Cask assembly for transporting radioactive material of different intensities

An improved cask assembly for forming a cask that is adapted to transport radioactive materials of a particular activity is disclosed herein. The cask assembly comprises an outer container having an opening leading to its interior, and a plurality of inner shield inserts, each of which includes an inner wall whose shape is substantially complementary to the shape of the interior of the outer container and which is insertable therein to form a cask. The exterior walls of the inner shield inserts are formed from different shielding compositions, such as depleted uranium or lead, and are also of different thicknesses. The particular shield inserted within the interior of the outer container is chosen to match the intensity and type of radiation emitted by the waste to be transported so that the assembled cask hold a maximum amount of the particular material to be transported without exceeding a surface radiation of 200 millirems at any point. To facilitate the insertion and removal of the shield inserts, the closure opening is at least as wide as the width of its interior. Either a screw-type closure means of a breech-lock closure means is used to seal the container opening.

BACKGROUND OF THE INVENTION 
This invention generally relates to casks for transporting radioactive 
materials, and is specifically concerned with an improved cask assembly 
for forming a cask adapted to transport a maximum amount of radioactive 
material of a particular activity within a given weight limit. 
Casks for transporting radioactive materials such as the waste products 
produced by nuclear power plant facilities are known in the prior art. The 
purpose of such casks is to ship radioactive wastes in as safe a manner as 
possible. Such casks may be used, for example, to ship high-level 
vitrified waste cannisters to a permanent waste isolation site or spent 
fuel rods to a reprocessing facility. At the present time, relatively few 
of such transportation casks have been manufactured and used since most of 
the spent fuel and other wastes generated by nuclear power plants are 
being stored at the reactor facilities themselves. However, the 
availability of such on-site storage space is steadily diminishing as an 
increasing amount of fuel assemblies and other wastes are loaded into the 
spent-fuel pools of these facilities. Additionally, the U.S. Department of 
Energy (D.O.E.) has been recently obligated, by way of the National Waste 
Policy Act of 1983, to move the spent-fuel assemblies from the on-site 
storage facilities of all nuclear power plants to a federally operated 
nuclear waste disposal facility starting in 1998. 
While the transportation casks of the prior art are generally capable of 
safely transporting wastes such as spent fuel to a final destination, the 
applicant has observed that there is considerable room for improvement, 
particularly With respect to vehicle-drawn, Type B casks. Specifically, 
the applicant has observed that, in many instances, the structure of these 
casks do not lend themselves to an optimal loading of radioactive wastes. 
The resulting less-than-optimum loading necessitates a larger number of 
trips by the shipper in order to complete the transportation of a given 
amount of radioactive waste, thus increasing both the time and the cost of 
transport. However, before the problems associated with optimizing the 
amount of waste carried by a particular cask may be fully appreciated, 
some understanding of the constraints imposed by NRC regulations is 
necessary. 
U.S. Department of Transportation (DOT) and state highway regulations limit 
the gross weight of the waste-carrying road vehicle to about 80,000 pounds 
for shipments without special permits. Since the typical tractor and 
trailer weighs approximately 30,000 pounds, the weight of a cask and its 
contents must not exceed approximately 50,000 pounds. These same 
regulations specify that the surface radiation of such cask be no greater 
than 200 millirems at any given point, and that the radiation emitted by 
the cask be no greater than ten millirems at a distance of two meters from 
the vehicle. Other DOT regulations require that the cask be capable of 
sustaining impact stresses of up to ten Gs in the longitudinal direction, 
five Gs in the lateral direction, and two Gs in the vertical direction 
without yielding the wastes. The end result of these regulations is that 
much of the 50,000 pounds must be expended in providing adequate shielding 
materials within the cask (which are usually formed from dense materials 
such as lead or depleted uranium), as well as a structurally strong outer 
shell that can withstand the designated impact stresses. The thicknesses 
of both the shielding material and the structural shell required to comply 
with federal regulations leaves only a relatively small amount of space in 
the center of the cask which can actually be used to contain and transport 
radioactive waste. To maximize the amount of carrying volume, the most 
effective shielding materials known are frequently integrated into the 
walls of the cask structure. Such materials include lead, depleted 
uranium, and tungsten. However, as these materials are of a very high 
density, the radius of the cask walls cannot be made too large, or the 
gross weight limitation of 50,000 pounds of the combination of cask and 
waste material will be exceeded. The end result of the foregoing 
constraints of structural strength, shielding effectiveness, and the 
density of the most effective known shielding materials renders the 
carrying space in such cask relatively small relative to the volume of the 
cask as a whole when high activity wastes such as spent fuel rods are 
being transported. 
