JPH0672947B2 - Cask for transporting radioactive materials - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to casks for transporting nuclear material to and from nuclear power plant facilities.

Conventionally, casks are known for transporting fuel rods to and from nuclear power plants. Such casks generally have a cylindrical transportable steel container and a basket structure insertable into the steel container, the outer basket structure being either a fuel assembly or a dense fuel rod. It has an array of cells containing a plurality of rectangular storage vessels each designed to house any of the spent fuel canisters that are assembled in a triangular pitch and integrated. Such transportable casks are fixed mounted on the trailer section of a trailer truck, usually in the safest possible way to dispose of spent fuel rods from a nuclear power plant to a permanent waste sequestration facility or reprocessing plant. Used for shipping in. Current,
The number of shipping casks used is relatively small. The reason is that most power companies that own nuclear power plants can store the spent fuel rods produced at nuclear power plants in the pool of spent fuel that was built into the reactor facility at the time of construction. It was not necessary to manufacture so many. However, as the number of fuel assemblies that can be put into the spent fuel pool every day is increasing, the available on-site storage space is steadily decreasing. Therefore, increase the number of storage facilities,
Ultimately, there is an increasing demand to transfer spent fuel assemblies from on-site storage facilities of nuclear power plants to off-site nuclear waste treatment facilities. A transport cask is needed to achieve this goal.

As a practical matter, the cask used to transport radioactive material by truck must meet at least five basic requirements set out below. The first requirement is that the cask wall shields both gamma and neutron radiation emitted by the payload sufficiently effectively that the total dose of radiation emitted from the cask surface is at a safe level to handle, eg,
It should be below 200 millirem at any given point on the cask surface and below 10 millirem 2m from the vehicle carrying the cask. As a second requirement, the cask needs to withstand a mechanical shock of the same magnitude as the mechanical shock generated in a vehicle accident. In this regard, it is not enough that the cask wall simply keeps the radioactive material stored even after it is subjected to a mechanical shock, as the cask wall does not allow water to enter the cask and It is necessary to maintain watertightness everywhere so that the neutrons emitted by the stored spent fuel rods and other substances do not heat up. As a third requirement, the basket structure inside the cask must be able to withstand the forces exerted by the inner wall of the cask on its periphery without significantly distorting the individual nuclear waste storage cells in the event of an accident. This is because if these cells are significantly deformed, the so-called neutron "traps" provided between the cells are damaged, which causes a critical state inside the cask. As a fourth requirement, the cask needs to be soaked in water that it does not enter, and it must be completely drainable. The reason why this is a requirement is that casks often move fuel rods in and out of the spent fuel pool of nuclear facilities to reduce the exposure of workers to potentially harmful radiation. Pool water usually contains radionuclides in dissolved form, and if such dissolved radionuclides enter and adhere to the gaps in the cask or micropores on the cask surface, it is difficult, if not impossible, to remove them. Is known. If such a radionuclide adheres to the gaps or surface micropores of the cask, the surface radiation dose of the cask exceeds the allowable limit, and thus the cask cannot be used. As a final and fifth requirement, the cask needs to be able to effectively dissipate the decay heat generated by the radioactive material inside the cask. The reason is that if there is no effective heat dissipation mechanism, the temperature inside the cask can be high enough to create a dangerous level of pressure.

Unfortunately, it is difficult to satisfy the above-mentioned five conditions at the same time, because constituent materials and mechanisms that satisfy different conditions are often intended to conflict with each other. For example, fins that project radially from the outer surface of the cask improve heat dissipation, but the presence of the fins reduces the ability of the cask to withstand large mechanical shocks without compromising the integrity of the cask wall. become. One of the most suitable and most economical known neutron shielding materials is high hydrogen cement, which is relatively brittle and tends to break up with the crushing forces experienced in an accident. Lead, depleted uranium, and Boro-Silicon® are also known as effective gamma-ray shielding materials, but they all have sufficient mechanical strength to withstand the conditions of an accident. Absent.
Furthermore, none of these materials are easy to weld to structurally strong metals or join by other known methods. Because they differ greatly in mechanical and metallurgical properties. Although stainless steel has good structural properties and corrosion resistance, it is not a particularly good heat transfer medium and is expensive. It has also been determined that the surface of stainless steel has micropores that can trap dissolved radionuclides and radioactive dust. Finally,
Carbon steel is a good and inexpensive structural material that has better heat transfer characteristics than stainless steel, but is susceptible to corrosion when exposed to water.

