel storage containers and more particularly to an arrangement for providing on-site storage of all spent fuel rods removed from a reactor during its lifetime.
In the operation of conventional nuclear reactors used for generating electric power, the nuclear fuel becomes spent and must be removed at periodic intervals. Although refuel- ing schedules vary, approximately one-third of the fuel rods must be removed annually so that replacement of all fuel rods in the core will take place over about a three-year period. Since nuclear reactors are designed for a life extending to approximately forty years, it is apparent that spent fuel storage facilities at the reactor site must accommodate about thirteen full cores of fuel rods, i.e., about twenty-six hundred fuel rods to accommodate ail discharged fuel.
Currently, there is a dearth of fuel reprocessing facilities in the United States and throughout the world. Since the fuel reasonably cannot be reprocessed, the electric utilities who remove spent fuel rods from their nuclear reactors must provide for their safe storage so long as the fuel therein remains radioactive.
Historically, utilities have always provided fuel storage areas alongside the reactor to accommodate a small number of fuel rods. However, in view of the present uncertainties regarding reprocessing fuel and shipping spent fuel from the reactor site, utilities recently have taken positive steps to increase their on-site fuel storage capacities. However, the plant construction generally precludes increasing the actual size of spent fuel storage pools. The increase in capacity has been accomplished by locating fuel storage racks on a closer pitch in the spent fuel pool, to thereby increase the number of fuel storage racks in the available space. Although the fuel storage capacity is greater, present storage pools at most reactor sites will not accommodate all the fuel expected to be removed from a reactor over its lifetime, unless new structures or methods are developed. Since such space reasonably will not be available, an increase in spent fuel rods beyond that capable of being absorbed by the fuel pits could result in shut-down situations for particular reactor plants, and especially if off- site storage facilities also are not available.
It therefore is the principal object of the present invention to provide a container for storing spent fuel rods recovered from currently operating nuclear reactors which offers greatly increased storage capacity and which facilitates loading in minimum line designed to accommodate the flexibility in fuel rods and permit quick, efficient loading in minimum time.
With this object in view, the present invention resides in a storage container for holding spent fuel rods in a compact array, said container having a base arranged to support fuel rods and walls to contain the fuel rods, characterized in that two opposite end walls of said container have upwardly opening channels on their upper ends spaced from each other a distance equal the distance of the rows of fuel rods to be disposed in said container and that a guidance plate is provided in said container for removal, disposition and displacement from row to row in order to facilitate loading of fuel rods into the container in rows, said guidance plate having protruding brackets at its upper end to be received in said channels and said container and the guidance plate having corresponding means for retractably engaging the guidance plate at its lower end, the side of the said guidance plate facing the row of fuel rods to be inserted being equipped with grooves defining the respective positions of the fuel rods to be loaded in the row and providing a side support for the fuel rods of a row when being inserted wherein each fuel rod (26) is in substantial line contact with the adjacent fuel rods (26).
The invention will become more readily apparent from the following description of preferred embodiments thereof shown, by way of example only in the accompanying drawing, wherein:

Figure 1 is a view in elevation, partly in section, illustrating the design of a container located inside a spent fuel assembly storage rack, and which is used for holding a compacted array of fuel rods;
Fig. 2 is a top view of the arrangement of Figure 1, with a section cut to illustrate how a guiding plate is used for aligning fuel rods in the container illustrated in Figure 1;
Fig. 3 is a plan view of the container of Figure 1;
Fig. 4 is a partial view in elevation showing a section of the guiding plate used for orienting fuel rods in Fig. 2;
Fig. 5 is a plan view of the guidance plate illustrated in Fig. 4;
Fig. 6 is a side view of the plate of Fig. 4.
Fig. 7 is a modification of the storage container shown in Figure 1;
Fig. 8 is a plan view of the container of Fig. 7;
Fig. 9 is a detailed view illustrating how fuel rods are positioned in openings provided in a bottom plate in the container of Fig. 7;
Fig. 10 is a top view of the section shown in Fig. 9; and
Fig. 11 is a modification of the base plate shown in Fig. 9.

