Patent Number: 055679528
Section: summary

FIELD OF THE INVENTION The invention concerns a fixing means for the base of a transport and/or storage container for highly radioactive material, in particular for irradiated nuclear fuel assemblies or their highly active reprocessing waste. DESCRIPTION OF RELATED ART Containers for highly radioactive material generally comprise a thick shielded chamber which confines the material, stops gamma radiation and is mechanically strong, even under accidental severe Conditions. They are generally cylindrical with a cross section which is either completely circular or is provided with flats. They comprise a tube which is closed at one end by a base which is permanently fixed thereto. The other end constitutes the main opening which is closed by a removable plug or cover which is often complex, for filling and emptying the contents. These closures must remain sealed (sometimes even to helium) when under accidental severe conditions, in particular after regulation drop tests, for example a drop of 9 m along a tube generatrix, on a corner of the container, on the base or on the cover, including penetration drops. The main frame of the tube and closures can be formed of a thick metal wall of high mechanical strength, for example a steel, which can be several tens of centimeters thick; thus containers for transport and/or storage of nuclear fuel assemblies or vitrified waste, the steel tube can be more than 25 or 30 cm thick, similarly the base and the main cover, and the unladen weight of the container assembly can be up to 120 t; its laden weight can be 150 t or more. The radiological shielding provided by the frame can be supplemented by layers of appropriate materials outside or inside the container, on the tube or on the end closures. A known container type is illustrated in FIG. 1, which shows: thick tube (1), for example of steel, covered with further layers (2), (3) of radiation absorbing material. The main closure system comprises two covers, a primary cover (4) and a secondary protective cover (5), both removable. Particular fixing or monitoring apparatus are not shown; PA1 a non removable base (6) which in this case is fixed to tube (1) by a weld (7) through the entire thickness of the tube; PA1 handling lugs (8) which are generally welded across weld (7). A weld of type (7) is long and difficult to make because of the great thickness of the steel which must be welded; great care is necessary and many checks must be carried out during manufacture because the weld alone holds the base in position and provides the drop strength as well as sealing the container. This welding problem has been simplified by providing a further closure means as illustrated in FIG. 2. Base (6) has an external diameter which is equal to that of tube (1); it includes a Shoulder (9) at the periphery of the internal surface (19), which cooperates with the internal diameter of tube (1) and allows base (6) to be cold assembled with a light friction fit, fitted partially inside the tube and abutting the cross sectional surface of end (10) of tube (1). A peripheral weld seam, which is simpler to make and check than the weld described above, holds base (6) in place. It can be completed by a second weld seam (18) on the inner face. In this type of assembly, here again a weld ensures that base (6) is held in place and seals the tube; the welds are extremely stressed if the container is accidentally dropped along a tube generatrix or at an angle to the base. As before, then, they must be made with extreme care and closely checked. They constitute a weak point there is a risk of rupture which cannot guarantee a complete seal in the event of a severe shock. European patent EP-A-0 061 400 describes a closure means for a container for radioactive material in which the seal is provided solely by Shrink fitting the (removable) cover into a tube (p.2, 1.37-38 to p.3, 1.30) which is brought about by an absence of faults in the contacting surfaces (p.4, 1.5-6). It also describes a boss (7) located On the tube on either side of the sealing surfaces, which is not in permanent contact with the removable cover and whose size is linked to the expansion of the tube. This thus creates an obstacle which cannot resist axial displacement of the cover (p.3, 1.31-32, 35-36, 39). French patent FR-A-2 092 502 describes a vacuum seal which is obtained by shrink fitting, one of the shrink fitted pieces being provided with an edge which deforms on contraction. European patent EP-A 0 101 362 describes a sealed closure for a removable cover produced by shrink fitting two conical portions, for a container for the transport of radioactive material; the contacting surfaces must be made with care (p.2, 1.9-10); the closure comprises an axial abutment and the cover projects beyond the tube. These various assemblies cannot guarantee a seal in the event of a severe shock. SUMMARY OF THE INVENTION We have sought to overcome these problems and develop a means for assembling a base which is more secure and which avoids weak points with their associated rupture risks or affect the seal of the container in the event of the container being dropped, in particular horizontal or oblique drops, which are usually more severe, but also vertical drops, on the base or on the cover, including penetration drops. We have also developed a simple assembly which is economical to manufacture. The invention therefore concerns a means for fixing the base of a shielded container for transport and/or storage of highly radioactive material, comprising a tube and a non removable base of thick metal, for example steel, the tube and base having respectively, at least over a certain height, an internal wall and a lateral wall forming a right cylinder with a circular cross section in contact with each other, the base being held in place by shrink fitting its lateral wall with the portion of the internal wall of the tube in contact therewith, characterised in that the base is located at least partially inside the tube, in that said portion of the internal wall of the tube comprises a shoulder which cooperates with a corresponding opposed shoulder in the lateral wall of the base and in that the base is connected to the tube by a continuous weld seam on its external surface and by a further weld seam on its internal surface. The base-tube closure is permanent. The means of the invention ensures that the base will not displace relative to the tube, due to the shrink fitting in the event of a horizontal drop, due to the opposed shoulder in the event of a vertical drop on the base, and due to the combination of these two elements in the event of an oblique drop on the base. Thus, the weld seams which provide a perfect seal to helium only suffer weak stresses, for example due to the contents of the container rebounding in the event of a vertical drop on the cover. The welds can thus be smaller. In general, a first weld seam connects the peripheral edge of the external surface of the base to the internal edge of the end face of the