It is an object of the present invention to provide an improved transportation cask that satisfies the above five requirements, is relatively easy to manufacture, and is inexpensive.

In view of this object, the gist of the present invention resides in the cask for transporting radioactive material according to claim (1) in the scope of claims.

More specifically, a shipping cask embodying the present invention comprises an inner container with a metallic heat conducting wall, preferably formed of low alloy steel, and a respective inner edge of the heat conducting wall of the container. A plurality of heat-conducting ribs parallel to each other, which are preferably made of carbon steel, bonded to the outer surface of the container, and the radiation-absorbing property of covering the outer surface of the heat-conducting wall of the container while being positioned between the ribs parallel to each other. A layer of cementitious material and a plurality of circumferentially-disposed flat fins each joined to the outer edges of two adjacent ribs, each with its entire longitudinal edge, preferably also carbon steel. And a member. The circumferentially arranged fin members advantageously serve both to dissipate the heat generated by the radioactive material in the inner container and to form a watertight barrier that supports and protects the layer of cementitious material.

The shipping cask further includes a removable basket assembly that defines an array of cells that can be loaded and unloaded from the inner container and that can hold a quantity of radioactive waste. In the preferred embodiment, the cell structure of the basket assembly is formed of two sets of parallel stainless steel plates placed orthogonally to each other and fitted in a "chicken crate" shape.

The perimeter of the basket assembly is defined by outer or peripheral cell corners and angled formers that prevent the cells from deforming under mechanical shock applied to the exterior of the cask. The contact surface is enlarged to provide good heat transfer from the basket assembly to the thermally conductive wall of the container. The formers are spaced so as not to significantly interfere with heat transfer due to convection and radiation from the radioactive material contained in the cell structure of the basket assembly to the inner container, and the formers are separated from each other. The total area is about 30% or less, preferably 20% or less of the total area of the outer peripheral surface of the basket assembly. The formers may each be formed of several plates arranged in tandem in the circumferential direction of the basket assembly, each edge connected to the corners of two adjacent cells.

In order to further enhance the deformation resistance of the cell structure of the basket assembly in the unlikely event that the cask receives a strong mechanical impact, the basket assembly is surrounded by the outer peripheral surface of the basket assembly and the inner surface of the inner container. There are a plurality of separate, evenly spaced contact surfaces that evenly distribute the impact forces that may occur therebetween. In the preferred embodiment, these separate contact surfaces conform in shape to the corners of cells located around the basket assembly.

By making the inner container from low alloy steel and the rib and fin members from carbon steel, all of the components of the cask are easier to weld together. In order to prevent surface corrosion and prevent dissolved radionuclides or radioactive dust from entering the pores of the metal forming the circumferential fin, a corrosion-resistant coating is applied to the outer surface of the fin. It may comprise a layer of zinc-containing primer, a layer of epoxy polyamide and a layer of polyester polyurethane.

Preferred embodiments of the present invention, which are exemplary only, will now be described with reference to the accompanying drawings.

With particular reference to FIGS. 1, 2 and 3, the illustrated transport cask 1 has an elongated generally cylindrical shape with a thermally conductive wall 4 made of metal, preferably low alloy steel. Has an inner container 3 of which the outer surface of the thermally conductive wall 4 of the container is
A plurality of lengthwise heat-conducting ribs 5 which are parallel to each other and are equally spaced and oriented in the radial direction are joined and preferably welded. A layer 7 of neutron absorbing cement is deposited and formed on the outer surface of the container 3 between the ribs 5 which are equally spaced apart. Preferably, the layer 7 is made of cement having a high atomic hydrogen content percentage.