If the cost of transporting a particular amount of radioactive waste is to 
be minimized, then the use of the carrying space within the cask must be 
maximized, i.e., the space must be completely filled up with a waste 
having an activity which brings the surface radiation of the cask, as a 
whole, to just under the 200 millirem limit. If the carrying space within 
the cask is completely filled with a waste, but the resulting surface 
radiation of the cask is substantially below 200 millirems per hour, then 
the use of the cask is not being optimized. In such a case, a cask having 
thinner walls with less shielding materials and a larger cavity would be 
the optimum choice for the transportation of such a waste. If, on the 
other hand, only a small amount of the carrying volume may be filled with 
a particular kind of waste before the surface radiation of the case 
reaches 200 millirems, then the large ring of air-space between the waste 
and the shielding material results in a highly ineffective shielding 
geometry, wherein an excessively large weight of shielding material is 
being used to comply with the surface radiation limit of 200 millirems. In 
short, there is a single, optimum activity that every static-walled, prior 
art cask is matched to. Nuclear waste having an activity which is 
substantially below or above this optimum activity results in significant 
inefficiencies wherein the ratio of cask weight to waste weight is 
considerably higher than desired. 
Clearly, what is needed is a cask capable of optimally adjusting both the 
type and the amount of shielding materials contained within its walls to 
the particular type and activity of the waste material being hauled. 
Ideally, such a cask should be capable of quickly and conveniently 
adjusting the type and thickness of the shield materials used in its walls 
which are difficult to fabricate and machine, such as depleted uranium or 
tungsten. Finally, such a cask should be relatively simple and inexpensive 
to fabricate, and some sort of means for easily opening and closing the 
cask to effect loading and unloading operations, as well as a mechanism 
for reliably venting, purging, and draining the interior of the cask 
regardless of the particular type and thickness of shielding used in the 
cask interior. 
SUMMARY OF THE INVENTION 
Generally, the invention is an improved cask assembly for forming a cask 
adapted to transport a radioactive material of a particular activity. The 
improved cask assembly comprises an outer container having an opening 
leading to its interior, and a plurality of inner shield inserts, each of 
which includes an exterior wall whose shape is substantially complimentary 
to the shape of the interior of the outer container and which is 
insertable therein to form a cask. The exterior walls of the different 
inserts are formed from different shielding compositions, and may be of 
different thicknesses, and the particular shield insert placed within, the 
interior of the outer container is chosen to match the intensity and type 
of radiation emitted by the waste to be transported so that maximum amount 
of waste is loaded into the cask without exceeding the 200 millirem 
surface radiation limit. 
To handle wastes emitting high levels of gamma radiation, at least one of 
the inner shield inserts preferably includes a layer of depleted uranium. 
To handle wastes emitting neutrons, at least one of the inner shield 
inserts includes a layer of lead or boro-silicone or other neutron 
attenuating or absorbing material. Both the inner wall of the outer 
container and the outer wall of the insert is preferably lined with 
non-corrosive metal, such as stainless steel. 
To facilitate the insertion and removal of the different shield inserts and 
the radioactive materials contained therein, the access opening of the 
outer container is equal to or greater than the width of the interior. 
Additionally, a screw-type, double-lidded closure means may be used to 
selectively open and close both the outer container and the shield insert. 
Such a screw-type closure means includes an outer lid circumscribed by 
screw threads that are engageable with screw threads present around the 
access opening of both the container and the insert, as well as an inner 
lid circumscribed by a gasket that seats onto a ledge that circumscribes 
the opening. Alternatively, an improved, double-lidded breech-lock closure 
means may be used which includes an inner lid that is rotatably connected 
to an outer lid. As is the case with the screw-type closure means, the 
inner lid is circumscribed by a gasket which seats around a ledge which 
circumscribes the closure opening. However, instead of threads, the outer 
lid includes a plurality of flanges which are insertable between flanges 
which circumscribe the closure opening and which are further rotatable 
therebehind. Both closures allow the outer container and shield inserts to 
be closed and sealed without any rubbing between the gasket and the ledges 
surrounding the opening of these vessel. Of the two types of closures, the 
improved breech-lock closure is preferred since it is easier to machine, 
and effects a seal with a minimal amount of rotation between the outer lid 
and the outer container or shield insert. 
