EP0748506A1

EP0748506A1 - Device for securing the base of a container for the transport and or storage of radioactive materials

Info

Publication number: EP0748506A1
Application number: EP95907034A
Authority: EP
Inventors: Bernard Kirchner; René CHIOCCA
Original assignee: Societe pour les Transports de lIndustrie Nucleaire Transnucleaire SA
Current assignee: Societe pour les Transports de lIndustrie Nucleaire Transnucleaire SA
Priority date: 1994-02-01
Filing date: 1995-01-13
Publication date: 1996-12-18
Anticipated expiration: 2015-01-13
Also published as: WO1995021449A1; CZ225896A3; CZ288522B6; FR2715762B1; EP0748506B1; TW285741B; DE69512035T2; ES2137492T3; JPH09508472A; ZA95700B; FR2715762A1; US5567952A; DE69512035D1

Abstract

Device for securing the base of a shielded container for the transport and/or storage of highly radioactive materials comprising a sleeve (1) and a non-removable thick metal base (6). The sleeve (1) and base (6) respectively include a mutually engaging inner wall and side wall forming a straight cylinder having a circular cross section, the base (6) being held by shrink-fitting the side wall (12) of the base into the inner wall portion (13) of the opposing sleeve (1). According to the invention, the base is at least partially located within the sleeve (11), said inner wall portion (13) of the sleeve including a counter-shoulder cooperating with a matching shoulder in the side wall (12) of the base. The base (6) is linked to said sleeve by a continuous welding bead on its outer surface and by another welding bead on its inner surface.

Description

DEVICE FOR FIXING THE BOTTOM OF A CONVEYOR AND / OR STORAGE CONTAINER

RADIOACTIVE MATERIAL

TECHNICAL AREA

The invention relates to a device for fixing the bottom of a transport and / or storage container for highly radioactive materials, in particular for irradiated nuclear fuel assemblies or high-activity waste originating from their reprocessing.

STATE OF THE ART

Containers of highly radioactive materials generally include a thick, tight, shielded enclosure intended to contain the materials, stop gamma radiation and ensure particularly good mechanical resistance, even in severe accident situations.

Usually of cylindrical shape with a section either completely circular, or arranged with flats, they comprise a ferrule closed at one end by a bottom which is definitively fixed to it and, at the other end (the main opening), by a removable or often complex plug or cover for loading and unloading the contents. These closures must remain watertight (sometimes even with helium) in a severe accident situation, in particular after regulatory drop tests, such as a 9 m drop on a generator of the ferrule, on a corner of the container, on the bottom or on the lid, including scraps on a punch.

The main framework of the ferrule and the closures may consist of a thick wall of metal with high mechanical resistance, for example steel, which can reach a thickness of several tens of centimeters; thus for transport and / or storage containers for nuclear fuel assemblies or vitrified waste, the steel ferrule may exceed 25 or 30 cm in thickness, as well as the bottom and the main cover, the entire container being able to weigh empty up about 120t and full up to 150t or more. For radiological protection, the shielding provided by the framework can be supplemented by layers of suitable materials placed outside or inside the container, on the shell or on the end closures.

A known type of container is schematically illustrated in FIG. 1, where one can see:

. in (1) the thick ferrule, for example of steel, covered with other layers (2), (3) of radiation absorbing materials, the main closure system comprising two covers, a primary cover (4) and a secondary cover protective (5), removable, the particular fixing or control devices are not shown

- in (6) the irremovable base fixed, in this case, on the shell (1) by a weld (7) involving the entire thickness of the shell

- In (8) handling pins which are generally welded astride the weld (7).

A weld of this type (7) is long and difficult to perform given the large thickness of steel to be welded; it requires special care and multiple checks during preparation since it alone ensures both the maintenance of the bottom, therefore the resistance to falls, and the sealing of the container.

To simplify this welding problem, another closure device has been developed and is illustrated in FIG. 2. The bottom (6) has an outside diameter equal to that of the ferrule (1); it has a shoulder (9), formed at the periphery of its internal face (19), which cooperates with the internal diameter of the ferrule (1) and allows the bottom (6) to be positioned with gentle friction when cold, in partially fitting inside the ferrule, and resting on the face of the straight end section (10) of said ferrule (1). A peripheral weld bead (11) simpler to produce and to control than the previous one, keeps the bottom (6) in place. It can be completed by a second weld bead (18) on the internal face.