As best seen in Figures 2 and 4A, cask 1
The exterior of the is formed by a plurality of plate-like circumferential fins 9, each of which has its entire longitudinal edge in the rib 5.
Of the two adjacent ribs are joined, preferably welded, to form a rigid, watertight joint between the fin and its associated rib. Preferably, each fin 9 is an elongated rectangular carbon steel plate with a thickness of about 6.5 mm. Advantageously, the fin 9 serves three functions. As a first function, the fin 9 is an effective means for dissipating the heat input through the rib. As a second function, the fins 9 serve as a strong mechanical barrier that holds, supports and protects the neutron-absorbing cement layer 7 so that it will not be damaged in the event of an accident. As a third function, the fins 9 serve as a watertight barrier that prevents the cement layer from absorbing dissolved radionuclides even when the cask is dipped into the spent fuel pool. In the preferred method of manufacturing the cask, the fins 9 are first welded in place on the ribs 5, and then the neutron absorbing cement 7 is injected into the space between the container 3 and the fins 9. When such a manufacturing method is adopted, if water and cement leak to the outer surface of the cask 1 after the injection of the neutron absorbing cement 7, the manufacturer removes the cement in the problem area and It can be seen that it is necessary to re-weld the joint in question between the rib 9 and the rib 9 to make it watertight, so that a "leak test" can be performed on the weld formed between the fin 9 and the rib 5. There are advantages.

Referring now to FIGS. 6, 9A, 9B and 9C,
The cask 1 further comprises a basket assembly 11 which can be freely taken in and out of the inner container 3. Basket assembly
11 is a pair of mutually parallel plates 14a to 14h, 1
It is formed by 6a to 16h. Two sets of plates 14a-14
h, 16a-16h are each provided with slots 15, 17 which, when fitted together as best shown in FIG. 9C, form a crate-like structure for eggs that defines square cells 18a-18x. To be done. Preferably plates 14a-1
4h, 16a-16h are formed of a solid sheet of stainless steel about 1 cm thick. The plates 14a to 14h and 16a to 16h are fitted to each other, and the welded portions are further connected to the respective slots.
It is secured to each other by forming over its length along the edges of 15,17. As best seen in FIG. 9B, a set of parallel plates 14a-14h or a set of parallel plates 16a-16h are each a central plate composed of closely spaced plates 14d, 14e. It includes a central plate pair composed of pairs or plates 16d, 16e, which are centrally located within the basket assembly 11 and thus add extra strength to the basket assembly as a whole. Each of these central plate pairs
Plate-to-end part with four equiangularly spaced contact surfaces between the periphery of the basket assembly 11 and the inner surface of the container 3.
It ends with 20a, 20b, 21a, 21b. These contact surfaces not only equalize the transfer of heat from the radioactive material contained in the cells 18a-18x to the wall of the container 3, but also when the cask 1 is in an accident, the basket assembly 11 and the container 3 The impact force between the inner wall and the inner wall can be evenly absorbed. Further, at least one of the ends 20a, 20b, 21a, 21b of the plate pair will maintain the correct orientation of the basket assembly 11 within the container 3 when the container 3 is subjected to torque, as will be described below. In a manner, it cooperates with plate holding means 38 provided on the inner surface of the container 3.

As best seen in FIGS. 9A and 9B, a plurality of angular shape maintaining members (hereinafter referred to as “angular formers” or simply “formers”) 22.1a to around the basket assembly 11 are formed. 22.5a, 22.1b ~ 22.5b, 22.1c ~ 22.5c, 22.
1d to 22.5d are arranged. The formers are each arranged in tandem and the entire edge is formed by three tension plates 24a, 24b, 24c welded to the corners defined by the plate overhangs 26a, 26b, 26c, 26d. ing. In the preferred embodiment, each of the tension plates 24 is formed of stainless steel plate material having a thickness of about 1 cm. Such stainless steel material is advantageously a stainless steel plate 14a-14h, 16a used to form the basket assembly 11.
~ 16h and good compatibility with welding. As best seen in FIG. 9A, the angular formers 22.1a-22.5d are spaced from each other along the longitudinal axis of the basket assembly 11,
Preferably, they are arranged at equal intervals. Basket assembly
Angular former 22.1a-2 for 11 total external surface areas
The total area of 2.5d is only about 20%. The use of such angular formers 22.1a-22.5d in the basket assembly 11 is advantageous in two main respects. The first advantage is that the plate overhangs 26a, 26b,
The multiple shallow corners defined by 26c and 26d provide four large contact areas between the outer surface of the basket assembly 11 and the inner surface of the container 3. These numerous contact areas and the ends 20a, 20b of the plate pairs 14d, 14e and 16d, 16e.
And 21a, 21b, together with the contact area obtained by
A wide contact surface is obtained between the basket assembly 11 and the container 3, which improves heat transfer and equalizes the mechanical impact forces exerted on the outside of the cask 1 as a result of an accident. As a second advantage, the area of the tension plates 24a, 24b, 24c of these formers 22.1a to 22.5d is relatively small (20%) as a whole as compared with the area of the peripheral portion of the basket assembly 11. %), The degree of hindering the transfer of heat from the radioactive substance contained in the basket assembly 11 to the wall of the inner container 3 by the conduction phenomenon and the radiation phenomenon is very small.

