Fuel assembly for a boiling water reactor

A fuel assembly of a boiling water reactor contains a fuel assembly base closed at the top by a filter plate, a fuel assembly head, and, disposed between them, a bundle of long and medium-length fuel rods extending away from the fuel assembly base. A water channel is disposed inside the fuel rod bundle. A plurality of the medium-length fuel rods are fixed axially on a separate holding part which is connected to the upper side of the fuel assembly base.

BACKGROUND OF THE INVENTION

Field of the Invention

The invention relates to a fuel assembly for a boiling water reactor. Such a fuel assembly, known for example from German Utility Model DE 201 05 913 U1 and Austrian patent AT 365 828 C, contains a fuel assembly base closed at the top by a filter plate, a fuel assembly head and, disposed between them, a bundle of long and medium-length fuel rods extending away from the fuel assembly base, and a water channel disposed inside the fuel rod bundle. The long fuel rods are held between the fuel assembly base and head with a small axial play. This play allows the fuel rods to expand in a longitudinal direction, without mechanical stresses occurring. The medium-length fuel rods, which extend starting from the fuel assembly base only as far as a position below the fuel assembly head, are used in boiling water reactors substantially for reasons of thermohydraulic stability and neutron moderation. In order to prevent the medium-length fuel rods from floating up during the reactor operation and reaching an unfavorable placement position, they are held on the fuel assembly base. This is conventionally done by incorporating bores, into which a sleeve is welded, in the filter plate (fuel guard) closing the fuel assembly base on the upper side and used to trap foreign bodies in the cooling water. A spring assembly, for instance sleeve-shaped, is fixed at the lower end of medium-length rods and is fitted into the bore provided with a sleeve in the filter plate and held there by a friction-fit or form fit. Producing the pores in the filter plate, soldering in the sleeves and fastening spring assemblies on the lower end of medium-length rods entail relatively high manufacturing and assembly outlay.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a fuel assembly for a boiling water reactor which overcomes the herein-mentioned disadvantages of the heretofore-known devices of this general type, which provides for a simple fastening of a medium-length fuel rod on a fuel assembly base.

With the foregoing and other objects in view there is provided, in accordance with the invention, a fuel assembly for a boiling water reactor. The fuel assembly contains a filter plate, a fuel assembly base having an upper side with a top and closed at the top by the filter plate, a separate holding part connected to the upper side of the fuel assembly base, a fuel assembly head and a bundle of fuel rods, including long fuel rods and medium-length fuel rods, extending away from the fuel assembly base and disposed between the fuel assembly base and the fuel assembly head. A plurality of the medium-length fuel rods are fixed axially on the separate holding part. A water channel is disposed inside of the bundle of fuel rods.

The object is achieved by a plurality of the medium-length fuel rods being fixed axially on a separate holding part which is connected to the upper side of the fuel assembly base. Introducing bores into the filter plate of a fuel assembly base and soldering in sleeves can be obviated. It is admittedly necessary to produce one or more separate holding parts. These, however, can be configured relatively simply, for example as stamped sheet metal parts, and can then be produced with correspondingly little outlay. Furthermore, a plurality or even all of the medium-length fuel rods can be fixed simultaneously on such a holding part. The holding part can likewise be fastened on the fuel assembly base or on the filter plate in a straightforward way, for example by welding at a few positions.

A further simplification relates in configuring the holding part so that long fuel rods can also be supported thereon, with or without axial fixing. A plurality of medium-length and long fuel rods are thus simultaneously assigned to one holding assembly. For example, a holding assembly may then extend over an entire fuel rod row without this entailing particular requirements for the configuration of the holding assembly. In particular, a single holding assembly may be used for fixing all the fuel rods of a fuel assembly.

A holding part may also be configured so that at least some of the long fuel rods are fixed at least radially thereon. Lateral holding of the long fuel rods thereby takes place in the region of the fuel assembly base, so that the bottom spacer, i.e. the one lying closest to the fuel assembly base, can be obviated. In this context, it may be advantageous for at least some of the medium-length fuel rods also to be fixed radially on a holding part.