The outer container of the improved cask assembly may include a vent, 
purge, and drain assembly mounted in the side wall thereof. The primary 
purpose of this assembly is to allow the seal effected by the closure to 
be checked for leakage. This assembly may in turn include a drain pipe and 
a vent pipe and plugs removably insertable in each pipe. A drain tube that 
fits into a groove provided in the inner walls of the outer container may 
also be provided for draining any liquids that collect on the floor of the 
container. The drain tube may communicate with the drain pipe by way of a 
fitting. Such a configuration renders the vent, purge, and drain assembly 
effective regardless of the type of shield insert used in conjunction with 
the outer container.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
With reference now to FIG. 1, wherein like numerals designate like 
components throughout all the several figures, the invention is a cask 
assembly 1 that is particularly useful in carrying radioactive materials 
of different activities aboard a vehicle such as a tractor-trailer. In 
use, the cask assembly is typically mounted within a novel biaxial 
restraint cradle 3, which in turn is secured onto the trailer of a 
tractor-trailer (not shown). Generally, the cask assembly itself has a 
cylindrical body 5 which is circumscribed on either end by toroidial 
impact limiters 7a, and 7b. Each of these impact limiters 7a, 7b is a 
donut-shaped shell of yieldable aluminum which is approximately one-half 
of an inch thick. Each of the toroidial impact limiters 7a, 7b is mounted 
around its respective end of the cylindrical body 5 by means of a support 
ring assembly 8a, 8b which in turn is secured to the cylindrical body 5 by 
a plurality of bolts 9. Disposed between the impact limiters 7a, 7b are a 
pair of opposing trunnions 11a, 11b and 11c, 11d. The two pairs of 
trunnions are disposed 180 degrees apart around the cylindrical body 5 of 
the cask assembly 1, and are receivable within two pairs of turn buckle 
assemblies 12a, 12b, and 12c (of which only 12a and 12b are visible) that 
form part of the cradle 3. The cylindrical body 5 is capped by a closure 
13 at one end, and an end plate assembly 15 (shown in FIG. 3) at the other 
end. As is best seen in FIGS. 3 and 5, the cylindrical body 5 of the cask 
assembly 1 is generally formed by an outer container 18 which is 
surrounded by a thermal protection shell 20 on its exterior, and which 
contains in its interior one of two different shield inserts 22 or 23, 
depending upon the activity and type of radiation emitted by the material 
to be transported. While only two specific types of shield inserts 22 and 
23 are specifically disclosed herein, it should be noted that the inserts 
22 and 23 are merely exemplary, and that the improved cask assembly may in 
fact be used with any number of different types of shield inserts formed 
of different shielding materials and of different wall thicknesses for 
handling radioactive material within a broad range of activity and 
radiation type. 
With reference now to FIGS. 2A, 2B, and 2C, the thermal protection shell 20 
which circumscribes the outer container 18 of the cask assembly 1 is 
formed from a pair of semi-cylindrical shell sections 24a, 24b which are 
rigidly interconnectable into thermal contact with one another. Each of 
the shell sections 24a, 24b includes a pair of cut-outs 26 for admitting 
the trunnions 11a, 11b, 11c, and 11d. Each of the shell sections 24a, 24b 
is formed from a metal having a thermal coefficient of expansion which is 
greater than that of the metal that forms the walls of the outer container 
18, and which is at least as heat-conductive as the metal which forms the 
walls 54 of the outer container 18. When the outer wall of the outer 
container 18 is formed from steel, the shell sections 24a, 24b are 
preferably formed from aluminum or magnesium or an alloy of either or both 
of these metals. The coefficient of thermal expansion of these metals is 
approximately twice that of the thermal coefficient of expansion of steel. 
Moreover, the high coefficient of thermal conductivity of each such metal 
insures that the thermal protection shell 20 will not significantly 
obstruct the conduction of decay heat conducted through the walls of the 
outer container 18 which is generated by the radioactive material held 
within the cask assembly 1. When the diameter of the outer container 18 is 
between forty and sixty inches, a wall thickness of approximately one-half 
of an inch is preferred for both of the shell sections 24a, 24b. Such a 
wall thickness renders the thermal protection shell 20, as a whole, thin 
enough to be conveniently retrofitted over many existing transportation 
casks without significantly adding to the weight thereof, yet is thick 
enough to maintain the structural integrity needed to expand away from the 
outer walls of the outer container when exposed to a source of intense 
thermal radiation, such as a fire. Finally, the preferred thickness of 
one-half of an inch provides enough mass to give the entire thermal 
protection shell 20 a significant latent heat of fusion, which will 
provide still more thermal protection through ablation should the cask 1 
be exposed to intense heat. 