Referring again to FIGS. 1, 2 and 3, the generally cylindrical inner container 3 of the cask 1 has a top open end.
28 and a bottom closed end 29, the top open end 28 of which
Has an inner annular shoulder portion 32 and an outer annular shoulder portion 34 in which bolt holes 33 and 35 are formed at equal intervals. The general purpose of providing these annular shoulders 32, 34 is to support, for example, a double lid closure assembly (not shown) of the type described in European Patent Application 0312870.

As mentioned above, the inner surface 36 of the container 3 is provided with at least one basket retaining means or assembly 38 (see FIGS. 6 and 7). In the illustrated embodiment, the basket retaining assembly 38 is formed of a pair of stainless steel dowel pins 42,43, preferably about 2.5 cm in diameter. When loading the basket 11 into the container 3, one of the ends 20a, 20b and 21a, 21b of the center plate pair is defined between the mating pins 42, 43 of the basket holding assembly 38 by a guide channel. , And slide the basket 11 into the guide channel while lowering it. The purpose of the basket holding assembly 38 is to prevent the basket assembly 11 from rotating relative to the container 3, as described above.

With particular reference to FIGS. 2, 3 and 4, the ribs 5 provided on the outer surface of the container 3, preferably made of carbon steel, are formed by forming fillet welds 48 on both sides of each rib 5. Container 3
Inner wall 47 joined to the low alloy steel wall 4. Forming these fillet welds 48 not only provides a strong mechanical joint between the walls 4 and ribs 5 of the container, but also facilitates the transfer of heat from the inner surface 36 of the container 3 to its outer surface 45. A high thermal conductivity bridge or passage is formed between the wall 4 and the rib 5 of the container.

The high hydrogen content cement 7, which is attached to the outer surface 45 of the container 3 between the ribs 5, has a large neutron capture cross section due to the high hydrogen content, which makes the cement layer 7 especially useful as a neutron absorbing medium. It is valid. This is a great advantage. The reason is that the low alloy steel of the wall 4 of the container 3 effectively shields gamma rays, but its shielding ability against neutron rays is not so effective. Similarly, as best seen in FIG. 4A, the surrounding fins 9 have fillet welds 53
Is applied to the edges 52a, 52b preferably continuously over the entire length of the edges 52a, 52b of the fin 9 so as to be joined to the outer edges 51 of the respective ribs 5. As mentioned above,
With such a configuration of the peripheral fin 9 and a method of mounting the peripheral fin 9 on the outer edge 51 of the rib 5, four remarkable advantages are obtained. As a first advantage, during the manufacture of the cask 1, the fins 9 can advantageously be used as a mold for depositing the cementitious material on the outer surface 45 of the container 3. Second
As an advantage, the fin 9 having a thickness of about 6.5 mm covers the relatively brittle cement layer 7, thereby preventing the cement layer 7 from being broken or crushed even when a mechanical shock is applied to the outside of the cask 1. Provides a strong mechanical barrier. Third
As an advantage of, the fin 9 is a watertight barrier that allows the cask 1 to be immersed in the spent fuel pool without the risk of dissolved radionuclides penetrating the porous and permeable cement 7 with the cement layer 7 covered. Becomes The fourth advantage is that
The peripheral fins 9 exert an excellent heat dissipation action in a state in which the degree of brittleness is considerably smaller than the structure formed by only the heat dissipation fins oriented in the radial direction.

As shown in FIG. 4B, the outer surface of the fin 9 preferably makes the fin 9 corrosion resistant and seals the micropores normally present on the surface of carbon steel, thereby preventing radioactive dust or dissolved radionuclides from finning. The surface of 9 is covered with a coating 54 that prevents it from entering the pores. In the preferred embodiment shown, the coating 54 comprises a base layer 55 of zinc-containing primer, an intermediate layer 56 of epoxy polyamide and a surface layer 57 of polyester polyurethane. Preferably, the primer is a Carboline Company (Ca), St. Louis, Mo.
Carbo Zinc-8 manufactured by rboline Company
inc-8), surface layer 57, and intermediate layer 56 are Series 66 High-Build Epoxo line (Series 66 High-Build
Epoxoline) and Series 70 and 71 Endura-Shield, both of which are from Tnemec Compan, Inc. of St. Louis, Missouri.
y, Inc.).

