EP0154770B1 - Storage structure for high-level radioactive materials using a dense asbestos cement - Google Patents

Description

The present invention relates to a structure for the storage of high-level radioactive materials by using a dense asbestos-cement.

• - d'une part, de séparer et de confiner sous un volume minimal,
• - d'autre part, éventuellement après une certaine durée de décroissance, ou d'évolution, de stocker dans des conditions de sécurité particulièrement strictes.

The development of nuclear energy leads to the production of ever greater quantities of effluents and radioactive waste of all kinds which are appropriate:

• - on the one hand, to separate and confine under a minimum volume,
• - on the other hand, possibly after a certain period of decay, or evolution, to store under particularly strict safety conditions.

In the first phase of these operations, the effluents will be brought, if they are not already so, in a solid form and they will be coated in a suitable solid matrix capable of withstanding the mechanical, physical or chemical aggressions coming from the radioactive materials themselves. - same or from the outside environment.

In the second phase, the effluents dispersed in said solid matrix will be “conditioned” so as to be able to be preserved for more or less long periods which will be necessary for the evolution of the radioactive material, then for the proper storage of said material.

It has already been proposed, in patent DE-A-2 730 729, to store high-level radioactive products in a structure composed essentially of vertical tubes, intended to receive said radioactive products, cooled externally by air circulating in various canals.

It has also been proposed, in patent FR-A-1 347 402, to produce packages of radioactive elements, for their transport, by using an armored container which is suitably positioned in a sealed box thanks to the use of a stuffing which can be glass fibers or asbestos impregnated with synthetic resin or cement.

In order to prepare a rigid structure for the storage of high activity radioactive materials, it was first necessary to select a suitable material.

It has been found, and this is the object of the present invention, that dense asbestos cement (that is to say by definition in the present application an asbestos cement with a density greater than 1.75 g / cm ³ ) is the material of choice for the construction of tubes and connection sleeves used for the storage of radioactive products.

The asbestos-cement chosen is a material comprising a high dosage of cement (of the order of 1500 kg / m ³ ), a fine particle size due to the mixture of approximately 85% of cement having approximately 3000 to 4000 cm ² / g BLAINE specific surface area and approximately 15% asbestos at 20,000 cm ² / g and a density greater than approximately 1.75 g / cm ³ obtained by applying a strong compression at the head of the forming machine bonding and densification of successive layers of material. The values given above are average values. Those skilled in the art will be able to adapt, on the one hand, the starting materials and, on the other hand, the conditions of implementation to obtain a material acceptable for the intended application.

Furthermore, the techniques for using this asbestos-cement material, in particular for the production of tubes or pipes, are known and industrialized and make it possible to obtain tubes or pipes having diameters, thicknesses and properties adapted to the problems. of packaging mentioned above.

• - une densité suffisante pour que l'on puisse envisager la réalisation d'une protection biologique raisonnable par utilisation d'une épaisseur d'amiante-ciment comprise entre 30 et 100 mm (cas de déchets de faible activité spécifique et des déchets émetteurs a);
• - des résistances mécaniques appréciables; ces propriétés seront spécialement intéressantes lors du transport de produits dans des conteneurs en amiante-ciment, ainsi que pour l'empilement des colis et leur enfouissement sous terre, car ces conteneurs devront résister à des efforts ou à ces chocs potentiels;
• - une stabilité dans le temps démontrée par des études portant sur près de 70 années;
• - une résistance élevée aux corrosions diverses (eaux souterraines, courants vagabonds, bactéries, gaz carbonique, etc.) que l'on peut rencontrer lors des stockages souterrains;
• - une étanchéité remarquable vis-à-vis des gaz et de l'eau due à une porosité extrêmement fine et au colmatage progressif de ces pores, au cours du temps, sous l'influence d'agents extérieurs (par exemple carbonatation);
• - une très grande résistance (et une bonne stabilité) lors des variations de température. Ainsi, l'amiante-ciment est considéré comme pouvant supporter, sans variation notable de ses propriétés, des températures comprises entre - 50 et +300°C environ.

