[0001] This invention relates to a cask for transporting nuclear materials to or from nuclear
power plant facilities.
[0002] Casks for shipping fuel rods to and from nuclear power plants are known in the prior
art. Such casks generally include a transportable steel vessel cylindrical in shape,
and a basket structure receivable within the steel vessel and having an array of cells
for holding rectangular storage containers each designed to hold either a fuel assembly
or a spent-fuel canister with fuel rods consolidated therein in a dense, triangular-pitch
arrangement. Such transportable casks are adapted to be mounted on and secured to
the trailer of a tractor-trailer, and they are typically used to ship spent fuel
rods from a nuclear power plant to a permanent waste isolation site or a reprocessing
facility in as safe a manner as possible. At present, relatively few such shipping
casks have been manufactured and used since most of the utility companies which own
nuclear facilities have been able to store the spent fuel rods in spent-fuel pools
initially built into the reactor facilities. However, the availability of such on-site
storage space is steadily diminishing as an increasing number of fuel assemblies are
being loaded into the spent-fuel pools every day so that there is a growing need for
more storage facilities, and eventually for transporting spent fuel assemblies from
on-site storage facilities of nuclear power plants to an off-site nuclear waste
disposal facility. For this purpose, shipping casks are needed.
[0003] In order to be practical, a cask for transporting radioactive material by truck
must meet at least five basic requirements. First the cask must have walls capable
of effectively shielding both the gamma and neutron radiation emitted by its payload
well enough in order for the total amount of radiation emitted from the surface of
the cask to be at a level safe for handling, e.g., not more than 200 millirems at
any given point of the cask surface, and not more than 10 millirems at a distance
of two meters from the vehicle carrying the cask. Second, the cask must be capable
of withstanding mechanical shock of a magnitude commensurate with that occurring
during a vehicular accident. In this regard, it is not enough that the walls of the
cask continue to contain the radioactive material after such a mechanical shock; they
must also remain water-tight at all points so that water cannot leak into the interior
of the cask and thereby thermalize the neutrons emitted by the spent fuel rods or
other material contained in the cask. Third, the basket structure within the cask
must be capable, in the event of an accident, of withstanding the forces applied to
its perimeter by the inner cask walls without any significant distortion of its individual
nuclear waste-containing cells, for if these cells were significantly deformed, the
effectiveness of the so-called neutron "traps" installed between them could be impaired
which in turn could result in a criticality condition within the cask. Forth, the
cask must be immersible in water without any incursion of water into it and, furthermore,
must be completely drainable. The reason for this requirement is that casks are often
loaded and unloaded in spent-fuel pools of nuclear facilities in order to reduce exposure
of the operating personnel to potentially harmful radiation, and the water in such
pools typically contains dissolved radionucleides which, if allowed to seep into the
crevices in the cask or to deposit themselves into micropores on the cask surface,
might prove difficult if not impossible to remove. The deposition of such radionucleides
in the crevices and surface pores of casks could well raise the surface radiation
of the casks beyond allowable limits, thus preventing the use of such casks. Finally,
the cask must be capable of effectively dissipating the heat of decay generated by
the radioactive materials within it, for if no effective heat dissipation mechanism
existed, the temperature within the cask could become high enough to generate dangerous
levels of pressure.
[0004] Unfortunately, the simultaneous achievement of these five criteria is difficult since
the materials and mechanisms for achieving the various criteria are often at cross-purposes
with one another. For example, fins radially projecting from the outer cask surface
will improve heat dissipation but impair the cask's ability to withstand large mechanical
shocks without detriment to its wall integrity. One of the best and most economical
neutron shielding materials known is high-hydrogen cement, but it is relatively brittle
and likely to break up under the shattering forces of an accident. Lead, depleted
uranium, and Boro-silicon
R are also well known as effective gamma shielding materials but none of these materials
has sufficient mechanical strength to withstand an accident condition by itself. Moreover,
none of these materials is good heat conductor, and none of them is easy to weld,
or to join by any other known means, to a structurally strong metal because of the
large differences in mechanical and metallurgical properties involved. While stainless
steel has good structural and corrosion-resistant properties, it is not a particularly
good heat transfer medium and is expensive. It has also been observed that the surface
of stainless steel contains micropores which are capable of capturing dissolved radionucleides
and radioactive dust. Finally, whilst carbon steel is a good and inexpensive structural
material having better heat transfer properties than stainless steel, it is liable
to corrode when exposed to water.