In this type of age, it is also a weld bead which maintains and supports the bottom (6) on the shell; the welds are highly stressed in the event of the container accidentally falling onto a generator on the shell or on a bottom angle. They must therefore, as before, be carried out with great care and particularly controlled. They represent a weak point which can lead to a risk of rupture and do not therefore offer a complete seal guarantee in the event of a severe impact.

Document EP 061400 also discloses a device for closing a container of radioactive material where the seal is only obtained by hooping the (removable) cover in a ferrule (p.2, 1.37-38 - p.3, 1-30 ) and due to the absence of defect in the contacting faces (p.4, 1.5-6). It also describes a boss (7) which is located on the shell on either side of the sealing faces, which is not in permanent contact with the removable cover and the size of which is linked to the expansion of the ferrule; it thus creates an insufficient obstacle to the axial movement of the cover (p.3, 1.31-32, 35-36, 39).

Document FR 2092502 describes a vacuum seal obtained by hooping, one of the hooped parts being provided with a cutting edge which deforms during withdrawal.

Document EP 101362 also describes a sealed closure of a removable cover obtained by shrinking two conical parts, for a container for transporting radioactive materials; surfaces in contact must be machined with care (p.2, 1.9-10); the closure has an axial stop and it can also be noted that the cover projects relative to the shell.

These different assemblies do not offer a seal guarantee in the event of a severe impact. The Applicant has sought to overcome these various drawbacks and to develop a device for mounting the bottom of high security which avoids the presence of weak points which can lead to risks of rupture or affect the tightness of the container in the event of the container falling. in particular falls in a horizontal or oblique position, which are generally the most severe, but also falls in a vertical position, on the bottom or the cover, including falls on a punch.

It also sought an easy and economical assembly to carry out.

DESCRIPTION OF THE INVENTION

The invention is a device for fixing the bottom of an armored container for transporting and / or storing highly radioactive materials comprising a ferrule and an irremovable bottom made of thick metal, for example steel, the ferrule and the bottom having respectively, at least over a certain height, an inner wall and a side wall forming a straight cylinder of circular section coming into contact with each other, the bottom being held in place by hooping of its side wall by the part of the inner wall of the ferrule facing it, characterized in that the bottom is at least partially located inside the ferrule, that said part of the inner wall of the ferrule comprises a shoulder cooperating with a corresponding counter-shoulder formed in the side wall of the bottom and the bottom is connected to the shell by a continuous weld bead on its outer face and by another on its inner face.

The bottom-shell closure is final.

The device according to the invention makes it possible to ensure that the bottom does not move relative to the shell, thanks to the hooping in the event of a horizontal fall, thanks to the counter-shoulder in the event of a vertical fall to the bottom, and thanks to the cooperation of these two elements in the event of an oblique fall on the bottom. Thus the weld beads which guarantee a perfect helium seal are only subjected to slight stresses, corresponding for example to the rebound of the content of the container in the event of a vertical fall on the cover, and can therefore be reduced in size.

A first continuous weld bead usually connects the peripheral edge of the outer face of the bottom to the inner edge of the end face of the ferrule, or possibly to the inner face of the ferrule, the recessed bottom thus providing a volume used for the installation, for example, of an additional incompressible neutron olining. Similarly, a second continuous weld bead generally connects the peripheral edge of the internal face of the base to the internal wall of the shell. These cords essentially have the role of guaranteeing the tightness of the container, in particular to ensure the confinement of radioactive materials in the cavity of the container or to avoid any contamination during immersion in a swimming pool; they are not highly stressed mechanically in the event of a fall, even in the event of a vertical or oblique fall on a lower edge of the container, or even during the rebound of the load contained against the bottom in the event of a fall on the cover.

Thus, not only does the device of the invention make it possible to significantly reduce the volume (up to -95%) and the size of the welds to be produced, but also said welds can be satisfied with less demanding specifications; in particular, the controls are simplified compared to those practiced on the welds of the prior art, the latter having an essential role in the mechanical strength of the bottom on the shell. It follows a facility of realization and a certain economic advantage.

The hooping essentially ensures the absence of play between the bottom and the ferrule, so as to prevent any relative movement between these two parts during falls, and thus to preserve the integrity of the welds.