Among the properties of this asbestos-cement especially interesting for said storage, one can mention:

• - a sufficient density so that we can envisage achieving reasonable biological protection by using a thickness of asbestos-cement between 30 and 100 mm (case of low specific activity waste and emitting waste a) ;
• - appreciable mechanical resistance; these properties will be particularly advantageous during the transport of products in asbestos-cement containers, as well as for the stacking of packages and their burial underground, since these containers will have to resist efforts or potential shocks;
• - stability over time demonstrated by studies covering almost 70 years;
• - high resistance to various corrosions (groundwater, stray currents, bacteria, carbon dioxide, etc.) that may be encountered during underground storage;
• - a remarkable seal against gases and water due to an extremely fine porosity and the progressive clogging of these pores, over time, under the influence of external agents (for example carbonation);
• - very high resistance (and good stability) during temperature variations. Asbestos-cement is therefore considered to be able to withstand, without significant variation in its properties, temperatures between - 50 and + 300 ° C approximately.

A container comprising asbestos cement is claimed in the initial application EP-A-37 312.

All of these properties show that dense asbestos-cement is indeed a material of choice for various embodiments of storage of waste and / or radioactive effluents dispersed in a solid matrix and placed in a container.

We have already mentioned the possibility of using materials based on asbestos and cement for certain achievements involving radioactive elements. This is the case in particular in patent DE-A-1,136,261 and in patent FR-A-1,347,402. Patent DE-B-1,136,261 describes the use of a refractory binder material of complex composition based on calcium sulphate mixed with asbestos, but does not describe the use of a dense asbestos-cement as in the present application. FR-A-1 347 402 describes the use of a rem pleating which consists of fibers impregnated with an adhesive and then compressed in the space between two containers; the material used in patent FR-A-1 347 402 is therefore not dense asbestos-cement as in the present application and does not play the same role with respect to radiation.

The storage structure according to the present invention consists of pipes of standard length, very thick, made of dense asbestos-cement, said pipes being arranged vertically next to each other; between said lines of pipes circulates the cooling air, said pipes being connected to each other using sleeves made of dense asbestos-cement.

• - l'on réalise lesdites colonnes à l'aide de tubes et de manchons, lesdites colonnes étant réalisées en amiante-ciment de densité supérieure à 1,75 g/ cm³, cesdites colonnes reposant sur un socle constitué par deux couches superposées de manchons de supports rangés en contact suivant une maille carrée ou hexagonale et l'axe des colonnes, l'axe des manchons de la colonne supérieure et l'axe des manchons de la couche inférieure étant décalés pour ménager des intervalles permettant la circulation de l'air de refroidissement.

The present invention therefore relates to structures for the storage of high-level radioactive products comprising columns intended to receive the radioactive products suitably packaged, said columns being externally cooled, characterized in that:

• - Said columns are made using tubes and sleeves, said columns being made of asbestos cement with a density greater than 1.75 g / cm ³ , said columns resting on a base formed by two superimposed layers of sleeves of supports arranged in contact according to a square or hexagonal mesh and the axis of the columns, the axis of the sleeves of the upper column and the axis of the sleeves of the lower layer being offset to provide intervals allowing the circulation of air cooling.

An embodiment of these self-supporting fractionation structures for the storage of radioactive materials is given below.

We know that high activity effluents (waste) must, after being dispersed in a suitable solid matrix (glass for example), be stored in intermediate storage where they lose most of their radioactivity, until the residual thermal release is compatible with final storage in geological formations. These effluents will then be stored in the same way as effluents of activity or less thermal load.

For the case of fractionation structures mentioned above, the example was taken of the intermediate storage of containers of high activity vitrified waste. This vitrified waste consists of cylindrical glass blocks, from 400 to 500 mm in outside diameter and from 1500 to 2000 mm in total height. Their unit heat release can reach a few kilowatts at the start of their storage period.

If one wishes to wait until their thermal release is no longer a nuisance for their final storage in a geological structure, comprising other radioactive waste a, but without thermal load, it will be necessary to envisage a cooling time of the order of 1 to 2 centuries. The use of ordinary steel metal structures will not offer all guarantees of durability. The use of noble alloys leads to excessive investment and the concrete does not have sufficient temperature resistance to be used as a fractionation structure in the case of compact storage.

• - de stocker un nombre important de conteneurs par m² de bâtiment;
• -d'effectuer un refroidissement par air et tirage naturel ou forcé, avec des températures de sortie de l'air de l'ordre de 200°C;
• - d'obtenir des structures résistantes mécaniquement, insensibles à la corrosion et résistantes à des températures d'air élevées;
• - de limiter la température à coeur des verres en absorbant dans la masse des conduits une partie notable des rayonnements gamma émis.

The use of pipe elements and sleeves made of dense asbestos-cement allows, according to the invention, to produce glass storage structures which allow:

• - to store a large number of containers per m ² of building;
• -to perform air cooling and natural or forced draft, with air outlet temperatures of the order of 200 ° C;
• - to obtain mechanically resistant structures, insensitive to corrosion and resistant to high air temperatures;
• - limit the core temperature of the glasses by absorbing a significant part of the gamma radiation emitted in the mass of the conduits.