[0005] It is the principal object of the invention to provide an improved shipping cask
which reasonably fulfills the five aforesaid criteria and is relatively easy and inexpensive
to fabricate.
[0006] The invention accordingly resides in a shipping cask for transporting radioactive
material, as defined and characterized in claim 1.
[0007] More particularly, the shipping cask embodying the invention comprises an inner vessel
having metallic, heat-conducting walls preferably formed from low alloy steel, a plurality
of heat-conductive, mutually parallel ribs, preferably formed from carbon steel, each
of which has an inside edge joined to the outside surface of the heat-conducting wall
of the vessel, a layer of radiation absorbing, cementitious material disposed between
the mutually parallel ribs and covering the outer surface of the heat-conducting wall
of the inner vessel, and a plurality of flat, circumferentially disposed fin members,
preferably also formed from carbon steel, each of which is joined along its longitudinal
edges to the outer edges of two adjacent ribs. The circumferentially disposed fin
members advantageously function both to dissipate heat generated by radioactive material
within the inner vessel, and to form a watertight barrier supporting and protecting
the layer of cementitious material.
[0008] The shipping cask includes further a removable basket assembly which is insertable
into and withdrawable from the inner vessel, and defines an array of cells each capable
of receiving a selected volume of radioactive waste. In the preferred embodiment,
the cell structure of the basket assembly is formed from two sets of parallel stainless-steel
plates which are orthogonally disposed with respect to one another and interfit in
"egg-crate" fashion.
[0009] The periphery of the basket assembly is defined by the corners of the outer or perimetric
cells and by angular formers which protect the cells from deformation under mechanical
shock applied to the outside of the cask, and in addition provide enlarged contact
surfaces for good heat transfer from the basket assembly to the heat- conductive
wall of the vessel. The formers are spaced apart so as not to interfere significantly
with the convective and radiative transfer of heat from the radioactive material in
the cell structure of the basket to the inner vessel, and the total area of the spaced-apart
formers is not more than about 30%, and preferably not more than 20%, of the total
area of the outer periphery of the basket assembly. Each of the formers may be formed
from several plates arranged in tandem circumferentially of the basket assembly and
with the edges of each plate connected to the corners of two adjacent cells.
[0010] In order to further increase the resistance of the cell structure of the basket assembly
to deformation in the event that the cask is subjected to an intense mechanical shock,
the periphery of the basket assembly includes a plurality of uniformly spaced, discrete
contact surfaces for uniformly distributing shock forces which may occur between the
outer periphery of the basket assembly and the inside surface of the inner vessel.
In the preferred embodiment, these discrete contact surfaces conform to the corners
of the peripherally located cells of the basket assembly.
[0011] Fabricating the inner vessel from low alloy steel and the ribs and fin members from
carbon steel allows all of the structural components of the cask to be easily welded
together. In order to prevent surface corrosion and the lodgment of dissolved radionucleides
or radioactive dust in the pores of the metal forming the circumferential fins, the
outer surfaces of the fins preferably are provided with an anti-corrosive coating
which may consist of a layer of zinc-containing primer, a layer of epoxy polyamide,
and a layer of polyurethane.
[0012] A preferred embodiment of the invention will now be described, by way of example
only, with reference to the accompanying drawings, in which:
Figure 1 is a side view of a shipping cask embodying the invention, showing the outer
closure lid of the cask raised above its top;
Figure 2 is a cross-sectional view of the cask as taken along the line 2-2 in Fig.
1, and with some of the fins and part of the layer of cement shown broken away for
clarity.
Figure 3 is a cross-sectional side view of the cask taken along line 3-3 in Figure
2, and with the basket assembly removed for clarity;
Figure 4A is an enlarged view of the area encircled in Figure 3;
Figure 4B is an enlarged view of the area encircled in Figure 4A which shows layers
of a protective coating applied to the exterior of the cask;
Figure 5 is a plan view of the outer lid of the cask as seen when viewed along line
5-5 in Fig. 1;
Figure 6 is a plan view of the upper end of the cask as seen when viewed along line
6-6 in Fig. 1;
Figure 7 is an enlarged view of the area encircled in Figure 6;
Figure 8 is a plan view of the bottom end of the cask as seen when viewed along line
8-8 in Fig. 1;
Figure 9A is a side view of the basket assembly of the cask, with part of the peripheral
basket wall broken away to expose some of the canisters disposed within the cells
of the basket assembly;
Figure 9B is a plan view of the basket assembly illustrated in Figure 9A;
Figure 9C illustrates the manner in which the two sets of plates from which the basket
assembly is formed are interfit with one another; and
Figure 9D is an enlarged view of the cell encircled in Figure 9B.