To obtain this result, it is not necessary that the facing surfaces during hooping have a surface condition as neat as that which would be necessary if this hooping were to seal the container.

It should be noted that, in the device according to the invention, the weld beads also make it possible to prevent corrosion from taking place at the interface between the bottom and the shell, corrosion which could occur when the container is immersed in a swimming pool or following atmospheric condensation and which would damage the bottom-shell connection; in the same way they prevent contamination from slipping into this same interface.

The shoulder, formed in the internal face of the ferrule, is in intimate contact with a corresponding counter-shoulder formed in the lateral wall of the bottom; it saves stresses on the welds mainly in the event of a vertical fall on a punch in the center of the bottom; it also allows precise positioning of the base relative to the shell.

The hooping is generally done on the small and the large diameter to obtain a double hooping. one can also realize it only on the small diameter (located towards the cavity of the container); in both cases it is advisable to adjust the height of the counter-shoulder of the base resting on the shoulder of the ferrule so that its shear strength is sufficient.

Furthermore, it is advantageous for the forged bottom to be substantially located inside the shell, the outer surface of said forged bottom or that of the possible additional neutron shield then flush with the end of the shell; such an arrangement allows the shock to be distributed over the bottom and the shell in the event of a vertical fall.

To ensure hooping, the side wall of the bottom has a diameter slightly greater than that of the corresponding inner wall of the ferrule, and the bottom is fitted into the ferrule after these two components have been brought to sufficiently different temperatures that a suitable temporary mounting clearance appears.

After fitting, the outer face of the bottom may advantageously not protrude from the plane comprising the end face of the ferrule, the bottom thus being entirely located in the ferrule. After cooling, the hooping force develops on the entire side wall from the bottom, or only over part of its thickness, and is enough to hold it in place.

The shrinking is all the more intense as the difference, when cold, between the diameter of the side wall of the bottom and that of the corresponding inner wall of the shell is important. It is however recommended that this difference remains below a limit value beyond which the tensile stresses in the shell and / or compression in the bottom would exceed the threshold deemed acceptable for the material used.

Thanks to the device according to the invention, it is therefore possible to adjust the hooping force by adjusting the value of the excess of the outside diameter of the bottom when cold, relative to the inside diameter of the corresponding inside wall of the shell also when cold.

As an indication, when the bottom and the ferrule are made of steel and for a difference between their diameter being between 0.5 and 1 per thousand (which may require a temperature difference of around 200 K during assembly) the hoop stress at the interface can be of the order of 100 MPa, value quite acceptable for many ^'steel grades.

In the case where the container cavity has a non-circular cross section, machining is generally carried out in the inner wall of the shell, in the part facing the bottom, so as to obtain, at a certain height, a straight section of the circular ferrule which will cooperate with the circular lateral wall of the bottom during the hooping.

The choice of thick metal to make the shell and the bottom is very varied; it can be guided by mechanical characteristics, corrosion resistance, protection against radiation, etc. If necessary, different metals can be used for the bottom and the shell. Preferably, the choice is made from steels (possibly stainless steel), copper and its alloys, for example bronzes, etc. The ferrule can advantageously include handling pins fixed on its outer wall near the bottom and the cover. With the method of fixing the bottom according to the invention, the pins located towards the bottom are fixed directly to the ferrule, for example by welding, and in this case the welding does not interfere with other welds contrary to what is goes into the prior art (see for example fig. 1 and 2). Indeed, intersecting welds generally present mutual risks of embrittlement, so the absence of weld interference is an additional advantage due to the invention.

The device according to the invention, comprising a bottom held by shrinking, is particularly suitable for fixing the bottom of containers of highly radioactive materials having walls (bottom and shell) of large thicknesses of metal, for example steel, typically ranging from 10 cm to 50 cm (usually 20 to 30 cm) and weighing more than lût (usually 70 to 150 t).

The cooperation of the means used in the device according to the invention thus makes it possible to obtain containers which are simple and therefore economical to produce, meeting the specifications of transport and / or storage packaging, including in a humid environment, and in particular requirements of international regulations relating to severe accidental fall or shock conditions (including horizontal fall on a generator of the ferrule or oblique on the angle of the ferrule near the bottom) while being satisfied with lightened controls in progress of achievement; in fact, during the major impact, no stress directly affects the weld seams of the bottom-shell interface.