Referring now to the drawings wherein like reference characters designate like or corresponding parts throughout the several views, there is shown in Figures 1-3 a fuel rack cell 10 of stainless steel or other non-corrosive material designed to receive a container 11 arranged to hold spent fuel rods removed from spent fuel assemblies used in a nuclear reactor. The fuel rack 10 which usually is positioned in a spent fuel pool for receiving an entire fuel assembly is essentially square in cross-section and consists of a base 12 and side walls 14. The upper ends of the walls have outwardly extending flanges 16 for guiding a fuel assembly into the cell 10.
A container 11 is fitted in the fuel cell 10, with a small space 21 therebetween to accommodate flow of coolant. It is not essential that the container be located within the fuel rack cell since it is designed as a single entity with sufficient structural integrity to independently perform a fuel rod storage function. Figure 1 shows that the container has side walls 18 and end walls 20, the side walls 18 of the container 11 extending upwardly above the end walls 20 to support a cap 22. Located inside the container 11 and supported on the base 12 is a pyramidal support structure 43 having steps 24 which rise from each of the side walls 18 to the center of the container 11. The purpose of these steps is to receive and support multiple rows of fuel rods which are positioned vertically on each step, as indicated in Fig. 2.
Since the objective is to store as many fuel rods as reasonably possible in the space provided inside the container, all the fuel rods are nested together in a triangular array, such that any one fuel rod is in substantially full line contact along its length with fuel rods in the same row and in each of the rows on opposite sides thereof, as clearly shown in the upper cut part of Fig. 2. As used herein and in the claims, the term "substantially line contact" means that most fuel rods in the container will have line contact along their length with adjacent fuel rods, and those instances where full line contact is not made results from differences in diameter from rod to rod, differential thermal expansion of the rods, and bow of some fuel rods.
Fuel rods 26 of the type shown in Figs. 1-3 used in contemporary reactors for electric power producing purposes, are long, thin rods, measuring less than about 2.5 mm in diameter, and up to 4.60 m in length. These dimensions may vary, however, and depend on the particular design of fuel assembly used in a particular design of reactor. Although each fuel rod has great strength in compression, it is difficult to handle because it is extremely flexible and will remain in a vertical position only when laterally supported along its length. For this reason, the fuel rods cannot be loaded conveniently into the container without support.
As shown in Figs. 1 and 2, the opposite end walls 20 which are shorter than the side walls 18 have stepped top edges forming channels 30 arranged directly above the corresponding steps 24 in the support structure 43. A guidance plate 32 as shown in Figures 1 and 4-6 carrying a support bracket 36 arranged to rest in the channels 30 is provided to extend over the length of the container to provide side support to the fuel rods as they are inserted into the container. The back side 34 of the plate is flat, as shown in Figs. 1 and 5. The side of the plate facing the fuel rods has grooves 38, each of a size complementary to the fuel rods to be loaded into the container. This side of the plate also carries a leaf spring 40 which projects outwardly beyond the grooved face and in a position to engage each fuel rod as it is loaded into the container. One end of the spring 40 is guided by a pin and slot arrangement 41 which permits the spring to compress and flatten out, and ride on the pin, when engaged by a fuel rod.
As loading of the fuel rods commences, the guidance plate 32 is placed in position adjacent the first channels 30 formed on the left side of the container as it is viewed in Figure 1. Each fuel rod is then separately loaded into the container. At one side the fuel rod is guided in a groove 38 while the other side of the fuel rod lightly engages the side wall 18 of the container 11. When fully insoled the bottom end of the fuel rod rests on the first step 24 of the support structure 43.
After the first row is filled with fuel rods the guidance plate 32 is moved to the next channel 30 to provide space for loading a second row of fuel rods into the container. In so doing, the spring 40 will ride outwardly from the grooved face on pin 41 and into the position shown in Fig. 6 where it effectively will hold the fuel rods in an upright position without buckling. The bulge 42 of the spring 40 stays in this position until it is again moved inwardly by a fuel rod. This arrangement prevents the rods from moving out of position and, importantly, provides an open area between the row of fuel rods just loaded into the container and the grooved surface of the guidance plate. This area is just sufficient to accept the second row of fuel rods.
The second row of fuel rods is loaded such that the axis of the fuel rod being loaded in column 2, falls between the axes of two adjacent rods in row No. 1, as shown in Fig. 2. As the first fuel rod is loaded into the second row, it establishes line contact with -the two adjacent fuel rods in the first row on one side, and engages the groove 38 and spring bulge 42 on the guidance plate on the other side of the fuel rod. The next fuel rod is similarly loaded, and in addition to making line contact with two adjacent fuel rods in the first row, also makes line contact with fuel rod just previously loaded in row No. 2.
