GB2289007A

GB2289007A - Nuclear fuel storage and transport cask internal structure

Info

Publication number: GB2289007A
Application number: GB9508889A
Authority: GB
Inventors: Miroslav Jilek; Petr Nemec; Josef Velkoborsky
Original assignee: SKODA JADERNE STROJIRENSTVI PL
Current assignee: SKODA JADERNE STROJIRENSTVI PL
Priority date: 1994-05-03
Filing date: 1995-05-02
Publication date: 1995-11-08
Also published as: DE4421486A1; FR2719699B3; CZ280289B6; CZ107194A3; GB9508889D0; SK279074B6; SK152094A3; FR2719699A3

Description

1 2289007 PATENTS ACT 1977

DESCRIPTION OF INVENTION

1 Title: "Nuclear fuel storage and transport cask internal structure" THE INVENTION relates to the internal structure of a cask intended f or the transport and dry storage of fresh or spent fuel assemblies or fuel rods from nuclear reactors.

It is known f or an internal structure of such a cask to be assembled from square or hexagonal tubes f or containing with the necessary assembly clearance the stored fuel assemblies. One known structure of this kind is manufactured by welding together strips of boron stainless steel. The boron, by neutron absorption, ensures the fuel assemblies or fuel rods within the cask remain subcritical. In order to ensure appropriate spacing of the tubes and provide for heat removal, longitudinal strips of aluminium alloy are usually placed between these tubes or transverse plates of aluminium alloy are provided with holes to receive said tubes. The technology used for the production of such cask internal structures generally involves pressing and welding but it is difficult to reach the required accuracy during assembly of the internal structure with respect to the installation into the cask body because there is also required, because of the requirements for heat removal, a minimal clearance between the outer surface of the internal structure and the cask body inner surface.

There are also known designs, again using arrays of tubes to contain the fuel elements but using boron aluminium alloy for the manufacture of tubes. In this case there is no need for separately formed heat manufacture of tubes. In this case there is no need for separately formed 2 heat dissipating sheets or plates as in the f irst-mentioned design but considering the f act that boron is not dissolved in the aluminium alloy, it is from a metallurgical point of view very difficult to ensure a uniform boron distribution in the aluminium alloy such as is necessary to ensure the fuel assemblies remain subcritical.

The above-mentioned disadvantages of the prior art are avoided or reduced by a nuclear fuel transport and storage cask internal structure embodying the invention, composed of profile tubes fitted to each other by a part of the outside wall,' each tube surrounding at least one fuel assembly and each outer side of each tube being shielded from the adjacent outer side of the next tube.

According to the invention there is provided an internal structure for a cask for transport and storage of nuclear fuel, said structure comprising a plurality of tubes packed together in substantially parallel relationship, said tubes being provided on their outer surfaces with retaining formations which retain in place, between adjoining said tubes, respective strips of neutronabsorbing material.

In a preferred embodiment of the invention, in each outside wall of each tube there is made a longitudinal dovetail recess and the bottom of each recess is provided with longitudinal projections, each recess defining, with the adjoining recess, a cavity containing a respective neutronabsorbing strip. Into both ends of each said cavity there is inserted a shrink link or expanding assembly, provided with a wedge-shaped hole into which is put an expanding wedge. Each neutron-absorbing strip is located in the space between the shrink links in the longitudinal dovetail recess between the projections from c A a 3 the f lat bottoms of the dovetail recesses, ensuring from each side a gap between the tube wall and the neutronabsorbing strip.

An advantage of the cask internal structure according to this invention is, due to its new design arrangement of tubes, the possibility of using, for the manufacture of the tubes, cheap materials, e.g. aluminium alloys without boron content, and a cheaper wasteless technology, e.g. extrusion of the tubes without the necessity of additional calibration and without decreasing the service life of the extrusion dies by the abrasive effect of boron. This is made possible by the fact that in the preferred embodiment practically the whole of each wall of each tube is lined with an absorption band or strip, e.g. of boron alloyed stainless steel, located in an optimum place from the point of view of utilization of its absorption characteristics. connection of individual tubes is mechanical, it does not require welding and with regard to the mutual contact between the tubes it is not necessary to use further heat conductive materials for heat removal.

An embodiment of the invention is described below by way of example with reference to the accompanying drawings, wherein:- FIGURE 1 shows. in cross section, part of the internal structure of a cask embodying the invention, FIGURE 2 is a view in longitudinal section of part of a cavity defined by cooperating dovetail recesses with a shrink link and expanding wedge f itted and part of a neutron absorbing strip, and 4 FIGURE 3 is a view in cross-section along the line III-III in Figure 2 showing a cross section of the cavity formed by cooperating dovetail recesses of two neighbouring tubes with the expanding wedge.

Nuclear fuel transport and storage cask internals consist according to Figure 1 of several tubes 1. The tubes are externally and internally of regular hexagonal cross-sectional shape.