A fuel rod is preferably fixed on the holding part with the aid of a snap connection with an axial assembly direction. This simplifies assembly of the fuel rods on the fuel assembly base. A fuel rod merely needs to be fitted axially into a correspondingly configured holding assembly on the holding part, without further fixing measures being necessary. According to a preferred configuration of such a type of connection, the lower end of a fuel rod has a waist behind which a latch assembly disposed on a holding part engages in an axially effective fashion. The latch assembly is preferably formed by two clip assemblies molded onto the holding assembly with their flat sides facing one another, these respectively having a precurved region which engages resiliently into the waist. The mutually opposing clips also retain a fuel rod laterally, so that to this extent there is already some degree of lateral or radial fixing. Lateral displacement could at most take place in a direction extending for instance along the clips. This, however, is prevented in a preferred configuration by two projections protruding laterally at diametrically opposing positions being provided on the lower end of a fuel rod, each of which engages in a recess of a clip.

In order not to further limit the flow cross section for the cooling water flowing through a fuel assembly base, which is already restricted by the filter plate, a holding part is disposed on the filter plate so that its filter openings remain substantially free. In the case of filter plates which contain bars extending mutually parallel, this is ensured by the holding parts being configured as rails which are disposed mutually parallel and with the grid period of the filter plate bars. It is in this case conceivable to use separate rails, i.e. ones which are not connected to one another. In such a case the individual rails are fixed, in particular welded on the grid plate. According to another advantageous configuration, the rails are components of an integral grid plate, and they are molded by their ends onto connecting bars aligned transversely to them. Such a grid plate can be fixed on the fuel assembly base with little assembly outlay. Particularly simple fixing is ensured by the grid plate containing an opening, through which the water channel passes, behind the opening edge of which the water channel engages in an axially effective fashion. Rotational fixing of the holding part, which is necessary in particular whenever the bottom spacer is intended to be obviated, may for instance be carried out by welding the grid plate on the fuel assembly base. Preferably, however, the grid plate is rotationally fixed on the water channel, which may be achieved by a form fit effective in the rotational direction between the water channel and the opening, or its opening edge.

Although the invention is illustrated and described herein as embodied in a fuel assembly for a boiling water reactor, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the figures of the drawing in detail and first, particularly, toFIG. 1thereof, there is shown a fuel assembly of a boiling water reactor which, as substantial components, contains a fuel assembly head1, a fuel assembly base2, a bundle of fuel rods3which is disposed between them, a water channel4disposed inside the fuel rod bundle and a plurality of spacers5which are spaced apart in the axial direction and hold the fuel rods3laterally.FIGS. 2 to 6show a first embodiment for fixing medium-length fuel rods3aon the fuel assembly base2. A grid plate6shown inFIG. 2is used for this. It is composed of a plurality of rails7extending mutually parallel and forming holding parts, and thereto connecting bars8aligned at a right angle, the ends of the rails7being integral with the connecting bars8. The rails7are spaced apart uniformly in a longitudinal direction of the connecting bars8. In the assembled state, the grid plate6rests on the upper side of a filter plate9closing the fuel assembly base2. The filter plate9contains a multiplicity of preforms10made of sheet steel or another suitable material, extending mutually parallel. The preforms10are connected to one another by bars12, extending transversely to their longitudinal extent, arranged on their upper and lower sides (see in particularFIGS. 3 and 7). The rails7of the grid plate6are disposed with the grid period of the bars12of the filter plate9. In the assembled state, the rails7do not therefore protrude into filter openings13present in the filter plate9, rather they extend above the bars12. The rails7and the connecting bars8are disposed in a plane, so that the grid plate6rests flat on the filter plate9in the assembled state. Both the long fuel rods3and the medium-length fuel rods3aare therefore supported not on the bars12but on the rails7. Each rail7thus extends in the direction of a fuel rod row14. A rail7a, which is assigned to a fuel rod row14acontaining medium-length fuel rods3a, forms a holding part and has a number of latch assemblies15corresponding to the fuel rods3a. A latch assembly15is formed by clips16molded at mutually opposing positions laterally on a rail7a. A grid plate6is produced by stamping from a for example 1 mm thick steel sheet. After stamping, the clips16extend in the surface plane of the grid plate6. From this situation, they are bent into their position shown inFIG. 2, in which they extend approximately in the longitudinal or axial direction of the fuel rod3a. Before this, however, embossing is carried out by which a constriction17, extending in the longitudinal direction of a rail7a, is produced in a central length section of the clips16. As can be seen particularly inFIG. 5, the constrictions17engage into an annular waist18at a lower end, formed by a plug19, of a fuel rod3. The waist18is followed upward by a conical region21. Below the waist18, the plug19has a region20radially widening toroidally. This is followed underneath by a conical region22with a rounded tip23. The end sections24, adjoining the constriction18at the top, of the clips16extend obliquely to a mid-longitudinal axis25of a fuel rod3aand include an upwardly opening acute angle (FIG. 5). The oblique setting or angle α corresponds approximately to the cone angle of the conical region21. The side edges26of a clip16extend obliquely and include a downwardly opening angle β (FIG. 6). An upper edge27of the clip16extends approximately parallel to the longitudinal direction of a rail7a. As can be seen fromFIGS. 5 and 6, the lower ends of the long fuel rods are configured like those of the medium-length fuel rods3a. The end of the long fuel rods3may, however, be configured differently.