When row No. 2 is filled a guidance plate is arranged at the right side of the container and a first row is loaded in the same manner as previously described relative to the loading of the first row on the opposite side of the container, and then the second row, and so on, until all rows are full except the centre rows. Two rows are then loaded on the ledge 43. At this time, the guidance plate 32 is removed, and since the only space remaining is that equal to the width of the plate, a dummy plate may be inserted in the void space, if desired, or it may be left open and thus provide some degree of looseness in the assembly.
It will be apparent that the steps 24 which appear in the stepped support structure 43 are not of a width equal to the diameter of a fuel rod because, as shown in Fig. 2, the fuel rods in the second and succeeding rows are nested between two adjacent fuel rods in the preceding row.
After the container is fully loaded with fuel rods, the top cap 22 is placed in the container and locked in position, thus providing a structure having the same design as the top nozzle of a fuel assembly, thereby permitting the container to be lifted by the same lifting apparatus which lifts fuel assemblies into and out of the reactor. The top cap 22 includes a lifting section 48 integrally joined with a bottom plate 51 by side walls 52. This top cap unit is held in place by a pair of oppositely disposed pivot pins 53 which extend from the top cap side walls 54 into the side walls of the container.
A rotatable lock plate 56 is slidably mounted on the top surface of bottom plate 51 and carriers a hex nut 58 which is welded or otherwise affixed to the lock plate 56. To secure the lock plate 56 to the bottom plate 51 of the top cap assembly 22, a pin 60 extends downwardly through the lock plate 56 and bottom plate 51 and is welded at its bottom end to the channel provided in the center of the bottom plate 51. As shown in both Figs. 1 and 3, the lock plate 56 is sufficiently long to extend into slots 64 formed in the side walls of the container. It will be apparent that as the lock plate 56 is rotated from a diagonal position to a locked position, as indicated in Fig. 3, the cap will be firmly locked to the container thus sealing the container and allowing the cap to perform a load carrying function. The upper portion of the top cap further is provided with an opening 66 and flanges 68 which are designed to be engaged by the lugs of a fuel assembly lifting mechanism to lift and transfer the container from one area to another at the reactor site. Spring biased pin 70 extends downwardly into hole 72 in bottom plate 51 to preclude inadvertent unlocking of the cap after it is secured to the container.
In the modification of Figures 7-11 showing a container with a base 12 and side and end walls 18, 20 the baseplate 12 has holes 50 aligned in rows and of a size slightly larger than the end of a fuel rod. The depth and spacing of the holes are such that when the end of a fuel rod is inserted it will not move laterally. Since the holes intersect at tangent points, each fuel rod will have substantial line contact along its length with fuel rods on opposite sides thereof and with those fuel rods in the next adjacent rows.
The top edges of end walls 20 are serrated to provide channels 30 which receive the flanged ends of a guidance plate 32 as illustrated in Figures 1, 4 and 5. However, the bottom end of each of two guidance plates 32, 33 is equipped with a chamfered end 35, which engages corresponding holes 50 in the base. When the plates 32, 33 are placed in position for loading fuel rods, the space thus provided between the face of the first guidance plate and the container wall, or a previously installed row of fuel rods, will be just sufficient to accept the fuel rods and guide them into position.
As indicated previously, although each fuel rod will withstand substantial compressive forces, the rod is extremely flexible and rod guidance into its position in the container must be carefully carried out. When rods are ready to be loaded, a first guidance plate 32 which is essentially the same as that of Figures 4-6, is set into position with just sufficient space between the alignment plate grooved face and the container wall. The second plate is then placed immediately behind the first plate such that its grooved face abuts the back of the first plate. The container is then tilted at a slight angle, up to about 15°, and the fuel rods then loaded into the low side of the container. The tilt provided by the container is just sufficient to furnish a container surface against which a fuel rod may slide, or at least slightly contact, to help minimize unwanted bending or lengthwise radial distortion which otherwise could be caused by a swaying fuel rod. The grooves 38 together with springs 40, on the guidance plate face also serve to help keep the fuel rod in vertical alignment as it enters the container while maintaining line contact with the preceding rod in the same row, and finally nests in its corresponding hole in the container bottom.
After the first row is filled, the first plate which has been occupying the space provided for the second row of fuel rods, is removed and installed behind the second plate 33 which is in the third row. The two alignment pins 35 of the first plate enter the fuel rod holes in row number 4 and thereby immovably locate the plate. When this plate transfer takes place, bulge 42 of spring 40 on the second plate, moves from its housed position to its fully projected position illustrated in Figure 6. Bulge 42 thereupon engages the sides of the fuel rods and holds them in an unbuckled vertical position. When the grooved face of first plate 32 engages the back of second plate 33, spring 40 in plate 32 engages the back of plate 33 and is again moved to a housed position. The spring 40 makes an angle of about 10° with the back 34 of the guidance plate. Although a pin and slot arrangement 41 is used, the pin may be eliminated to permit the spring to move in response to contact either by fuel rods or the back of the other plate.