A plurality of top and bottom connecting assemblies 28, 29 are used to 
rigidly interconnect the two semi-cylindrical shell sections 24a, 24b. 
Since each of the connecting assemblies 28, 29 are identical in structure, 
a description will be made only of the top connecting assembly 28 circled 
in FIG. 2A. 
This connecting assembly 28 is formed from a pair of opposing semicircular 
lugs 30a and 30b which are integrally formed along the edges of the shell 
sections 24a and 24b respectively. These lugs 30a, 30b include mutually 
alignable bore holes 31a and 31b for receiving a connecting bolt 32. The 
threaded end 33 of the bolt 32 is engaged to a tension nut 34 as shown in 
FIG. 2B. The distance between the two lugs 30a, 30b (and hence the 
distance between the edges of the shell sections 24a, 24b) is largely 
determined by the extent of which the end 33 of the bolt 32 is threaded 
through the tension nut 34. A lock washer 35 is disposed between the 
tension nut 34 and the lug 30a to prevent the nut 34 from becoming 
inadvertently loosened. A pair of lock nuts 36a, 36b are threadedly 
engaged near the center portion of the connecting bolt 32 between the two 
lugs 30a and 30b. These lock nuts provide two functions. First, when 
properly adjusted, they prevent the tension nut 34 from applying excess 
tensile forces between the two shell sections 24a and 24b which might 
interfere with their expansion away from the outer container 18 in the 
event the cask assembly is exposed to a fire or other source of intense 
heat. Second, the nuts 36a, 36b eliminate all slack or play between the 
lugs 30a, 30b, thus insuring that the connecting assembly 28 rigidly 
interconnects the two shield sections 30a, 30b. Again, lock washers 37a, 
37b are disposed between the lock nuts 36a and 36b and their respective 
lugs 30a and 30b to prevent any inadvertent loosening from occurring. 
An overlap 40 is provided between the edges of the two shell sections 24a 
and 24b to establish ample thermal contact and hence thermal conductivity 
between these shell sections. The overlap 40 is formed from an outer 
flange 42 and recess 44 provided along the edge of shell section 24a which 
interfits with a complementary outer flange 46 and recess 48 provided 
along the opposing edge of shield section 24b. The actual length of the 
overlap 40 will vary depending upon the distance between the two lugs 30a 
and 30b as adjusted by the bolt 32, tension nut 34, and lock nuts 36a and 
36b. 
In operation, the two sections 24a, 24b of the thermal protection shell 20 
are installed over the cask assembly 1 by aligning the various cutouts 
26a, 26b, 26c, and 26d with the corresponding trunnions of 11a, 11b, 11c, 
and 11d which project from the cylindrical body 5, and placing the 
sections 24a, 24b together so that the lugs 30a and 30b of each of the 
connecting assemblies 28, 29 are in alignment with one another and the 
flanges and recesses 42, 44, and 48, 46 of each overlaps 40 are 
interfitted. Next, the bolt 32, tension nut 35, lock nuts 36a, 36b, and 
lock washers 35, 37a, and 37b are installed in their proper positions with 
respect to the lugs 30a, 30b of each of the connecting assemblies 28, 29. 
The tension nut 34 is then screwed over the threaded end 33 of connecting 
bolt 32 until the interior surface of each of the shell sections 24a and 
24b is pulled into intimate thermal contact with the outside wall 54 of 
the outer container 18. In the preferred method of installing the thermal 
protection shield, the tension nut 34 of each of the connecting assemblies 
28, 29 is initially torqued to a selected maximum on the threaded shaft of 
the bolt 32 until the nut 34 imparts a significant tensile force between 
the two lugs 30a and 30b. This tensile force tends to squeeze the two 
shell sections 24a and 24b together around the outer wall 54 of the outer 
container 18 in a clamp-like fashion, which in turn removes any 
significant gaps between the outer surface of the wall 54 and the inner 
surface of the shell sections 24a and 24b by bending these sections into 
conformity with one another. In the next step, each of the nuts 34 is 
relaxed enough to prevent these tensile clamping forces from interfering 
with the expansion of the thermal protection shell 20 in the event of a 
fire, yet not so much as to cause the surfaces of the shell 20 and the 
outer container from becoming disengaged with one another. Thereafter, the 
lock nuts 36a and 36b are tightened against the faces of their respective 
lugs 30a and 30b to remove all slack in each connecting assembly 28, 29. 