[0013] With particular reference now to Figures 1, 2 and 3, the shipping cask 1 illustrated
therein comprises an elongated generally cylindrical inner vessel 3 which has a metallic,
heat-conductive wall 4, preferably formed of low alloy steel, and has a plurality
of heat-conductive mutually parallel and uniformly spaced and radially oriented longitudinal
ribs 5 joined, preferably welded, to its outer surface. A layer of neutron-absorbing
cement 7 is applied to and over the outer surface of the vessel 3 between the uniformly
spaced ribs 5. Preferably, the layer 7 is formed from a cement having a high percentage
content of atomic hydrogen.
[0014] As seen best in Figs. 2 and 4A, the exterior of the cask 1 is formed by a plurality
of plate-like circumferential fins 9, each of which is joined, preferably welded,
along its longitudinal edges to outer edge portions of two mutually adjacent ones
of the ribs 5 so as to form a secure and watertight joint between the fin and the
ribs 5 associated therewith. Preferably, each of the fins 9 is an elongate, rectangular
plate of carbon steel approximately 6.5 mm thick. The fins 9 advantageously perform
three functions. First, they provide an effective means for dissipating heat conducted
to them through the ribs 5. Second, they provide a strong mechanical barrier which
retains, supports and protects the layer 7 of neutron-absorbing cement from fracture,
even in the event of an accident. Third, these fins 9 provide a waterproof barrier
preventing the layer of cement from absorbing dissolved radionucleides when the cask
is lowered into a spent fuel pool. In the preferred mode of fabricating the cask 1,
the fins 9 are first welded into position on the ribs 5 and the neutron-absorbing
cement 7 is then poured into the spaces between the vessel 3 and the fins 9. Such
mode of manufacturing has the advantage of providing a "leak-test" for the welds formed
between the fins 9 and the ribs 5, in that any leaking of water and cement occurring
on the outer surface of the cask 1 after the neutron-absorbing cement 7 has been poured
will serve as an indication telling the manufacturer that the cement needs to be removed
in the affected areas, and the affected joints between fins 9 and ribs 5 need to be
rewelded to make them watertight.
[0015] Referring now to Figures 6, 9A, 9B and 9C, the cask 1 further includes a basket assembly
11 which is freely insertable into and withdrawable from the inner vessel 3. The basket
assembly 11 is formed from two sets of parallel plates 14a-14h and 16a-16h. The plates
14a-14h and 16a-16h of the two sets are provided with slots 15 and 17, respectively,
which interfit, as seen best from Figure 9C, to form an egg-crate like structure defining
square cells 18a-18x. Preferably, the plates 14a-h and 16a-h are formed from a solid
sheet of stainless steel approximately 1 cm thick. In addition to being interleaved,
the plates 14a-h and 16a-h are secured to one another by means of welds formed along
the edges of the respective slots 15 and 17 throughout their entire length. As seen
best from Fig. 9B, each set of parallel plates 14a-14h or 16a-16h includes a central
pair of closely spaced plates 14d, 14e or 16d, 16e, respectively, which due to their
central location within the basket assembly 11 lend extra strength to the latter as
a whole. These central pairs of plates terminate in plate-pair ends 20a, 20b, 21a,
21b, respectively, which provide four uniformly spaced contact surfaces between the
periphery of the basket assembly 11 and the inner surface of the vessel 3. These contact
surfaces will assure not only a uniform heat transfer from the radioactive material
within the cells 18a-18x through the walls of the vessel 3 but also a uniform absorption
of impact forces between the basket assembly 11 and the inner wall of the vessel 3
in the event the cask 1 is subjected to an accident condition. Additionally, at least
one of the plate-pair ends 20a, 20b, 21a, 21b coacts, as further described later herein,
with plate retaining means 38 on the inner surface of the vessel 3 in a manner such
as to maintain the proper orientation of the basket assembly 11 within the vessel
3 in the event the vessel 3 is subjected to torque.
