EP0022031B1 - Large-capacity basins for storing radioactive products - Google Patents

Abstract

1. New reservoirs for storing radioactive materials in water, characterized in that they are of toric shape and a ventilation shaft is provided in the central free part of the reservoirs to allow an adequate ventilation of said reservoirs and of the exchangers used.

Description

The present invention relates to large-capacity basins for the storage of radioactive products.

The radioactive waste is generally stored, at the exit of the piles, in basins filled with water which one compared to swimming pools.

The basins currently used have the shape of a parallelepiped; seen in plan this parallelepiped has the shape of a rectangle whose width is limited by the normal range of the handling bridges which must put in place and remove the "baskets" of radioactive material and whose length is limited by problems of expansion materials (concrete) in which the basins are made. Consequently, in the current state of the art, there is a dimension of the basins which cannot be exceeded.

However, the multiplication of radioactive waste likely to be immersed in such basins is such that it appears desirable to be able to have in the near future of containment basins (that is to say in fact surface) significantly greater than that of the current basins. This is one of the aims of the present invention.

In addition, the current basins all have walls made up of a thick concrete veil lined, on its internal face, with a stainless steel wall. This stainless steel wall is produced on site using flat sheets which are welded to each other edge to edge and on the other fixed in vertical uprights (wooden or metallic) integral with the web of concrete. This arrangement of the stainless steel inner lining has a number of advantages, but it also has notable drawbacks which will increase as the dimensions of the basins increase, due to differences in expansion between the concrete and the stainless steel. It therefore appeared desirable, in particular for larger basins, to use a novel device for coating the interior walls of the concrete of the basins by means of stainless steel elements. This is another object of the present invention.

Finally, it goes without saying that any new design of the basins leads to new and / or preferential possibilities for the development of the ancillary devices and the easements necessary for the operation of these basins. The basins according to the invention have proved to be particularly advantageous for the organization of cooling and ion exchange devices; such a rational organization of these devices also constitutes an important secondary characteristic of the invention.

The present invention therefore relates to basins for storage of radioactive products in water, characterized in that they have a toric shape and that the free central part of these basins is surmounted by a chimney allowing proper ventilation of the basins and exchangers used.

This toric shape is most generally circular, the diameter of the inner wall of the torus being at least 30 m and the diameter of the outer wall up to 120 m being understood that the distance between the inner wall and the outer wall will generally be compatible with the spans of handling bridges. The reasonable dimensions seem, under current technological conditions, to be: diameter of the interior wall 46 m, diameter of the exterior wall 90 m.

The depth of these basins can be arbitrary; in general, the same depth will be used as that of the current basins, that is to say of the order of 9 m.

The construction of basins of this type is within the reach of those skilled in the art; the walls of the basin are of reinforced concrete of sufficient thickness to ensure on the one hand biological protection and on the other hand resistance to deformations and earthquakes; these walls could for example be 1.5 m thick.

The basins are preferably of the underground type, that is to say that they are mounted on studs, the upper part of which is provided with flexible connection devices which allow on the one hand the expansion of the concrete and of on the other hand, a first resistance to horizontal earthquakes.

The inlet and outlet structures of the basins are advantageously constituted by "hop jumps" constructed so as to be separated in structure from the basins and connected to the lateral ground.

According to an advantageous embodiment of the invention the internal faces of the basins are coated with a continuous coating of stainless steel by means of a vertical corrugated wall fixed to the upper part of the walls of the basins and by means of a plate bottom, the connection between said vertical corrugated wall and said bottom plate being produced by means of an expansion wave, also made of stainless steel, welded to said corrugated wall and to said bottom plate.

The bottom plate is obviously made by welding elementary stainless steel plates; this plate is welded around its edge to an edge of a stainless steel expansion wave.

The vertical corrugated wall is formed by the in situ welding of unitary elements of suitable shape; these elements have a shape such that the vertical weld line, between two adjacent elements, is at a certain distance from the interior face of the concrete wall when the adjacent unit elements are pressed against said internal face. An example of usable unit elements is constituted by "boards".

The lower part of the vertical corrugated wall is welded to a stainless steel expansion wave; the purpose of this expansion wave is to deform when, on the one hand, the corrugated wall and, on the other hand, the bottom plate expand or retract under the influence of a change in the temperature of the water.

The upper part of the vertical corrugated wall is fixed, for example by means of a flap, to the upper part of the concrete wall.

This internal coating system for basins has significant advantages over the coating techniques currently used in basins.

The toric arrangement of the basins according to the invention gives rise to certain new possibilities with regard to the additional devices necessary for the operation of said basins. It is essentially a matter of using the free space located in the center of the torus as rationally as possible and of placing there the heat and / or ion exchangers and the ventilation elements.

The ventilation, which contributes to the cooling of the basin, can be easily and advantageously carried out by the installation of a chimney covering the free space located in the center of the torus; this chimney is provided with a propeller creating forced ventilation and an upward air flow; this current of air coming from around the basin and passing under this basin can be guided so as to lick the underside of said basin.