The dense asbestos-cement tubes according to the invention are therefore used for carrying out intermediate storage of high activity effluents, said tubes serving to fractionate the mass of the products to be stored and to guide the cooling air around said products.

The entire repository will be in the form of a buried building, preferably of vertical cylindrical shape, comprising a bundle of dense asbestos-cement tubes, of triangular or square mesh.

The design is based on the use of standard length pipes in pressure class, very thick, the pipes being connected to each other using dense asbestos-cement sleeves. These sleeves will also serve as spacers, at different levels, with a shift from one tube to another, so as to constitute a rigid and compact structure, resistant to the forces and stresses resulting both from the load, the temperature and the seismic constraints.

For the example considered, we will choose pipes with an internal diameter of 600 mm.

By superimposing four or five elements of pipes 5 m long each, we can store a dozen glass containers in each vertical channel thus formed.

In the case of the triangular (or hexagonal) mesh structure, the sleeves of two neighboring tubes will be declared by a third of the length, so as to provide suitable bracing. In the case of a square mesh structure, the sleeves of two adjacent tubes will be offset by half a length of tube.

The space between the tubes will serve, at least for part, for the descent of the fresh air along the wells, this same cooling air then rising inside the tubes, in the annular space left free between the glass containers and the inner wall of dense asbestos-cement tubes. The heating of the air during the descent is sufficient to allow the dew point temperature to be exceeded at the start of the licking of the containers by the air, thus avoiding any risk of corrosion of stainless steel glass containers. The speed of ascent of the air, of the order of 2 m / s, is substantially the same as that chosen for the descent between the tubes. The hot air will come out at a temperature of around 200 ° C at the top of the storage. An air mattress formed by the incoming fresh air protects the concrete parts on the upper surface of the storage and on the outer shell. With a triangular mesh, the number of wells that can thus be installed is approximately 2 per m ² of storage area, a figure substantially double that corresponding to the installations currently carried out. A square mesh storage also makes it possible to obtain very good results from the point of view of the compactness of the storage.

The core glass temperatures remain below 400 ° C., therefore clearly below the critical recrystallization temperature. As for dense asbestos-cement pipes, their highest wall temperature remains below 200 ° C, which is significantly below the permissible limits.

Guiding the containers inside the asbestos-cement tubes can also be carried out with dense asbestos-cement elements (plate elements fixed inside the tubes for example), without this being a compulsory solution. Other variants are possible such as, for example, containers provided with metal guide fins, or even by fixing metallic parts, made of noble metal, adapted and fixed on the pipes or on the glass containers.

Finally, suitable notches and holes, fitted with adjustable valves if necessary, make it possible to adapt the flow of circulating air to the thermal power to be evacuated in each of the channels, according to conventional arrangements for such types of storage.

The preferred cylindrical shape for storage makes it possible, if necessary, to partition it into sectors, and to circulate the arrival of cold air from the periphery towards the center.

Such a type of storage can be just as suitable for transient cooling of the glasses, for a few years after their production, as for longer cooling, while waiting for their final storage in a geological structure.

Such a fractionation structure can also be adapted to the storage of dissolving shells or fines after solidification and placing in a container.

• -la figure 1 est une coupe partielle transversale d'un silo de stockage;
• - la figure 2 est une section selon II-II de la figure 1;
• - la figure 3 est une vue partielle en perspective, en partie éclatée, du socle du silo de stockage;
• - la figure 4 est une vue partielle en perspective, en partie éclatée, de la partie du silo où se réalise l'évacuation de l'air chaud et une traversée d'air froid; et
• - la figure 5 est une vue en coupe longitudinale d'un raccordement entre deux tubes, en amiante-ciment dense, superposés.

A nonlimiting but preferred example of this embodiment of the invention is illustrated diagrammatically in FIGS. 1 to 5:

• FIG. 1 is a partial cross section of a storage silo;
• - Figure 2 is a section along II-II of Figure 1;
• - Figure 3 is a partial perspective view, partly exploded, of the base of the storage silo;
• - Figure 4 is a partial perspective view, partly exploded, of the part of the silo where the evacuation of hot air and a crossing of cold air is carried out; and
• - Figure 5 is a longitudinal sectional view of a connection between two superimposed asbestos-cement tubes.