The cask 1 also extends around a lid 59 with a stud and nut assembly 60 (FIG. 5) for fastening to the open end 28 of the container 3 and around the upper end of the cask 1 with a lower edge 64 of the fin 9. A lid assembly 58 consisting of an upper ring 62 (FIG. 3) which is welded to the upper edges of the fins 9 (this weld is not shown) to form a strong, watertight joint between them. . The outer edge of the lid 59 abuts the upper edge 63 of the ring 62 when the lid is fixed in place on the cask.

As can be seen in FIGS. 1, 3 and 8, the cask 1
Is also used to help balance the load of the disc 66 and the cask 1 that is applied to the disc 66 when standing on the ground.
It has a floor assembly 65 consisting of a spoke assembly 67 mounted on the ground plane of 66 and a lower ring 68 which is similar to the upper ring 62 but which surrounds the bottom of the cask 1. The lower ring 68 is rigid and mechanical between the fin 9 and the bottom edge 69, which is welded around the periphery of the disc 66 to form a strong, watertight connection with the lower disc 66. An upper edge welded to the lower edge of the fin 9 to form a solid and watertight joint.

Next, referring to FIGS. 9A, 9B and 9D, each of the cells 18a to 18x of the basket assembly 11 contains a spent fuel rod container 74. As best seen in Figure 9A, around each container 74, at its top edge,
An inlet flange 75 is formed for the purpose of facilitating the insertion of fuel rods into the container 74. Also, as shown in FIG. 9D, the four walls of the container 74 are each lined on their outer surfaces with sheets 76a-76d made of Boral® or other material with a large neutron capture cross section. ing. Sheets 76a-76d with high neutron absorption capacity (these are called "poison plates" in the industry)
By providing a neutron flux trap between adjacent cells 18a-18x, these traps significantly reduce the transfer of thermal neutrons between the various cells 18a-18x of the basket assembly 11. Finally, inside each of the cells 18a-18x, corner brackets 78a-78d are provided for fixing the container 74 in place. Preferably, the container 74 is made of corrosion resistant stainless steel sheet material.

[Brief description of drawings]

FIG. 1 is a side view of a transportation cask embodying the present invention, and is a diagram showing a closing outer lid in a state of being lifted above the uppermost portion of the cask. FIG. 2 is a transverse cross-sectional view of the cask taken along line 2-2 of FIG. 1 with some fins and a portion of the cement layer removed for clarity. FIG. 3 is a cross-sectional side view of the cask at line 3-3 of FIG. 2 with the basket assembly removed for clarity. FIG. 4A is an enlarged view of a portion surrounded by a circle in FIG. Figure 4B is an enlarged view of the encircled portion of Figure 4A showing the various layers of protective coating applied to the exterior of the cask. FIG. 5 is a plan view of the outer lid of the cask taken along the line 5-5 in FIG. FIG. 6 is a plan view of the upper end portion of the cask as seen along the line 6-6 in FIG. FIG. 7 is an enlarged view of a portion surrounded by a circle in FIG. FIG. 8 is a plan view of the lower end portion of the cask taken along the line 8-8 in FIG. FIG. 9A is a side view of the basket assembly of the cask with a portion of the peripheral basket cut away to reveal some of the canisters housed within the cells of the basket assembly. 9B is a plan view of the basket assembly shown in FIG. 9A. FIG. 9C is a diagram showing how two sets of plates forming a basket assembly are fitted together. FIG. 9D is an enlarged view of the circled cell of FIG. 9B. [Explanation of main reference numbers] 1 ... Transport cask 3 ... Container 4 ... Thermal conductive wall 5 ... Rib 7 ... Radiation absorbing cement layer 9 ... Fin 11 ... Basket assembly 18 ... Cell 22 …… Former 38 …… Basket holding means 58 …… Lid assembly 65 …… Floor assembly

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location 8607-2G G21F 5/00 J

Claims (17)