The heat exchangers can conventionally be constituted by plate exchangers, located for example in the free space forming the center of the torus, which receive on the one hand, by means of a pump, the water from the basin and on the other share a fluid allowing cooling.

But, and this is one of the complementary aspects of the invention, it has been found more convenient to carry out the various exchanges (thermal, ionic, filtration) that the water in the basin must undergo using autonomous exchangers immersed in said basin. By autonomous exchanger means an individualized, independent exchanger and therefore interchangeable; moreover, these submerged autonomous exchangers are by definition supplied with water from the basin using known devices providing water at low or medium pressure.

Thus, to carry out heat exchanges, it is advantageous to use autonomous submerged exchangers receiving, for example by means of removable tubes, a fluid allowing the cooling conveyed by means of a pump, said exchanger being traversed for the water of the basin by a pump. with integrated propeller. Such exchangers will be removable. These submerged exchangers can be used by supplying them with cooling fluid, with a condensed fluid that will vaporize in the exchanger by evacuating the calories from the water in the basin; such a system (which will employ, for example, butane or isobutane as heat transfer fluid) is known to technicians.

It is also possible to use a similar system of exchangers immersed in the basin to carry out the ion exchange of the water of said basin.

The following nonlimiting example illustrates a preferred embodiment of the invention; a basin according to this example is shown diagrammatically in FIGS. 1, 2, 3, 4 and 5.

Figure 1 is a sectional view of a basin according to the invention.

Figures 2 and 3 are diagrams, in plan and in section, of a wall portion of a basin according to the invention with its coating of stainless steel.

Figures 4 and 5 are diagrams, in section, of two exchangers, one thermal and the other ionic immersed in the basin.

• - en 1 le bassin proprement dit construit en béton, avec revêtement intérieur d'acier inoxydable; ce bassin a la forme d'un tore de section rectangulaire ouverte à sa partie supérieure; les deux parois verticales sont circulaires, la paroi interne a par exemple un diamètre de 40 m et la paroi externe un diamètre d'environ 85 m; la hauteur du bassin est de 11 m; ce bassin torique repose sur des supports en béton 2 convenablement disposés avec intercalation éventuelle, entre lesdits supports et le bassin, d'éléments connus permettant la dilatation normale du bassin sous l'influence de changements de température,
• - en 3 les murs du bâtiment entourant le bassin,
• - en 4 un pont de manutention des substances actives,
• - en 5 la cheminée centrale couvrant le vide central du bassin; cette cheminée d'aération est de préférence munie d'un dispositif connu 6 (par exemple hélice) permettant d'y assurer une ventilation forcée; on peut cependant concevoir cette cheminée de façon qu'une ventilation naturelle suffisante soit assurée,
• - en 7 un dispositif d'échange thermique ou ionique qui sera ultérieurement décrit.

In Figure 1 is shown in section a diagram of a basin according to the invention; there are shown in particular:

• - in 1 the basin itself, constructed of concrete, with an internal coating of stainless steel; this basin has the shape of a torus of rectangular section open at its upper part; the two vertical walls are circular, the inner wall for example has a diameter of 40 m and the outer wall a diameter of about 85 m; the height of the pool is 11 m; this toroidal basin rests on concrete supports 2 suitably arranged with possible intercalation, between said supports and the basin, of known elements allowing normal expansion of the basin under the influence of temperature changes,
• - in 3 the walls of the building surrounding the basin,
• - in 4, a handling bridge for active substances,
• - in 5 the central chimney covering the central void of the basin; this ventilation chimney is preferably provided with a known device 6 (for example a propeller) making it possible to provide forced ventilation; we can however design this chimney so that sufficient natural ventilation is ensured,
• - In 7 a heat or ion exchange device which will be described later.

A basin of this type, possibly fitted with accessory input or output devices called "chip jumps" which are preferably separated in structure from the basin, would be capable of receiving approximately 10,000 t of active material to be stored.

Figures 2 and 3 are diagrams, in plan and in section, of a portion of basin wall showing a method of interior coating, using stainless steel, of the walls of this basin.

In Figure 3 we see the circular concrete wall 8 which, thanks to the coating device according to the present invention, we can leave the internal surface to be coated in the state of raw concrete; at 9 is shown the internal coating of stainless steel; this coating is made from unitary elements which welded to each other constitute a corrugated surface. These unit elements can have very different shapes, but it is important that the welds 10 between two neighboring unit elements are made on ends of these elements which are at a certain distance from the wall when said elements are pressed against said wall; thanks to this latter arrangement it can be verified that the welds between the various elements are properly carried out.

In Figure 2, it appears that the coating 11 of the bottom of the basin is made using flat sheets welded together; between the covering 11 and the vertical elements 9 it is important to provide an expansion wave 12; this expansion wave, which can play in a groove made for this purpose in the lower part of the wall of the basin, allows the vertical stainless steel wall to be hooked to the only upper part of the concrete wall, this attachment being shown schematically in 13; thus, unlike the techniques currently used for making stainless steel coverings for basins, the stainless steel wall is not linked, at points other than the attachment points, to the concrete wall to be coated.

