Induced nuclear reactions: processes – systems – and elements – Handling of fission reactor component structure within... – Storage container systems for new and/or irradiated core...
Reexamination Certificate
2000-04-17
2001-07-03
Behrend, Harvey E. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Handling of fission reactor component structure within...
Storage container systems for new and/or irradiated core...
C250S507100
Reexamination Certificate
active
06256363
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a storage/transport container for spent nuclear-fuel elements. More particularly this invention concerns such a container used for spent fuel rods.
BACKGROUND OF THE INVENTION
A transport/storage container for spent nuclear-fuel elements typically has a vessel having a side wall with an inner surface defining an interior extending along an axis and a multilevel basket extending substantially a full axial length of the interior and forming a plurality of axial full-length rectangular-section wells adapted to receive the spent fuel elements. The lower end of the interior is closed by a permanent floor and the upper end by a massive but removable cover.
It is essential to maintain the rods held in such a container in a subcritical state. Thus their neutron emissions must be controlled. This is normally accomplished as described in U.S. Pat. No. 5,032,348 by forming each level of the basket of a plurality of fitted-together plates. Each such set of plates forms a plurality of openings that together form the wells that receive the spent fuel rods. All the plates fit together at slot joints at the corners of all the wells. The neutron-absorbing plates are normally of a relative poor thermal capacity so it is necessary to alternate layers made of plates of neutron-absorbing material with layers made of plates of a more thermally conductive material so that the heat generated in such a container can be conducted to the side walls.
Such a system therefore trades dissipating heat off against suppressing neutron emissions. One function can only be made better by making the other worse. In addition fitting together the numerous plates making up each level is an onerous task, involving meticulously fitting together long plates simultaneously at multiple joints so that the container is expensive to manufacture of a large number of pieces of different sizes.
In another system borated-steel plates form the baskets. Plates of this material must be welded or screwed together, as the slot joint described above is not usable because of the brittleness of borated steel. Thus assembly of the basket becomes extremely expensive since it is too complex to automate, even with welding which is the preferred and cheaper solution. Furthermore the container is extremely heavy when its basket is made of steel.
It is further known for some of the wells in a borated-steel basket to be water filled, acting as neutron traps. Such use of water makes the container quite large and of course also increases its weight, while making the gaps necessary for flow of the water increases the cost of the plates. Furthermore using some of the wells for water only reduces the number of wells usable for fuel rods, reducing the capacity of the container. The basket furthermore is not particularly strong and has difficulty meeting requirements in this regard, as the basket must keep the fuel rods apart even if dropped or otherwise subjected to some serious axial and/or radial stress. Finally, such a system is difficult to decontaminate.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to provide an improved container for spent nuclear-fuel rods.
Another object is the provision of such an improved container for spent nuclear-fuel rods which overcomes the above-given disadvantages, that is which is of inexpensive and light construction and considerable neutron-absorbing capacity yet which has a high load capacity and is easy to decontaminate.
SUMMARY OF THE INVENTION
A transport/storage container for spent nuclear-fuel elements has according to the invention a vessel having a side wall with an inner surface defining an interior extending along an axis and a plurality of like basket sections forming a stack extending substantially a full axial length of the interior and forming a plurality of axial full-length rectangular-section wells adapted to receive the spent fuel elements. Each of the basket sections is formed of two long light-metal neutron-absorbing plates crossing each other, each having a pair of outer ends directly engaging the inner surface of the side wall in heat-transmitting contact therewith, and subdividing the interior at the respective section into a plurality of segments. A plurality of short light-metal neutron absorbing plates are fitted together in each of the segments and form with the main plates of the respective section rectangular-section axially throughgoing openings forming the wells with the plates of the other sections.
The short plates are not actually connected to the long plates. Thus these long plates can fulfill the heat-conducting function that is preferred at each level. In addition the use of interfitted short plates in each segment of each level means that these short plates are easily assembled together. In a standard system with eight wells per segment there will be at most two joints between any short plate and the other plates. Such a system is still as strong as needed, even though the short plates are not actually joined to the long plates. The wells are normally defined on at least two sides by the plates and, toward the center of the container they are defined on all four sides by the plates.
For maximum strength and lightness the plates are made of a borated light metal, preferably aluminum or an aluminum alloy. Normally the plates are all of the same material, since the long plates will also work effectively to conduct heat to the wall of the container. Preferably according to the invention the long plates are thicker than the short plates for best heat conduction and strength. More particularly the long plates are between 9 mm and 15 mm, preferably 11 mm and 13 mm, thick. The short plates are between 6 mm and 10 mm, preferably 7 mm and 9 mm, thick. With a system having 32 wells, only two different lengths of short plates are needed, plates having a length equal to twice the width of a well, and plates with three times that width.
In accordance with the invention the inner surface is generally cylindrical and centered on the axis. The long plates are perpendicular to each other and subdividing the interior at the respective level into four quadrants. In addition at least one respective filler block is provided in each of the quadrants. The blocks each have a curved outer surface complementarily engaging the inner surface at the respective level and at least one planar inner surface forming walls of respective openings. Furthermore each filler block is provided with shielding. These blocks can be made of cast borated aluminum with lead bodies imbedded in them.
Furthermore the plates each have upper and lower edges which, except for at the bottom and top levels, engage lower and upper edges of the overlying and underlying plates. The wells therefore basically continuous side walls so that any neutrons will be intercepted. Ends of the short plates bear in heat-transmitting relationship on the main plates and on the filler blocks so that, even though these short plates are normally somewhat thinner than the long plates, they also transmit heat effectively. There is no actual connection of the short plates to the long plates, for instance by means of half-width joint slots; at most the short plates bear longitudinally on the main plates. Thus these filler blocks, which have the same axial dimension as the plates, have three functions: conducting heat away from the contained rods to the container walls, blocking neutrons, and transversely bracing the basket sections and tubes in the container.
According to the invention a respective light-metal tube extends substantially the full length of each of the wells. The spent nuclear-fuel elements are within the light-metal tubes. These tubes are each provided with axial guide passages and is provided therein with neutron-absorbing rods. Each tube is of the same square or rectangular section as the respective well and fits tightly therein so that these tubes considerably rigidify the system, holding the basket sections in accurate axial alignment with one anothe
Methling Dieter
Spilker Harry
Behrend Harvey E.
Dubno Herbert
GNB Gesellschaft fur Nuklear-Behalter mbH
Wilford Andrew
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