Fixing means for the base of a radioactive material transport and/or storage container

A container for transport and storage of highly radioactive material, the container being made of thick metal and comprising a tube having an internal wall and a base having a lateral wall, the base being non-removably sealed to one end of the tube, and the internal wall and lateral wall forming a right cylinder with circular cross-section in contact with each other. The base is fixed to the container by shrink fitting the lateral wall with a portion of the internal wall in contact with the lateral wall, the base being disposed such that the lateral wall is entirely inside the tube with a portion of the internal wall comprising an opposed shoulder which cooperates with the corresponding shoulder in the lateral wall. A first continuous seam weld is provided on an external surface of the base in contact with the tube and a second continuous weld seem is provided on an internal surface of the base in contact with the tube.

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; 
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; 
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.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 3 shows a container with a non removable base in accordance with the 
present invention. FIG. 3a shows an enlargement of the base--tube 
interface. FIG. 3b shows a detail of the base--tube interface in the 
particular instance where the base is recessed within the base. 
Reference numerals 1, 2, 3, 4, 5, 6, 8, 10, 18, 19 have the same meanings 
as in FIGS. 1 and 2 of the prior art. Base (6) is located inside tube (1), 
of steel or other strong metal, and has a peripheral lateral wall (12) 
which is cylindrical with a circular cross section and is enclosed over 
its entire height by an identical cylindrical portion (13) machined in the 
internal wall (20) of tube (1). In the figure, the external surface (14) 
of base (6) does not extend beyond the plane containing the end face (10) 
of tube (1). Base (6) is then held in place by shrink fitting using the 
tube (1) itself. 
The cavity (22) of the container can be of any shape (for example with a 
polygonal cross section) such that its internal wall (20) requires a 
countersink (21) (FIG. 3a) to hold the base in place while other cavity 
(22) shapes (for example a circular cross section) would not require this 
countersink (see FIG. 3, 18a). 
The following can also be seen: 
the external weld seem (17) connects the peripheral edge of the external 
surface (14) of base (6) to the tube; 
the internal weld seam (18, 18a) which connects the peripheral edge of the 
internal surface (19) of base (6) to the tube; and at (18a), the internal 
wall (20) of the cavity has no countersink (as described above). 
Two cooperating shoulders (15) can also be seen, one machined in the inner 
wall of tube (1) and the other in the lateral (12) of base (6). 
It can be seen from FIG. 3b that the external surface (14) of base (6) is 
not flush with the end face (10) of tube (1), but is within it: this forms 
a circular disk which can, for example, be used to hold additional 
incompressible neutron shielding (23). The sealing weld (17) thus connects 
the external surface (14) of base (6) to the internal surface of tube (1). 
It should be noted that it could be advantageous to provide an allowance 
for assembly between base (6) and tube (1) in the vertical larger diameter 
portion (arrow 13) from the shoulder and it is preferable that this 
portion is of reduced height with respect to the smaller diameter portion 
(arrow 12). This is preferably sufficient to hold the base (6) in place 
even if it forms no part of the secure shrink fitting. 
It should also be noted that, as described above, the countersink (21) of 
the tube can be partially or completely absent over the internal surface 
of the tube and may or may not coexist with shoulder (15). It may also 
replace shoulder (15).