• - une partie support 16 qui repose sur le fond du bassin avec une fixation éventuelle, sur ce fond, à l'aide d'un dispositif simple tel qu'un dispositif baïonnette par exemple; ce support se présente par exemple sous la forme d'un tube vide muni d'une embase et d'un (ou plusieurs) orifice d'évacuation 17; la partie supérieur de ce support a généralement une forme évasée de façon à faciliter la mise en place, sur ce support, de la partie échange du groupe,
• - une partie échange 18 qui comporte les éléments d'échanges proprement dits c'est-à-dire soit des faisceaux de tubes pour l'échange thermique, soit un lit de particules pour les échanges ioniques. Pour cette partie d'échange on peut utiliser tout dispositif connu qui ne nécessite pas, pour la circulation de l'eau du bassin à travers ledit dispositif, une pompe à haute pression. Dans le cas des échangeurs thermiques on utilisera par exemple des faisceaux de tubes, certains de ces tubes étant alimentés à l'aide d'un fluide réfrigérant extérieur, les autres tubes étant parcourus par le liquide du bassin; l'alimentation des tubes du réfrigérant à l'aide d'un fluide réfrigérant extérieur s'effectue, à partir d'une prise 19, au moyen de préférence de tubes souples 20; ce fluide réfrigérant peut être de l'eau ou un liquide susceptible de se vaporiser dans l'échangeur en absorbant les calories de l'eau du bassin. L'alimentation des "éléments d'échanges" avec l'eau du bassin (pour refroidir cette eau ou lui faire subir des échanges ioniques) s'effectuera à l'aide d'un dispositif susceptible d'assurer une circulation d'eau sous basse ou moyenne pression; ainsi on utilisera par exemple soit un moteur immergé soit un moteur 21 situé hors du bassin commandant une hélice plongée dans l'eau du bassin, cette hélice assurant la circulation de l'eau à travers la partie d'échange. Les avantages particuliers de ce système d'échange sont notamment:
• l'absence de circulation de l'eau légèrement radioactive à l'extérieur du bassin et,
• -la facilité d'intervention sur ces échangeurs qui sont aisément démontables.

Figures 4 and 5 show the autonomous exchange groups (thermal or ionic) usable in the basins according to the invention. These groups 14 and 15 are essentially characterized in that they are immersed directly in the basins, preferably in the vicinity of one of the vertical walls of these basins. These groups include:

• - A support part 16 which rests on the bottom of the basin with a possible fixing, on this bottom, using a simple device such as a bayonet device for example; this support is for example in the form of an empty tube provided with a base and one (or more) discharge orifice 17; the upper part of this support generally has a flared shape so as to facilitate the installation, on this support, of the exchange part of the group,
• - An exchange part 18 which includes the actual exchange elements, that is to say either bundles of tubes for thermal exchange, or a bed of particles for ionic exchanges. For this exchange part, any known device can be used which does not require a high pressure pump for the circulation of the water in the basin through said device. In the case of heat exchangers, for example, bundles of tubes will be used, some of these tubes being supplied with the aid of an external cooling fluid, the other tubes being traversed by the liquid from the basin; the refrigerant tubes are supplied with an external coolant from a socket 19, preferably by means of flexible tubes 20; this refrigerant may be water or a liquid capable of vaporizing in the exchanger by absorbing the calories from the water in the basin. The "exchange elements" are supplied with water from the basin (to cool this water or subject it to ion exchange) using a device capable of ensuring circulation of water under low or medium pressure; thus, for example, either a submerged motor or a motor 21 located outside the basin controlling a propeller immersed in the water of the basin will be used, this propeller ensuring the circulation of water through the exchange part. The particular advantages of this exchange system are in particular:
• the absence of circulation of slightly radioactive water outside the basin and,
• -the ease of intervention on these exchangers which are easily removable.

The basins according to the invention have considerable advantages compared to current basins; for the most part the new basins are much more balanced, more resistant to deformations and earthquakes, easier to build and use and, for the same footprint, much more efficient as regards the quantities of radioactive materials which are storable in said basins.

Claims (4)

1. New reservoirs for storing radioactive materials in water, characterized in that they are of toric shape and a ventilation shaft is provided in the central free part of the reservoirs to allow an adequate ventilation of said reservoirs and of the exchangers used.

2. Reservoirs according to claim 1, characterized in that they are of circular toric shape, the inside diameter of the torus being at least about 30 metres whereas the outside diameter can reach about 120 metres.

3. Reservoirs according to any one of claims 1 and 2, characterized in that the inside vertical faces of the walls are lined with a vertical undulated wall in stainless steel fastened to the upper part of said walls and connected to the base-plate, also in stainless steel, by way of an expanding ridge.

4. Reservoirs according to any one of claims 1 to 3, characterized in that the ion and heat exchangers needed to operate the reservoirs are immersed in the water of said reservoirs.

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