When a fuel rod3ais being mounted, it is fitted with its plug19into the latch assembly15. The end sections24extending obliquely to one another then form an installation funnel. The conical region22and the region20adjoining it push the clips16apart. Once the plug19has been fitted fully into the holding assembly15, the constrictions17latch into the waist18and fix the fuel rod3ain the axial direction.

As can be seen fromFIG. 2, at a decentralized position of the grid plate6there is an opening28whose opening edge is formed by bars29. The bars29are shaped and connected to two rails7a′ in such a way as to form an octagonal opening28. Two short rail sections11, fitting into the grid period of the other rails and respectively with a bar29molded on their mutually opposing ends, extend between the two rails7a′. In the assembled state a non-illustrated longitudinal section of the water channel4, which has a circumferential shape formed complementarily with the opening shape, passes through the opening28. The grid plate6is thereby fixed so that it can rotate on the water channel4. The grid plate6is fixed in the axial direction by the non-illustrated longitudinal section of the water channel4, which passes through the opening28, undercutting the grid plate in an axially effective fashion. This is achieved by a non-illustrated radial shoulder, on the water channel4, which protrudes radially beyond the bars29and those sections of the rails7a′ which are connected thereto.

FIG. 7represents a second embodiment, which likewise contains a plurality of holding parts configured as rails7b. The rails7bare disposed on the bars12of the filter plate9and are welded in the region of their ends onto the filter plate9, or the fuel assembly base2. In the region of a recess32used for fixing the water channel4, two short rails7b′ between whose mutually opposing ends there is a section which is at least as large as the internal width of the recess32, are assigned to a bar12present there and formed of two subsections12a,12b, or to a fuel rod row14. A difference from the exemplary embodiment described above is that each rail7bcarries as many latch assemblies15aas there are fuel rods3,3ain a fuel rod row14. The latch assemblies15aare configured so that they also fix a fuel rod3,3ain a direction extending in the longitudinal direction of a rail7b. To this end, they have an approximately circular recess33in the region of their constriction17. The toroidally widened region20, the surface of which may for example also be configured as part of a spherical surface, engages into the recesses33of the clips16of a latch assembly15a. It is also conceivable for the clips16to have a bulge (not shown) receiving the region20. All the fuel rods3,3aare thus fixed on the fuel assembly base2in any radial directions. The fuel rods3,3a′ in the lower region of the fuel rod bundle are thereby fixed laterally so that the bottom spacer5a(FIG. 1) may be obviated. It is also conceivable for the grid plate6according toFIG. 2to be equipped with a number of latch assemblies15and/or15acorresponding to the number of fuel rods3,3aof a fuel assembly.

In order to facilitate mounting of the rails7bon the filter plate9, they are molded onto a frame35which has an outer contour corresponding to the circumferential surfaces of the filter plate9. The short rails7b′ are molded with their inner end on a plate36. A structure formed of rails7b,7b′ and the frame35may, like the alternative embodiment ofFIG. 2, be stamped from a for example 1 mm thick sheet steel preform, in which case as well the clips16are bent out of the surface plane of the original sheet metal preform into their position shown inFIG. 11. The auxiliary construction thereby obtained is put onto a filter plate9and then the individual rails7b,7b′ are welded at suitable positions, for example at their ends, to the filter plate9or the fuel assembly base2. The plate36and the frame35are subsequently separated.