The end result is a rigid interconnection between opposing edges of the 
shield sections 24a and 24b, wherein each of the opposing lugs 30a and 30b 
is tightly sandwiched between the tension nut 34 and lock nut 36a, or the 
head of the bolt 38 and lock nut 36b, respectively. 
If the outer container has no trunnions 11a, 11b, 11c, 11d, or other 
structural members which would prevent the surfaces of the shell 20 and 
outer container 18 from coming into intimate thermal contact, the shell 20 
may assume the form of a tubular sleeve which may be, in effect, heat 
shrunk into contact over the container 18. This alternative method of 
installation comprises the steps removing the impact limiters 7a, 7b, of 
heating the shell to a temperature sufficient to radially expand it, 
sliding it over the wall 54 of the outer container 18, allowing it to cool 
and contract into intimate thermal contact with the wall 54, and 
reinstalling the impact limiters 7a, 7b. 
FIG. 2C illustrates the typical gap condition between the inner surface of 
the thermal protection shell 20 and the outer surface of the outer 
container 18. Under ambient conditions, these two opposing surfaces are 
either in direct contact with one another, or separated by only a tiny gap 
50 which may be as much as one mil. Such a one mil separation at various 
points around the cask assembly 1 does not significantly interfere with 
the conduction of heat between the wall 54 of outer cask 18, and the 
thermal protection shell 20. However, when the cask assembly 1 is exposed 
to a source of intense thermal radiation such as a fire, the. 
substantially higher thermal coefficient of expansion of the aluminum or 
magnesium forming the shell 20 will cause it to expand radially away from 
the outer surface of the outer container 18, leaving an air gap 53 (shown 
in phantom) between the two surfaces. Moreover, since the thermal 
protection shield 20 is formed from a metal having good heat conductive 
properties, this differential thermal expansion is substantially uniform 
throughout the entire circumference of the shield 20, which means that the 
resulting insulatory air gap 53 is likewise substantially uniform. When 
this gap exceeds approximately two and one-half mils, the primary mode of 
heat transfer switches from conductive and convective to radioactive. Thus 
the three mil gap provides a substantial thermal resistor between the fire 
or other source of intense infrared radiation in the outer container 18 of 
the cask 1. 
With reference now to FIGS. 3, 4A, 4B, and 5, the side walls of the outer 
container 18 of the improved cask 1 are a laminate formed from the 
previously mentioned outer wall 54, an inner wall 56, and a center layer 
58 of shielding material. In the preferred embodiment, the outer wall 54 
is formed from low alloy steel approximately one-forth of an inch thick. 
Such steel is economical, easy to manufacture, and a reasonably good 
conductor of heat. In the alternative, stainless steel may be used in lieu 
of low alloy steel. While the use of stainless steel would be more 
expensive, it provides the additional advantage of corrosion-resistance. 
The inner wall 56 is preferably also formed from low alloy steel. However, 
the inner wall 56 is made two inches thick in order to provide ample 
structural rigidity and strength to the outer container 18. Disposed 
between the outer wall 54 and the inner wall 56 is a layer of 
Boro-Silicone This material advantageously absorbs neutrons from 
neutron-emitting radioactive materials (such as transuronic elements), and 
further is a relatively good conductor of heat. It is a rubbery material 
easily cast, and may be melted and poured between the inner and outer 
walls 54, 56 of the outer container 18 during its manufacture. 
Boro-Silicone is available from Reactor Experiments, Inc., and is a 
registered trademark of this corporation. 
The bottom of the outer container 18 is formed by an end plate assembly 15 
that includes an outer plate 60, an inner plate 62, and a layer of center 
shielding material 64. In the preferred embodiment, the outer plate 60 is 
again formed from a low alloy steel approximately one-forth inch thick. 
