CA1236936A - Apparatus for storing fuel elements in a pool - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
The disclosure herein describes an apparatus for storing fuel elements which are stored in racks placed at the bottom of a pool and separated by sheets of water, the walls of the racks being externally covered with a neutrophage material layer. According to the invention, this layer is constituted by a thin cadmium sheet continuously covering the outer surface of the walls in accordance with the periphery of the rack. This thin sheet is held in place and protected by a thin metal plate, preferably of stainless steel.
Application to the storage of nuclear fuel elements.

Description

- ~Z36936 APPARATUS FOR STORING FUEL ELEMENTS IN A POOL

BACKGROUND OF THE INVENTION
The present invention relates to an improve-ment made to apparatus for storing nuclear fuel elements in pools.
In nuclear power stations, the fuel elements not being used in the reactors are stored in means constituted by metal racks regularly distributed in the storage pools. At present, the spacings between the racks are relatively large, because the water between the racks only acts as a neutrophage moderator.
However, increasing efforts are being made to increase the capacity of pools by reducing the spacings between the racks, particularly in order to permit a greater flexibility with respect to reprocessing delays. This leads to a reduction in the thickness of the sheets of water separating the walls of two adjacent racks. It is conse-quently necessary to compensate the loss due tothis reduction in the thickness in the sheet of water. The presently most frequently used solution consists of covering the walls of the racks with a neutrophage product. The most frequently used products are boron and its derivatives (boral, boron graphite, boron carbide, etc.), some of which are used by projecting fine particles directly on to the outer walls of the rack, and the necessary adhesion requirements may make it necessary for plasma projection spraying with the use of chrome and nickel as the binder. Another ~Z3693~

known solution consists of eliminating the sheet of water between the racks and placing the neutrophage product in a tight recess constitu-ted by peripheral welding of contiguous walls of adjacent racks. The first solution is difficult to carry out. The second solution without the sheet of water between the racks excludes the use of certain economically and technically interesting neutrophage materials.
SUMMARY OF THE INVENTION
The object of the present invention is to eliminate the disadvantages indicated herein-before, using an apparatus for the storage of fuel elements which is easier to realize with regards to the neutron protection material.
The present invention therefore relates to an apparatus for the storage of fuel elements in which the latter are placed in racks defined by metal walls, each rack being separated from adjacent racks by a sheet of water, theouter surface of said walls being covered by a layer of neutrophage material, wherein said layer is constituted by a thin cadmium sheet covering in a continuous manner the outer surface of said metal walls, in accordance with the circumference of the rack and wherein said thin sheet is held in place and protected by a thin metal plate, said thin sheet and said thin metal plate extending over at least part of the rack.
According to another feature of the apparatus according to the invention, the thickness of the cadmium layer is between 0.1 and 1 mm approximate-ly and preferably between 0.1 and 0.5 mm. The ~236936 thin metal plate is preferably made from stainless steel and its thickness is between 0.5 and 1 mm approximately.
_ IEF DESCRIPTION OF THE DRAWINGS
The invention is described in greater detail hereinafter relative to non-limita'tive embodiments and the attached drawings, wherein show:
Fig. 1 a diagrammatic perspective view of an apparatus for storing fuel elements in a pool.
Fig. 2 a larger scale, part sectional, perspective view of the upper part o*
a rack of the apparatus according to Fig. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Fig. 1 shows a storage apparatus 1 comprising a large number of vertically arranged racks 2, into which are introduced fuel assemblies 4. The number of racks in an apparatus like apparatus 1 is approximately 100 to 200 and there are generally several apparatuses of this type in a pool, each resting on the bottom of the pool via a not shown support system.
Fig. 2 shows in greater detail the upper part of a rack such as 2. In this case, the racks have a square cross-section and are bounded by metal plates such as 6 and which are generally made from stainless steel. The upper part 8 of each plate 6 forms an angle with the remainder of the plate, in such a way that the cross-section of the rack is larger in the upper part of the storage apparatus. This facilitates the intro-duction of assemblies into the racks. Moreover,in order to ensure a better rigidity of the system, the racks are joined in their upper parts by longitudinal beams 10, welded to part 8 of plates 6.
The racks are arranged very close to one another in order to increase the storage capacity of the apparatus, the thickness of the sheet of water between the two adjacent racks being reduced to approximately 50 mm and it is necessary to coat the outer surface of walls 6 with a , continuous layer of neutrophage product 12 which, in the preferred embodiment of the invention, is constituted by a thin cadmium sheet. The latter is held in place by a thin metal plate 14, which is generally of stainless steel and which also protects it against corrosion.
Practical tests have shown that cadmium, in certain special configurations, and particularly in the case where the racks are separated by a sheet of water between 40 and 60 and preferably between 45 and 55 mm thick, have excellenent neutrophage properties, even if the thickness of the layer is small, i.e. below 1 mm. For example, with a rack spacing of 280 mm, the racks having a square cross-section of internal dimensions 225 x 225 mm, 2 mm thick metal plates 6 and 1 mm thick metal plates 14, a 0.5 mm thick cadmium layer makes it possible to obtain, with a sheet of water 48 mm thick, a Keff multiplication factor equal to or below 0.95 for fuel assemblies with an enrichment equal to or below 4.5%.

Thus, the apparatus according to the invention has particularly interesting advanta-ges, because cadmium in sheet form is easier to fit in a continuous manner around racks than the compact prior art material. Thus, it is possible to reduce the cost of the complete storage apparatus, whilst retaining the same neutron protection characteristics.

'I ' :~ :
,:,,

Claims (3)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An apparatus for the storage of fuel elements in which the latter are placed in racks defined by metal walls, each rack being separated from adjacent racks by a sheet of water, the outer surface of said walls being covered by a layer of neutrophage material, wherein said layer is constituted by a thin cadmium sheet covering, in a continuous manner, the outer surface of said metal walls, in accordance with the circumference of the rack and wherein said thin sheet is held in place and protected by a thin metal plate, said thin sheet and said thin metal plate extending over at least part of the rack; the thickness of the cadmium sheet being between 0.1 and 1 mm; the thickness of the thin metal plate being between 0.5 and 1 mm;
each rack being separated from an adjacent rack by a sheet of water with a thickness between 40 and 60 mm.

2. An apparatus according to claim 1, wherein the thickness of the cadmium sheet is between 0.1 and 0.5 mm.

3. An apparatus according to claim 1 or claim 2, wherein the thickness of the sheet of water is between 45 and 55 mm.