EP0843318A1 - Spline arrangement for externally cooling containers for radioactive material - Google Patents

Abstract

The container consists of a thick inner wall (3), e.g. of steel, a hydrogenated resin neutron shielding layer, and an outer wall (5) with grooves (7) to enhance air cooling. The grooves are separated by relief portions (6), are 1-30 mm wide and 1-15 mm deep, and are spaced no more than 50 mm apart. The outer wall of the container can be of steel, cast iron, aluminium, copper or alloys of these, and grooves in its surface can lie perpendicular or parallel to its generating line, or they can run in one or more spiral lines. They can be made by moulding, extruding or machining.

Description

TECHNICAL AREA

The invention relates to a device for cooling the external surface of containers for transport or storage of radioactive materials, in particular fuel assemblies irradiated nuclear power, said cooling consisting in dissipating the thermal power generated by said radioactive materials packaged by convection in ambient air.

STATE OF THE ART

Radioactive materials, especially. irradiated nuclear fuel assemblies, are usually transported and / or stored in containers (also called packaging) watertight cylindrical shape with very thick walls (several tens of cm), mainly base of cast iron or forged or cast steel, which provide holding functions mechanical (resistance to impact or severe falls), radiological protection, and transfer from the inside to the outside.

The walls can also be composite and include additional elements allowing improve container performance: for example resin, lead to improve the radiological protection, copper to improve heat transfer, etc.

Packaged radioactive material giving off heat, dissipating power temperature in the ambient air must be such that the components of the container and its contents are not brought in any place to temperatures prejudicial to their maintenance in an adequate state.

As the transfer of heat through the wall of the container is generally done by conduction, that is to say in proportion to the thermal power of the content, it follows that, for a given power, the outside temperature of said wall must also remain less than a certain value.

In order not to exceed this maximum value, the main parameter on which we can play to dissipate the required thermal power is the surface area of the exposed container convection of ambient air.

• des éléments en acier ou cuivre (bandes, picots, profilés...) peuvent être soudés perpendiculairement à la surface exteme des emballages
• des profilés en aluminium ou ses alliages, par exemples en forme de U peuvent être fixés selon une génératrice (par vissage, rivetage...) contre ladite surface exteme pour assurer un bon contact thermique
• des ailettes peuvent être usinées dans la masse de la paroi, soit selon une génératrice, soit circonférentiellement ; ceci est actuellement réalisé en disposant au départ de parois épaisses permettant d'y tailler des ailettes de grande taille, ce qui entraíne une perte de matière importante
• des emballages peuvent être obtenus par coulée directement avec des ailettes.

To increase this area, it is known to use fins, for example according to the following embodiments:

• steel or copper elements (strips, pins, profiles ...) can be welded perpendicular to the outer surface of the packaging
• aluminum profiles or its alloys, for example U-shaped can be fixed according to a generator (by screwing, riveting ...) against said outer surface to ensure good thermal contact
• fins can be machined in the mass of the wall, either according to a generator, or circumferentially; this is currently achieved by having at the start of thick walls to cut large fins, which results in a significant loss of material
• packages can be obtained by casting directly with fins.

Typically the fins are spaced 5 to 10 cm apart and have a height, perpendicular to the outer surface, about 10 cm or more.

The presence of fins increases the overall size of the packaging and constitutes a serious handicap when there are severe constraints relating to the space or surface available where the packaging must be stored, handled or transported.
In addition the presence of fins on the one hand can constitute contamination traps difficult to clean, in particular when the fins are fixed against the outer surface of the packaging, on the other hand can constitute a significant mass of material that 'It is not possible to take into account to carry out the radioactive shielding calculations because of the gamma radiation leaks which can occur between two consecutive fins if they are too far apart from each other.

The plaintiff has therefore sought how to increase the exchange surface by limiting increasing the size and ensuring that all of the material used is taken into account for the shielding.

DESCRIPTION OF THE INVENTION

The invention is a device for the external cooling of containers for the packaging of radioactive materials having an outer metal wall, characterized in that it has grooves made in the outer face of said wall.

The walls can be more or less thick (from a few cm to a few tens of cm thick) homogeneous or composite metals and generally based on steel or cast iron, but also aluminum, copper or their alloys.

Thus the grooves are made directly in the outer face of the metal wall thick, or when the thick wall is covered with a neutron shield (for example at hydrogenated resin base), which in turn is covered with a metallic shell more thin (for example in sheet steel 10 to 30 mm thick), said grooves are practiced in the outer face of said envelope, the thermal continuity between the thick wall and the envelope then being usually produced using metal parts serving as thermal conductors and passing through the neutron shield.

As the containers are generally cylindrical with a circular or polygonal cross section, the splines constituting the device according to the invention can be practiced there according to generating the cylindrical surface, so in a plane parallel to the axis of the cylinder, or according to the perimeter (or circumference) of said cross section, therefore in a plane perpendicular to the axis of the cylinder. The splines can also be helical, rotating around the container in one direction, or the other, or both.

