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
2002-05-01
2003-12-16
Jordan, Charles T. (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...
C376S260000, C376S261000, C250S506100, C250S507100, C250S515100, C250S518100
Reexamination Certificate
active
06665365
ABSTRACT:
TECHNICAL DOMAIN
The invention relates to a storage basket containing several adjacent compartments, each of which is designed to contain conditioned radioactive materials such as fuel elements from nuclear reactors or others.
The storage basket according to the invention may be used in particular for the transport or storage of fuel elements from nuclear reactors or other nuclear materials in a wet or dry environment. In particular, in order to achieve this the basket according to the invention may particularly be placed in a transport or storage container, or in a nuclear reactor pool or inside a building. It may also be buried in geological layers.
Furthermore, the invention is particularly suitable for the manufacture of compact baskets with a regular prismatic polygonal section, for example hexagonal. In particular, this type of basket may contain fuel elements with a hexagonal cross section such as VVER type elements used in some nuclear reactors, in an optimised volume. Obviously, the basket according to the invention may also be formed of compartments with a simpler cross section, for example square or rectangular, that in particular may contain fuel elements such as elements frequently used in light water nuclear reactors.
STATE OF THE ART
Fuel elements for nuclear reactors and other radioactive materials to be transported or stored are generally arranged firstly in receptacles or compartments of a basket (also called rack or storage rack), that will itself be placed in the inner cavity of a transport container or storage container or in an appropriate storage installation.
This type of basket must fulfil several functions. These functions include particularly mechanical strength and packing of radioactive materials, and ease of handling.
Furthermore, depending on the nature of the radioactive materials, the basket must perform different functions related to nuclear safety during transport or storage. These functions include mainly the need to dissipate heat produced by the materials contained in the basket and control of nuclear criticality, when these materials are fissile materials that could provoke a chain reaction.
The purpose of the mechanical strength function is to maintain the geometry of the basket during handling operations under the effect of accelerations encountered during transport and also in the case of an accidental collision or if the basket is dropped, to maintain control of the nuclear criticality under these circumstances.
Existing baskets normally comprise straight line compartments usually delimited by composite sandwich type partitions formed from successive layers of different materials in contact with each other such that each of them fulfils at least one of the above mentioned functions.
Thus for example, a layer of a good heat conducting material such as aluminium, copper and aluminium and copper alloys may be used, together with a layer of a structural material to provide the mechanical strength of the basket in the case of an accidental shock, and a layer of a material containing a neutron absorbing element such as boron or cadmium. In particular, the structural material may be a stainless steel, or a carbon steel, or an aluminium or one of its alloys. The material containing the neutron poison element is usually a stainless steel, an aluminium or its alloys or a sintered material, for example based on boron carbide B
4
C. In the case of stainless steel or aluminium, the neutron poison element is usually added directly into this material, and does not significantly reduce its mechanical strength. A single layer of material is then sufficient to provide mechanical strength and control over criticality.
In a first type of existing basket, composite partitions of the compartments are obtained by putting flat or profiled elements called “strips” adjacent to each other and making them intersecting in the longitudinal direction. Each strip is then composed of a multi layer placement of materials like those mentioned. Another known method of achieving a regular stack that is robust in the longitudinal direction is to form notches in the strips that cooperate with each other in order, such that the strips intersect perpendicular to the centre line of the compartments and are rigidly fixed to each other.
In another type of existing basket, the composite strips are replaced by successive layers of different materials alternated in the longitudinal direction of the compartments. More precisely, strips with the same geometry are manufactured from different materials and placed alternately, in order to create a sequence of components in the longitudinal direction each fulfilling at least one of the above mentioned functions. U.S. Pat. No. 5,032,348 describes a basket made in this manner.
In some cases and under some conditions, it is possible to make a basket from a single material. For example, aluminium is a good heat conductor, and it is easy to add a neutron absorbing element such as boron into it. In order to give the aluminium elements sufficient mechanical strength, these elements are then made in the form of sufficiently thick plates, or beams with a rigid straight H or U type cross section.
However, the reduction in the number of materials used which simplifies the manufacture and reduces costs, can lead to a certain loss of performance. Thus, in the above mentioned example of a basket composed entirely of a stack of aluminium components, it is difficult to maintain the mechanical strength, particularly at the high temperatures that are possible in a basket containing radioactive materials. Consequently, the thickness of the aluminium components has to be increased, or particularly thick shapes are necessary as mentioned above. The result is that the basket becomes excessively heavy or large which is a disadvantage, unless the number of compartments and therefore the capacity of the basket are reduced.
Conversely, when a high performance basket has to be made, for example due to the nature of the radioactive materials or due to stringent mass or dimensional constraints, baskets with a composite structure, in other words formed of several materials adapted to each of the functions, are usually better adapted despite their complexity and their higher cost.
Regardless of the number of materials used, a number of difficulties can arise with manufacturing of existing baskets. The strips must be manufactured with a very high precision, so that they can be perfectly aligned when they are stacked. This condition is essential to obtain compartments with a constant cross section and with perfectly smooth walls, in order to prevent any risk of radioactive materials getting trapped when they are added and extracted. Furthermore, when notches are formed in the strips, the notches must be machined very precisely with a small assembly play, to ensure that the basket has the required stiffness without reducing the alignment of the strips.
When the compartments have a square or rectangular cross section, these difficulties can be overcome by precision machining, although the cost is not negligible. As shown in
FIG. 6
in U.S. Pat. No. 5,032,348, when the basket contains this type of compartment, single piece strips are usually used which pass from one side of the straight section of the basket to the other without any discontinuity in the material between partitions in adjacent compartments. This configuration improves the cohesion and the mechanical strength of the basket.
The use of these techniques is much more difficult for baskets with polygonal compartments (for example hexagonal compartments) with a larger number of sides. Considering the example of hexagonal compartments, strips folded in the form of broken lines then have to be used, that are then put into order along appropriate directions as illustrated in
FIG. 5
in document EP-A-0 329 581 mentioned above. In this case, it is impossible to use single piece strips passing through the straight section of the basket and therefore a large number of elements of strips have to be placed in this str
Dallongeville Maurice
Vallentin Christophe
Richardson John
Societe pour les Transports de l'Industrie Nucleaire-Transn
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