[0016] As seen best from Figures 9A and 9B, the basket assembly 11 is circumscribed by a
plurality of angular formers 22.1a-22.5a, 22.1b-22.5b, 22.1c-22.5c and 22.1d-22.5d,
each of which is formed from three strut plates 24a, 24b, and 24c arranged in tandem
and welded, along their edges, to corners defined by plate overhangs 26a, 26b, 26c
and 26d. In the preferred embodiment, each of the strut plates 24 is formed from stainless
steel plate material approximately 1 cm thick. Such material is advantageously weld-compatible
with the stainless steel plates 14a-14h and 16a-16h used to form the basket assembly
11. As seen best from Figure 9A, the angular formers 22.1a-22.5d are spaced apart,
preferably uniformly, along the longitudinal axis of the basket assembly 11. The total
area of the angular formers 22.1a-22.5d relative to the total exterior area of the
basket assembly 11 preferably is only about 20 percent. The use of such angular formers
22.1a-22.5d in the basket assembly 11 is advantageous in two major respects. First,
the multiple, shallow corners which the formers provide over the respective plate
overhangs 26a, 26b, 26c and 26d offer four broad areas of contact between the outer
surface of the basket assembly 11 and the inner surface of the vessel 3. These multiple
areas of contact, together with the contact areas presented by the ends 20a, 20b and
21a, 21b of plate pairs 14d, 14e and 16d, 16e, provide an ample amount of contact
surface between the basket assembly 11 and the vessel 3, both for thermal conduction
and for the equilibration of any mechanical shock forces applied to the exterior of
the cask 1 as the result of an accident. Second, due to the fact that the area of
the strut plates 24a, 24b and 24c in these formers 22.1a-22.5d is relatively small
(20 percent) as compared to the area of the periphery of the basket assembly 11 as
a whole, it interferes very little with the conductive and radiative transfer of heat
from the radioactive materials disposed within the basket 11 to the walls of the inner
vessel 3.
[0017] Referring once more to Figures 1, 2 and 3, the generally cylindrical inner vessel
3 of the cask 1 has an open top end 28 and a closed bottom end 29 and, at its top
end 28, has an inner annular shoulder 32 and an outer annular shoulder 34 each with
uniformly spaced bolt holes 33 or 35, respectively, therein. The general purpose of
these annular shoulders 32, 34 is to support a double-lid closure assembly (not shown),
e.g., of the type described in EP-A-0312870.
[0018] As mentioned hereinbefore, the vessel 3 has disposed on its inner surface 36 at least
one basket retaining means or assembly 38 (Figs. 6 and 7). In the embodiment shown,
the basket retaining assembly 38 is formed from a pair of stainless steel dowels 42,
43 measuring preferably about 2.5 cm in diameter. When the basket 11 is to be loaded
into the vessel 3, one of its central plate-pair ends 20a, 20b or 21a, 21b is inserted
into the guide channel defined between the dowels 42, 43 of the basket retaining assembly
38 so as to slide down therein as the basket is lowered. The purpose of the basket
retaining assembly 38 is to prevent the basket assembly 11 from rotating relative
to the vessel 3, as mentioned earlier herein.
[0019] With particular reference to Figs. 2, 3 and 4, the ribs 5 on the outer surface of
the vessel 3 which preferably are made of carbon steel have their inner edges 47 joined
to the low-alloy steel walls 4 of the vessel 3 by means of fillet welds 48 formed
on both sides of each rib 5. These fillet welds 48 not only provide a strong mechanical
joint between the vessel wall 4 and the ribs 5 but also form highly heat-conductive
bridges or paths between the vessel wall 4 and the ribs 5 which will facilitate the
conduction of heat from the inner surface 36 of the vessel 3 to its outer surface
45.
[0020] The high-hydrogen content cement 7 applied to the outer surface 45 of the vessel
3 between the ribs 5 provides, due to its high hydrogen content, a high neutron capture
cross-section which renders the layers 7 particularly effective as a neutron-radiation
absorbing medium. This is highly advantageous since the low-alloy steel of the wall
4 of the vessel 3 shields effectively against gamma radiation but not so effectively
against neutron radiation. Likewise as seen best from Fig. 4A, the peripheral fins
9 are joined to the outer edges 51 of the respective ribs 5 also by means of fillet
welds 53 which are applied to the edges 52a, 52b of the fins 9 and preferably are
continuous for the full length of the edges 52a, 52b. As mentioned hereinbefore, the
peripheral fins 9 and the manner in which they are attached to the outer edges 51
of the ribs 5 offer four distinct advantages. First, during fabrication of the cask
1, the fins 9 may advantageously be used as molds for applying the cementitious material
7 upon the outside surface 45 of the vessel 3. Second, the fins 9, having a thickness
in the order of about 6.5 mm, form a strong mechanical barrier covering the relatively
brittle layer of cement 7, thereby protecting it from fracturing or shattering in
the event of mechanical shock applied to the exterior of cask 1. Third, the fins 9
form a watertight barrier over the layer of cement 7 which renders the cask 1 immersible
in a spent-fuel pool without any danger of dissolved radio-nucleides soaking into
the porous and water-permeable cement 7. Finally, the peripheral fins 9 provide excellent
heat dissipation in a structure which is considerably less fragile than one formed
by radially oriented heat dissipating fins alone.