tube or the internal surface of the tube, the recessed base thus providing a volume which can usefully accommodate, for example, an additional incompressible neutron shield. Similarly, a second weld seam connects the peripheral edge of the inner surface of the base to the inner wall of the tube. These weld seams seal the container, in particular to confine radioactive material in the container cavity or to avoid contamination when immersed in a cooling pond. They are not highly stressed mechanically in the event of a drop, even in the event of a vertical or an oblique drop on a lower edge of the container, or event in the event of the load rebounding against the base in the event of a drop on the cover. Thus the means of the invention not only significantly reduces the volume (up to -95%) and size of the welds, but also the specifications for these welds are less demanding. In particular, checks are simplified compared with those carried out of prior art welds, which latter play a major mechanical role in holding the base on the tube. This facilitates manufacture and provides cost advantages. Shrink fitting ensures the absence base and the tube, by preventing any relative movement between these two pieces during a drop, thus maintaining the integrity of the welds. To obtain this result, the facing surfaces during shrink fitting do not have to be as carefully prepared as would be necessary if the shrink fitting had to provide the seal for the container. It should be noted that, in the means of the invention, the weld seams can also prevent the onset of corrosion at the base--tube interface, which corrosion can occur when the container is immersed in a cooling pond or as a result of condensation from the atmosphere and which would damage the base--tube joint; they can also prevent contamination from entering this interface. The shoulder in the inner surface of the tube is in intimate contact with the corresponding opposed shoulder in the lateral wall of the base; it reduces stresses in the welds, primarily in the event of a vertical penetration drop in the centre of the base. It also ensures exact location of the base with respect to the tube. Shrink fitting is generally carried out on the small and the large diameter to produce double shrink fit. It can also be effected solely on the small diameter (towards the container cavity); in both cases, the height of the opposed shoulder of the base resting on the shoulder of the tube can be adjusted so that its transverse strength is sufficient. Advantageously, the forged base is located substantially inside the tube, the external surface of said forged base or that of the optional complementary neutron shielding being flush with the end of the tube; this disposition distributes the shock over the base and tube in the event of a vertical drop. Shrink fitting is effected by ensuring that the lateral wall of the base has a diameter which is slightly greater than that of the corresponding internal wall of the tube. The base is fitted into the tube after the two components have been heated to temperatures which are sufficiently different to provide a suitable temporary allowance for assembly. After fitting, the external surface of the base advantageously does not go beyond the plane containing the end face of the tube, the base thus being located entirely within the tube. After cooling, the shrink fitting force develops over the whole of the lateral wall of the base, or over only a portion of its thickness, and is sufficient to hold it in place. The shrink fitting is the stronger the larger the difference, when cold, between the diameter of the lateral wall of the base and that of the corresponding internal wall of the tube recommended, however, that this difference is kept below a limiting value above which the tensional strains in the tube and/or compressional strains in the base would go beyond the accepted threshold for the material used. With the means of the invention, the shrink fitting force can be regulated by adjusting the value of the excess of the external diameter of the base when cold over the internal diameter of the corresponding internal wall of the tube, also when cold. By way of example, when the base and tube are of steel and for a difference between their diameters of between 0-5 and 1 per thousand (which would necessitate a temperature difference of the order of 200K during assembly), the shrink fitting force at the interface can be of the order of 100 MPa, which is an acceptable value for most types of steel. When the cavity of the container has a non circular cross section, in general the internal wall of the tube which contacts the base is machined to produce a circular cross section which cooperates with the circular lateral wall of the base during shrank fitting A wide variety of thick metals can be selected to form the tube and the base. The choice can be guided by mechanical properties, corrosion resistance, protection against radiation. etc. . . If required, different metals could be used for the base and tube- Preferably, the metal is selected from steels (optionally stainless) copper and its alloys, for example bronzes, etc. . . Advantageously, the tube comprises handling lugs fixed on the external wall close to the base and the cover. With the fixing mode of the invention, the lugs near the base are fixed directly to the tube, for example by welding, and the weld does not interfere with other welds, unlike those of the prior art (see, for example, FIGS. 1 and 2). Welds which cross over One another generally run the risk of mutual embrittlement, and thus the absence of weld interference is an additional advantage of the invention. The means of the invention, comprising a base which is held in place by shrink fitting, is particularly suitable for fixing the base of containers for highly radioactive material with very thick walls (base and tube) of metal, for example steel, typically 10 cm to 50 cm thick (generally 20 to 30 cm thick) and weighing more that 10 t (generally 70 to 150 t), Cooperation of the means employed in the invention also means that containers can be produced which are simple and thus economical to make, which satisfy specifications for transport and/or storage containers, including those for liquids, and in particular satisfy the requirements imposed by international regulations governing drop conditions or accidental severe shocks (including horizontal drops along a tube generatrix or obliquely at an angle to the tube near the base), while at the same time reducing the checks required during manufacture. The stresses occurring during a severe impact do not directly affect the weld seams of the base--tube interface. Thus with the means of the invention, the forces which occur during severe shocks or drops do not damage the base--tube joint, thus improving not only the seal security but also corrosion protection and contamination protection of said base--tube interface. In addition, a simpler method of manufacture is employed which uses only known machining or welding techniques.