Thus, the device according to the invention is such that the forces brought into play during severe shocks or falls do not cause damage to the bottom-shell connection, and that consequently not only the sealing guarantee but also the protection against corrosion and contamination of said bottom-shell interface are improved, and this using a simple manufacturing process using only routine machining or welding operations. FIG. 3 illustrates a container fitted with a non-removable bottom according to the present invention. FIG. 3a represents a magnification of the bottom-shell interface. FIG. 3b represents the detail of the bottom-shell interface in the particular case where the bottom is set back relative to the shell.

The references 1,2,3,4,5,6,8,10,18,19 have the same meaning as in Figures 1 and 2 of the prior art.

We see that the bottom (6) is located inside the ferrule (1) of steel, or other resistant metal, has a peripheral side wall (12) cylindrical with circular section which is enclosed over its entire height by a part identical cylindrical (13) machined in the inner wall (20) of the ferrule (1); in the case of the figure the external face (14) of the bottom (6) does not project beyond the plane comprising the end face (10) of the ferrule (1). The bottom (6) is thus held in place by hooping using the ferrule (1) itself.

The cavity (22) of the container can have a shape such (for example with a polygonal cross section) that its inner wall (20) requires a counterbore (21) - (fig.3a) for the establishment of the bottom whereas for d 'other forms of the cavity (22) (for example with a circular cross section) counterbore is not necessary (see fig. 3 in 18a).

We also see:

- the outer weld bead (17) which connects the peripheral edge of the outer face (14) of the bottom (6) to the shell,

- the inner weld bead (18, 18a) which connects the peripheral edge of the inner face (19) of the bottom (6) to the shell; we see that (18a) the inner wall (20) of the cavity has no counterbore (as has already been said).

We also see in (15) two shoulders cooperating with each other, one being machined in the internal wall (13) of the ferrule (1) and the other in the side wall (12) of the bottom ( 6).

In Figure 3b we see that the outer face (14) of the bottom (6) is not flush with the end face (10) of the ferrule (1), but is located set back; this results in a volume in the form of a circular wafer which can be used, for example, to install there an incompressible additional neutron shield (23). The sealing weld (17) then connects the outer face (14) of the bottom (6) to the inner face of the ferrule (1).

It should be noted that it may be advantageous to leave, when cold, a mounting clearance between bottom (6) and ferrule (1) in the vertical portion of large diameter (location of arrow 13) of the shoulder and provide that this part is preferably of reduced height compared to the vertical part of small diameter (location of the arrow 12) which must preferably ensure sufficient hooping to keep the bottom (6) in place even if it is not not used shrinking to justify holding the fund in the safety analysis.

It should also be noted, as has already been said above, that the counterbore (21) of the ferrule may be absent, partial or total on the inner face of said ferrule and that it may or may not coexist with the shoulder (15); it can also replace said shoulder (15).

Claims

1. Device for fixing the bottom of an armored container for transport and / or storage of highly radioactive materials comprising a ferrule (1) and a non-removable thick metal bottom (6), the ferrule (1) and the bottom (6) having respectively an inner wall and a side wall forming a straight cylinder of circular section coming into contact with each other, the bottom (6) being held by hooping of the side wall (12) of the bottom (6) by the part of the inner wall (13) of the ferrule (1) which faces it, characterized in that the bottom (6) is at least partially located inside the ferrule (1), that said part of the wall interior (13) of the ferrule (1) comprises a counter-shoulder cooperating with a corresponding shoulder formed in the side wall (12) of the bottom (6), that the bottom (6) is connected to the ferrule by a weld bead continuous on its external face and by another on its internal face.

2. Device according to claim 1 characterized in that the outer face (14) of the thick metal bottom is flush with the end face (10) of the ferrule (1).

3. Device according to claim 1 characterized in that the outer face (14) of the thick metal bottom is disposed recessed relative to the end face (10) of the ferrule (1).

4. Device according to any one of claims 1 to 3 characterized in that the inner wall (20) of the cavity has a countersink (21).

5. Device according to any one of claims 1 to 4 characterized in that the metal is chosen from steels, copper, its alloys, bronzes.

6. Device according to any one of claims 1 to 5 characterized in that the pins (8) for handling are fixed in the full outer wall of the shell near the bottom (6).