Typical increased fuel storage potential

Since commercial power reactors conventionally are designed for about a 30 year life span, it is evident that the design of fuel rod storage described herein will accommodate all of the spent fuel rods over the lifetime of the reactor.
The additional benefits which will flow from this design include the elimination of the need to ship spent fuel assemblies from a reactor area to a remote storage area which, likely, will be located at a large distance from the reactor site. If spent fuel assemblies are stored at a remote site the possibility exists that the spent fuel which still has a useful life in a different type of reactor, may not be recoverable from the storage area. As reprocessing of nuclear fuel materializes and facilities are set in place for reprocessing purposes, the number of fuel shipments will not be as great as it would be if fuel assemblies alone were shipped to a reprocessing facility.
In the base plate modification shown in Figure 11, instead of using holes 50 to receive the chamfered ends 35 of an alignment plate 32, 33, a series of parallel grooves 60 are machined in the container base plate. These grooves form pedestals 62 on which fuel rods also are placed. Since nesting of the fuel rods takes place, the grooves in the alignment plate face are shown by dotted lines in Figure 11.

Claims

1. A storage container (11) for holding spent fuel rods in a compact array, said container having a base arranged to support fuel rods (26) and walls (18, 20) to contain the fuel rods, characterized in that two opposite end walls (20) of said container (11) have upwardly opening channels (30) on their upper ends spaced from each other a distance equal the distance of the rows of fuel rods to be disposed in said container (11) and that a guidance plate (32) is provided in said container (11) for removal disposition and displacement from row to row in order to facilitate loading of fuel rods (26) into the container (11) in rows, said guidance plate (32) having protruding brackets (36) at its upper end to be received in said channels (30) and said container and the guidance plate (32) having corresponding means (24, 50, 35) for retractably engaging the guidance plate (32) at its lower end, the side of said guidance plate (32) facing the row of fuel rods to be inserted being equipped with grooves (38) defining the respective positions of the fuel rods (26) to be loaded in the row and providing a side support for the fuel rods (26) of a row when being inserted wherein each fuel rod (26) is in substantial line contact with the adjacent fuel rods (26).

2. A container according to Claim 1, characterized in that said guidance plate (32) includes springs (40) for engaging the fuel rods (26) being loaded into the container (11) and urging them into alignment with fuel rods (26) previously loaded into the container (10).

3. A container according to Claim 1 or 2, characterized in that said means for engaging the guidance plate (32) at its lower end is a stepped support structure (43) for the fuel rods 26, wherein said rows on said base are at different elevations and said channels (30) at the top of said opposite end walls are correspondingly stepped such that, after loading of fuel rods into a row, said guidance plate (32) may be moved in place for the next row by lifting the guidance plate one step.

4. A container according to Claim 1 or 2, characterized in that the base of said container (11) is flat and has holes (50) therein of a size sufficient to accept the end of a fuel rod (26), such that fuel rods (26) having their ends therein will lie in substantial line contact with adjacent fuel rods (26) to thereby provide support for each other.

5. A container as claimed in any of Claims 1 to 4, characterized by a cap (22) including a locking member (56) movably mounted thereon to permit completely closing and locking the cap (22) on said container (11); and means (66) on said cap (22) adapted to be engaged by lifting lugs on a lifting device for raising and moving the container (11) from one position to another.