[0021] As shown in Figure 4B, the exterior surfaces of the fins 9 preferably are covered
with a coating 54 rendering the fins 9 corrosion-resistant and sealing the micropores
normally existing in the surface of carbon steel, thereby preventing radioactive dust
or dissolved radionucleides from lodging in surface pores of the fins 9. In the preferred
embodiment illustrated, the coating 54 consists of a base layer 55 of a zinc-containing
primer, an intermediate layer 56 of an epoxy polyamide, and top layer 56 of polyester
polyurethane. Preferably, the primer is Carbo Zinc -8 manufactured by the Carboline
Company located in St. Louis, Missouri, and the top and intermediate layers 56 and
57 are a Series 66 High-Build Epoxoline and a Series 70 and 71 Endura-Shield, both
manufactured by Tnemec Company, Inc., located in St. Louis, Missouri.
[0022] The cask 1 includes also a lid assembly 58 comprising a lid 59 with stud-and-nut
assemblies 60 (Fig. 5) for fastening it to the vessel 3 at the open end 28 thereof,
and an upper ring 62 (Fig. 3) which extends around an upper end portion of the cask
1 and has its lower edge 64 welded (not shown) to the upper edges of the fins 9 so
as to create a strong mechanical and watertight joint therebetween. The outer edge
of the lid 59 abuts the upper edge 63 of the ring 62 when the lid is secured in place
upon the cask.
[0023] As seen from Figs. 1, 3 and 8, the cask 1 also includes a floor assembly 65 comprising
a disk 66, a spoke assembly 67 provided over the ground-engaging surface of the disk
66 to help in evening out the load applied by the cask 1 upon the disk 66 when standing
upright on the ground, and a lower ring 68 which is similar to the upper ring 62 but
circumscribes the cask 1 at the very bottom thereof. The ring 68 has its bottom edge
69 welded all around to the peripheral edge of the bottom disk 66 so as to form a
strong, watertight connection therebetween, and has its upper edge welded to the lower
edges of the fins 9 so as to form a strong mechanical and watertight joint therebetween.
[0024] With reference now to Figures 9A, 9B and 9D, each of the cells 18a-18x of the basket
assembly 11 has disposed therein a container 74 for spent fuel rods. As seen best
from Figure 9A, each of the containers 74 has a lead-in flange 75 disposed thereabout
at the upper end thereof in order to facilitate the insertion of fuel rods therein,
and has each of its four walls lined on its outer surface, as indicated in Fig. 9D,
with a sheet 76a-76d of Boral
® or some other material having a high neutron capture cross-section. The provision
of such high neutron-absorbing sheets 76a-76d, or "poison plates" as they are known
in the art, creates neutron-flux traps between adjacent cells 18a-8x which greatly
attenuate the transmission of thermal neutrons among the various cells 18a-18x of
the basket assembly 11. Finally, each of the cells 18a-18x has disposed therein corner
brackets 78a-78d for securing the container 74 therein in place. Preferably, the
containers 74 are made of anti-corrosive stainless-steel sheet material.
1. A shipping cask (1) for transporting radioactive material, comprising a vessel
(3) having a metallic heat-conductive wall (4), a plurality of substantially parallel
spaced, heat-conductive ribs (5) disposed on and projecting from the outer surface
of said heat-conductive wall, a layer (7) of radiation-absorbing material covering
the outer surface of the heat-conductive wall between the ribs, and a removable basket
assembly (11) disposed in said vessel and adapted to receive radioactive material,
characterized in that said vessel (3) includes a plurality of circumferentially disposed,
flat, heat-conductive fin members (9) joined along opposite edges thereof to outer
edge portions (51) of the respective ribs (5) and bridging the spaces between the
latter so as to form a watertight barrier supportingly and protectively overlying
said layer (7) of radiation-absorbing material; and said basket assembly (11) includes
a plurality of heat-conductive formers (22) joined to the basket assembly so as to
structurally reinforce same, said formers (22) being located at the perimeter of the
basket assembly and arrayed in spaced relationship with respect to each other in a
manner such as to enable the formers to conduct heat from the basket assembly (11)
to said heat-conductive wall (4) of the vessel (3) without objectionably impeding
the convective and radiative transfer of heat therebetween.