In these figures, it can be seen that the storage silo according to the invention consists of a closed concrete enclosure 2 in which are arranged a series of columns 3, 3a, 3b made of dense asbestos-cement.

Each of these columns consists of a stack of superimposed unit tubes 4, 4a, 4b, 4c.

The junction 5 between two tubes is shown in detail in FIG. 5. The tube 4a rests end to end on the lower tube 4b by means of a seal 6 held in the circular groove 7 of the connection sleeve 8.

In FIG. 5, it can be seen that the ends 9 and 10 of the tubes rest on the O-ring 6 made of asbestos and glass fiber.

As can be seen more particularly in FIGS. 1 and 2, the connection sleeves are offset by a height corresponding to a half-height of unitary tubes on two adjacent columns. Using this arrangement, a sleeve is only in contact with the four adjacent columns and never with another sleeve. Consequently, the sleeves also act as a spacer between adjacent columns.

The upper end of each column is closed by a plug 11, 11a, 11b housed in the corresponding openings of the slab 12 of reinforced concrete. The cap plus slab assembly provides protection between the intervention zone 13 located above the silo and the interior of the enclosure 2.

The lower end of each column, as seen more particularly in FIG. 3, rests on a base 14 constituted by a double row 15 and 16 of adjacent support sleeves 15a, 15b and 16a, 16b whose average diameter is equal to £ times the square mesh of the network made up of all the columns.

The bottom 17 of the columns has a stool 18 formed by a blank made of dense asbestos-cement. This bottom can be fitted with a container fall-arresting device such as, for example that described in French patent application FR-A-2 500 098 filed on February 13, 1981 and published on August 20, 82.

The storage containers 19 are stacked on top of each other inside the columns, their centering being able to be achieved by guide fins not shown in the figures and arranged radially inside the columns.

As can be seen more particularly in FIGS. 1 and 4, the fresh air is distributed at the upper part of the enclosure thus protecting the upper concrete slab and penetrates into the isthmus 20, 21 formed between adjacent columns or between the columns and the side walls of the enclosure by means of inlet tubes 22 asbestos-cement. These tubes 22 are held by two plates 23 and 24 made of asbestos-cement, these plates resting on the corresponding sleeves 25, 25a, 25b of the neighboring columns and delimiting an enclosure 26.

This fresh air during its descent heats up in contact with the columns, arrives at a temperature higher than the dew point temperature at the base and rises in the free annular space formed between the columns and the stack of containers. the double row 15 and 16 of base support sleeves provides better protection of the concrete floor 27 and better distribution of air at the bottom of the silo.

The hot air opens into the enclosure 26 through the lights 28, 28a formed in the part of the columns passing through the enclosure 26. These lights can be more or less obscured by an appropriate adjustment device not shown, for example by a cover. horizontal sliding in the pierced part of the column and actuated in the intervention zone 13 by means of a rod sealingly passing through the plug.

As seen more particularly in Figure 2, the columns are arranged in a square mesh network, the two axes D ₁ and D ₂ are respectively parallel to the walls P ₁ P ', and P ₂ P' ₂ of the pregnant. The network is made up of two sets of columns.

A first assembly 29 is constituted by the columns 30, 30a, 30b situated in contact with the walls P ₁ P ' ₁ P ₂ P' ₂ of the enclosure; the columns of this set are preferably empty (therefore do not contain radioactive materials) and they are swept by a downward flow of fresh air; the columns of this set act as a spacer between the walls of the enclosure and the columns of the second set. A second set 31 is made up of all the columns which are not in contact with the walls P. The columns of the second set 31 of columns contain the containers.

It is noted that, in the case shown, the four corners of the enclosure are cut to maintain the rigidity of the assembly. This gives excellent cohesion and therefore good resistance to earthquakes.

Claims (2)

1. Structure for the storage of highly active radioactive products comprising columns (3) designed to receive the suitably packed radioactive products, said columns being externally cooled, characterized in that:

- the columns are formed with tubes (4) and sleeves (8),

- said columns being produced in asbestoscement of density higher than 1.75 g/cm³, and

- said columns resting on a base (14) constituted by two superposed layers of support sleeves (15 and 16) arranged so as to be in contact one with the other and to form a square or hexagonal mesh pattern, the axis of the columns, the axis of the sleeves of the upper column and the axis of the sleeves of the lower column being offset to provide gaps and to allow the flow of the cooling air.

2. Structure according to claim 1, characterized in that it contains a radioactive product in the form of a glass enclosing the fission products issued from the treatment of waste nuclear fuels.

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