[Claims] 1. A container provided with a heat conductive wall made of metal, and a plurality of heat conductive ribs projecting from the outer surface of the heat conductive wall and arranged substantially parallel to each other and spaced apart from each other. And a layer of radiation absorbing material covering the outer surface of the heat conducting wall between the ribs and a retractable basket assembly housed in a container with the radioactive substance in it. Therefore, the container is joined to the outer edges of each rib along the entire opposite edges and bridges the spaces between the ribs, supporting the radiation absorbing material layer and surrounding it in a watertight manner. Having a plurality of circumferentially arranged flat thermally conductive fin members forming a barrier, a plurality of thermally conductive formers joined to the basket assembly to structurally reinforce the basket assembly, Sex former
The basket in such a manner that heat can be transferred from the basket assembly to the heat conducting wall of the container without disturbing the heat transfer by convection and radiation of heat between the basket assembly and the heat conducting wall of the container. A cask for transporting radioactive material, characterized in that it is located around the assembly and arranged in spaced relation to each other. 2. The cask for transporting radioactive material according to claim 1, wherein the rib is welded to the metal heat conductive wall of the container. 3. The radioactive material according to claim 1 or 2, wherein the rib and the fin member are made of substantially the same metal, and the fin member is welded to the rib. Cask for substance transport. 4. The container according to claim 1, wherein the metal heat conductive wall is made of low alloy steel and the rib is made of carbon steel.
Cask for transporting radioactive material according to item (2) or (3). 5. The flat heat-conducting fin member has an outer surface coated with a corrosion-resistant and pore-sealing coating, as claimed in any one of claims (1) to (4). The cask for transporting radioactive material according to any one of the items. 6. The coating is a primer layer containing zinc,
6. The cask for transporting radioactive material according to claim (5), which is composed of a layer of epoxy polyamide and a layer of polyester polyurethane. 7. The radioactive substance according to any one of claims (1) to (6), characterized in that the formers are arranged at substantially equal intervals. Transport cask. 8. The former covers substantially 30% or less of the surrounding area of the basket assembly, and the former is any one of claims (1) to (7). The cask for transporting radioactive material according to. 9. The basket assembly has an outer peripheral shape defining several corners, and the former each includes a plurality of protrusions each connected to the basket assembly at two adjacent corners of the corner. The cask for transporting radioactive material according to any one of claims (1) to (8), characterized in that the cask is composed of a veneer. 10. The basket assembly is located at its periphery and cooperates with an inner surface portion of the thermally conductive wall of the container to act between the basket assembly and the thermally conductive wall, or A section having separate contact surfaces for transferring heat from the basket assembly to the thermally conductive wall and absorbing impact forces when operating between them. ) -A cask for transporting radioactive material according to any one of (9). 11. The contact surface-provided portion of the basket assembly comprises:
The cask for transporting radioactive material according to claim (10), wherein the cask is arranged around the basket assembly at equal intervals. 12. The basket assembly comprises a cell structure composed of plates fitted together in a substantially orthogonal configuration, the contact surface-providing portion of the basket assembly comprising a central portion of the fitted plates. Formed with opposite edges of the plate, the container has at least one plate holding means for holding at least one of the contact surface-equipped parts in a received state. The cask for transporting radioactive material according to item (10) or (11). 13. The plate retaining means includes a pair of mating pins provided on the inner surface of the heat conductive wall of the container, the mating pins slidably between the contact surface-provided portions of the basket assembly therebetween. The cask for transporting radioactive material according to claim (12), wherein the cask protrudes from the inner surface of the heat conductive wall in a relationship in which they are substantially parallel to each other and spaced apart from each other. 14. The container has a top open end, the shipping cask has a closure assembly for sealingly closing the top open end of the container, the closure assembly sealingly coupled to the upper edge of the fin member. The radioactive material transport cask according to any one of claims (1) to (13), wherein the cask has a lower edge. 15. The closure assembly comprises a ring having a lower edge and a lid releasably and sealingly connected to an upper edge formed on the ring.
Cask for transporting radioactive material as described in 4). 16. A floor assembly wherein the container has a bottom closed end and a shipping cask is provided at the bottom closed end of the container and has a top edge sealingly connected to the bottom edge of the fin member. The cask for transporting radioactive material according to any one of claims (1) to (15), characterized in that it has. 17. The radioactive material transport of claim 16 wherein the floor assembly has a bottom plate and the layer of radiation absorbing material extends between the bottom plate and the bottom closed end of the container. For cask.