The spline device according to the invention therefore has very great flexibility since it can be adapted to the type of container used and to the manner of using it.
Thus a container for storage will be used vertically and will have splines according to generators; on the other hand a container for transport will be used especially in horizontal position and will have a system of circumferential grooves, or a system of helical grooves wound in one direction or the other, or in both, which cannot be obtained with a conventional fin cooling system.

The splines can be engraved by machining (turning, milling ...) or obtained directly by molding, extrusion or any other process.

In general the grooves are regularly spaced. Their cross section (recessed in the wall metallic) is advantageously polygonal, preferably square or rectangular, to obtain better surface growth, but may have other profiles, for example circular etc ... They are separated from each other by a portion of the outer surface of the wall in relief in relation to the hollow grooves.

Figure 1 illustrates a composite wall of a straight section cylindrical container circular where the grooves according to the invention were made according to generators (parallel to the cylinder axis).

In (1) we see the interior cavity of the container into which is introduced for example a plurality of irradiated nuclear fuel assemblies, in (2) the thick wall, for example in steel ensuring the confinement of nuclear material, in (3) a neutron shielding in resin hydrogenated, in (5) an outer steel casing covering said shielding (3) and in (4) metal parts ensuring the thermal transfer from the wall (2) to the outer casing (5) which, in turn, ensures the dissipation of thermal power in the ambient air by convection natural.

In the outer surface (6) of the envelope (5), grooves (7) were made open and in contact with ambient air, rectangular section (in this case), of width (8), of depth (10) and whose re-entrant angles (9) are rounded.

It is particularly important to note that the width (8) of the grooves is less than 30 mm and their depth (10) to 15 mm; under these conditions it is easy to obtain that no variation significant dose rate resulting from the presence of splines is not measurable as well on the surface of the container only 1 m or 2 m from its surface (as required by the regulations applicable): thus all the material of the wall or of the external envelope, after shaping of the splines, can actually be taken into account in the shielding calculations.

Furthermore, so that the grooves have sufficient efficiency for cooling by ambient air, it is advantageous that their width and depth are greater than 1 mm and preferably between 5 and 10 mm, so that air can circulate efficiently in said grooves.

On the other hand, the pitch of the grooves (that is to say the distance each of which should be moved to coincide with the next) must be determined in relation to their width and their depth according to the increase in surface area that the we want to get.
Typically the pitch is between 1 and 50 mm, preferably between 10 mm and 20 mm.
Of course the pitch must be greater than the width of the grooves.

Thus longitudinal grooves with a rectangular cross section 10 mm wide, depth 5 mm in steps of 20 mm, allow a 50% increase in the external surface of the container subjected to ambient air cooling, while the section grooves square right of the same width, 10 mm deep with 20 mm pitch allow an increase 100%.

Simultaneously with this increase in surface there is no measurable variation in the flow of dose around the container, compared to a smooth wall of equivalent average thickness, then that the outer diameter of the container (size) has only been increased by the depth a groove (i.e. twice the half-depth of a groove) relative to said smooth wall providing equivalent shielding performance.

In summary, a container with splines according to the invention will have the same weight and the same shielding performance than a container without a groove, while its cooling surface will be significantly increased and that its size (for example its diameter) will not be increased only a few mm.

In comparison, a container having a smooth wall of the same thickness provided with fins 10 mm thick, 50 mm high, 100 mm pitch, providing a 100% increase in surface area, leading to an increase in overall diameter of 100 mm, the dose rate being determined by the zones between the fins; this means that the material of the fins cannot be taken into account in the shielding calculations and therefore constitutes a dead weight from this point of view.

In addition to the advantages concerning the size and the better efficiency of the shielding, which have just been seen, it should be noted that the making of the grooves is much easier to make and less expensive, in particular by molding, extrusion or machining, because in this last case of their small dimensions and therefore the production of a quantity of chips scaled down. For the same reason decontamination is greatly facilitated, especially when the re-entrant angles of the grooves are rounded.

The invention can be used for any type of container intended for materials releasing a significant thermal power; it is particularly suitable for large containers dimensions, for example for transporting or storing a plurality of assemblies irradiated nuclear fuel, and in particular to containers with a wall of thick primary containment (20 to 50 cm) in steel or cast iron, possibly covered with a neutron shielding, generally made of hydrogenated resin, itself protected by an envelope steel which is generally several centimeters thick.

Claims (6)

External cooling device for a material packaging container radioactive having an outer metal wall (5) characterized in that it comprises grooves (7) formed in the hollow in the outer face (6) of said wall (5) and separated the from each other by a raised portion of said external face of the wall, the width of the grooves being between 1 and 30 mm and their depth between 1 and 15 mm.,. Device according to 1 claim 1 characterized in that the pitch of the grooves (7) is less at 50 mm. Device according to any one of claims 1 or 2 characterized in that, when the container is cylindrical with circular or polygonal cross-section, the grooves are parallel or perpendicular to a generator, or helical around the axis of the container and wound in at least one sense. Device according to any one of Claims 1 to 3, characterized in that the splines result from casting, extrusion or metal machining. Device according to any one of Claims 1 to 4, characterized in that the wall The outer metallic container is made of steel, cast iron, aluminum, copper or their alloys. Radioactive material packaging container comprising the device of one any of claims 1 to 5.

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