2. A shipping cask according to claim 1, characterized in that said ribs (5) are welded
to the metallic heat-conductive wall (4) of said vessel (3).
3. A shipping cask according to claim 1 or 2, characterized in that said ribs (5)
and said fin members (9) are made of a metal of substantially the same type, and the
fin members are welded to the ribs.
4. A shipping cask according to claim 1, 2 or 3, characterized in that the metallic
heat-conductive wall (4) of said vessel (3) consists of low alloy steel, and said
ribs (5) consist of carbon alloy steel.
5. A shipping cask according to claim 1, 2, 3 or 4, characterized in that said flat,
heat-conductive fin members (9) have disposed on their outer surfaces an anti-corrosion
and pore-sealing coating (54).
6. A shipping cask according to claim 5, characterized in that said coating (54) is
composed of a layer (55) of zinc-containing primer, a layer (56) of epoxy polyamide,
and a layer of polyester polyurethane.
7. A shipping cask according to any one of the preceding claims, characterized in
that said formers (22) are spaced apart substantially uniformly.
8. A shipping cask according to any one of the preceding claims, characterized in
that said formers (22) cover substantially not more than 30% of the perimetric area
of the basket assembly (11).
9. A shipping cask according to any one of the preceding claims, characterized in
that said basket assembly (11) has a perimetric outline defining several corners (26),
and each of said formers (22) is composed of strut plates (24) each connected to the
basket assembly at two mutually adjacent ones of said corners.
10. A shipping cask according to any one of the preceding claims, characterized in
that said basket assembly (11) has portions (20,21) located at its perimeter and presenting
discrete contact surfaces adapted to coact with inner surface portions of the heat-conductive
wall of said vessel (3) to transfer heat from the basket assembly to said heat-conductive
wall and to absorb impact forces if and when acting therebetween.
11. A shipping cask according to claim 10, characterized in that the contact-surface
presenting portions (20,21) of the basket assembly (11) are spaced apart substantially
uniformly about the perimeter of the basket assembly.
12. A shipping cask according to claim 10 or 11, characterized in that said basket
assembly (11) comprises a cell structure (18a-18x) composed of substantially orthogonically
interleaved plates (14,16), said contact-surface presenting portions (20,21) of the
basket assembly being formed by opposite edge portions of central ones (14e-f, 16e-4)
of said plates, and said vessel (3) including at least one plate retaining means (38)
for receiving and retaining at least one (20a) of said contact-surface presenting
portions (20,21).
13. A shipping cask according to claim 12, characterized in that said or each plate
retaining means (38) comprises a pair of dowels (42,43) disposed on the inner surface
(36) of said heat-conductive wall (4) of the vessel (3), and projecting therefrom
in substantially parallel spaced relationship with respect to each other so as to
receive said or the respective contact-surface presenting portion (20a) of the basket
assembly (11) slidably therebetween.
14. A shipping cask according to any one of the preceding claims, characterized in
that said vessel (3) has an open top end (28), and the shipping cask includes a closure
assembly (58) for sealingly closing said top end (28) of the vessel, said closure
assembly (58) including a lower edge (64) adapted to be sealingly connected to upper
edges of said fin members (9).
15. A shipping cask according to claim 14, characterized in that said closure assembly
(58) includes a ring (62) having thereon said lower edge (64), and a lid (59) which
is sealingly and detachably connected to an upper edge (63) of said ring (62).
16. A shipping cask according to any one of the preceding claims, characterized in
that said vessel (3) has a closed bottom end (29), and the shipping cask includes
a floor assembly (65) disposed at the bottom end of the vessel and having an upper
edge (70) which is sealingly connected to bottom edges of said fin members (9).
17. A shipping cask according to claim 16, characterized in that said floor assembly
(65) includes a bottom plate (66), and said layer (7) of radiation-absorbing material
extends between said bottom plate (66) and the closed bottom end (29) of the vessel
(3).