Extending longitudinally along each of the outer faces of each tube 1 is a respective groove 3. The tubes 1 are packed together in a honeycomb- like array with each face of each tube engaging an adjacent face of an adjacent tube. Because of the grooves 3, the tubes engage one another only in the regions of the corner edges of the hexagons. Each groove 3 has a base wall or bottom parallel with the plane of the respective hexagon face of the tube and has opposing edge walls which converge outwardly from the tube so that, as best shown in Figure 3, each groove 3 is of dovetail shape in cross section. The two cooperating grooves 3 of adjoining faces of adjoining tubes together define a cavity which receives a strip, of rectangular section, of a neutron-absorbing material such as boron alloyed stainless steel. The bottom or base wall of each groove 3 is provided with projections 30, for example in the form of shallow longitudinally extending ribs, which engage the opposing faces of the neutron-absorbing strip 4. The strip 4 is held in place in its cavity by expandable closure assemblies which are expanded by wedging means to be firmly engaged in the cavity. In the preferred embodiment shown, each said expandable assembly comprises two edge blocks 5 each engaging a respective lateral edge of the respective cavity, the blocks 5 defining therebetween a tapering slot 50 receiving a complementary C z expanding wedge 6 which is driven into the recess 50 to thrust the blocks 5 laterally against the edges of the cavity defined between the cooperating grooves 3. The wedge 6 is provided with a screw threaded bore 60 at the outer end of the wedge. The bore 60 is intended to receive a correspondingly screw-threaded end of a dismantling screw (not shown) used for disassembly of the core structure. Once the cask internal structure has been assembled, the wedges 6 are secured in place by weld beads 70.

As best shown in Figure 5, the edges of the blocks 5 which engage the lateral edges of the recesses defined by cooperating grooves 3 are themselves provided with longitudinally extending V-section grooves which cooperate with the inclined edge walls of the groove 3 to draw adjacent tubes 1 together as the assembly 5, 6 is expanded. It will be understood that, for convenience, the two blocks 5 of each wedging assembly may be replaced by a single slab-like element having a central tapering longitudinal passage therethrough (and thus, in effect, constituting two blocks 5 connected by narrow interconnecting webs which will be stretched as the slab is expanded by insertion of wedge 6). In the preferred embodiment illustrated the tubes 1 comprise lengths cut from an aluminium alloy extrusion of the constant cross sectional shape shown in Figure 1.

To summarise, nuclear fuel transport and storage cask internals consist according to Figure 1 of several tubes 1. The tubes are hexagonal and their outside walls 2 are fitted to each other in the hexahedron corners. In each outside wall there is a longitudinal dovetail recess 3 the bottom of which is provided with several longitudinal projections 30. on the projections 30 rests the absorption band 4 which is according to Figures 2 and 3 secured in 6 longitudinal direction from both sides by shrink links 5 which are inserted into the cavity created by at least two tubes on each of both ends of the longitudinal dovetail recesses 3 of both mutually connected tubes. Each shrink link 5 is provided with a wedge-shaped hole 50 f or the expanding wedge 6 which is in this case provided with a pulling orifice 60 with a thread for screwing up the nonplotted dismantling screw and secured by weld joints 70 in the desired position. Nuclear fuel transport and storage cask internals are assembled so that between individual tubes 1 are one by one inserted the absorption bands 4 and from both ends the shrink links 5 which after inserting the expanding wedge 6 join the tubes 1 to each other. Number and configuration of tubes 1 are practically optional according to the cask size.

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Claims

1. An internal structure for a cask for transport and storage of nuclear fuel, said structure comprising a plurality of tubes packed together in substantially parallel relationship, said tubes being provided on their outer surfaces with retaining formations which retain in place, between adjoining said tubes, respective strips of neutron-absorbing material.

2. A structure according to claim 1 wherein said tubes are of aluminium alloy and said strips are of boron alloyed stainless steel.

3. A structure according to claim 1 or claim 2 wherein said tubes are externally of regular hexagonal form and are of the same size externally and are pushed together faceto-face in a honeycomb-like array, each said tube having a respective groove extending longitudinally along each face thereof, whereby there is defined between adjoining said faces, by cooperating said grooves, a respective cavity accommodating a respective said strip of neutron absorbing material.

4. An internal structure according to claim 3 wherein each said groove is of dovetail shape in cross-section whereby the mouth of the groove is narrower than the base wall thereof.

5. An internal structure according to claim 3 or claim 4 wherein the base wall of each said groove is provided with projections.

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6. An internal structure according to any of claims 3 to 5 wherein an expanding retaining assembly is fitted in each said cavity at each end thereof to retain the respective strip of neutron absorbing material in place in the respective cavity.

7. A nuclear fuel transport and storage cask internal structure consisting of at least two tubes, each tube surrounding at least one fuel assembly, are noted for the fact that the tubes are f itted to each other by at least parts of the outside walls and that in each outside wall (2) of each tube (1) is a longitudinal dovetail recess (3) the bottom of which is provided with longitudinal projections (30) and into each of both ends of the longitudinal dovetail recess (3) which together with the other tube creates a cavity, is inserted a shrink link (5) provided with a wedge-shaped hole (50) for the expanding wedge (6), the absorption band (4) being inserted into the longitudinal dovetail recess (34) between the shrink links (5).

8. An internal structure for a cask for transport and storage of nuclear fuel, substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

9. A nuclear fuel transport and storage cask incorporating an internal structure according to any of claims 1 to 7.

10. Any novel feature or combination of features described herein.

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