The inner plate 62, like the inner wall 56, is again formed from a layer 
of low alloy steel approximately two inches thick. The center shielding 
material 64 is again preferably Boro-Silicone for all the reasons 
mentioned in connection with the center shielding material 58 of the side 
walls of the container 18. The low alloy steel inner plate 62 is joined 
around the bottom edge of the inner wall 56a 360.degree. via weld joint 
66. The top of the outer container 18 includes a forged ring of low alloy 
steel 68. This ring 68 is preferably four inches thick throughout its 
length, and is integrally connected to the inner wall 56 of the container 
18 by a 360.degree. weld joint 69. The upper edge of the ring 68 is either 
threaded or stepped to accommodate one of the two types of improved 
closures 115b or 117b, as will be explained in detail hereinafter. 
With specific reference now to FIGS. 3 and 5, the cask assembly 1 is formed 
from the outer container 18 and shell 20 in combination with one of two 
different shield inserts 22 (illustrated in FIG. 3) or 23 (illustrated in 
FIG. 5). Each of the shield inserts 22, 23 is formed from an outer 
cylindrical wall 72 which is preferably one inch thick and a cylindrical 
inner wall 74 which is approximately one-fourth of an inch thick. Both 
walls are formed from A1 S1 304 stainless steel. The corrosion resistance 
of stainless steel prevents the outer dimensions of the outer wall 74 from 
becoming distorted as a result of rust, which in turn helps advantageously 
to maintain a relatively tight, slack-free fit between the shield inserts 
22, 23 and the interior of the outer container 18. 
Each of the shield inserts 22 and 23 includes a layer of shielding material 
76 between their respective outer and inner walls 72, 74. However, in 
shield insert 22, this shielding material is formed from a plurality of 
ring-like sections 78a, 78b, and 78c of either depleted uranium or 
tungsten. These materials have excellent gamma shielding properties, and 
are particularly well adapted to contain and shield radioactive material 
emitting high intensity gamma radiation. Of course, a single tubular layer 
of depleted uranium or tungsten could be used in lieu of the three stacked 
ring-like sections 78a, 78b, and 78c. However, the use of stacked 
ring-like sections is preferred due to the difficulty of fabricating and 
machining these metals. To effectively avoid radiation streaming at the 
junctions between the three sections, overlapping tongue and groove joints 
79 (see FIG. 4A) are provided at each junction . By contrast, in shield 
insert 23, a layer of poured lead 80 is used as the shielding material 76. 
While lead is not as effective a gamma shield as depleted uranium, it is a 
better material to use in connection with high-neutron emitting materials, 
such as the transuranic elements. Such high neutron emitters can induce 
secondary neutron emission when depleted uranium is used as a shielding 
material. While such a secondary neutron emission is not a problem with 
tungsten, this metal is far more difficult and expensive to fabricate than 
lead, and is only marginally better as a gamma-absorber. Therefore, lead 
is a preferred shielding material when high-neutron emitting materials are 
to be transported. It should be noted that the radius of the interior of 
the shield inserts 22 and 23 will be custom dimensioned with a particular 
type of waste to be transported so that the inner wall 74 of the insert 
comes as close as possible into contact with the radioactive material 
contained therein. The Applicant has noted that fulfillment of the 
foregoing criteria provides the most effective shielding configuration per 
weight of shielding material. Additionally, the thickness and type of 
shielding material 76 will be adjusted in accordance with the activity of 
the material contained within the shield insert 22, 23 so that the surface 
radiation of the cask assembly 1 never exceeds 200 mr. The fulfillment of 
these two criteria maximizes the capacity of the cask assembly 1 to carry 
radioactive materials while simultaneously minimizing the weight of the 
cask. 
FIGS. 4A and 4B illustrate the vent, purge, and drain assembly 90 of the 
outer container 18. This assembly 90 includes a threaded drain pipe 92 for 
receiving a drain plug 94. The inner end 96 of the drain plug 94 is 
conically shaped and seatable in sealing engagement with a complementary 
valve seat 97 located at the inner end of the pipe 92. Wrench flats 98 
integrally formed at the outer end of the drain plug 94 allow the plug 94 
to be easily grasped and rotated into or out of sealing engagement with 
the valve seat 97. A vent pipe 100 is obliquely disposed in fluid 
communication with the end of the drain pipe 92. A threaded vent plug 102 
is engageable into and out of the vent pipe 100. A screw head 103 is 
provided at the outer end of the vent plug 102 to facilitate the removal 
or insertion of the threaded plug 102 into the threaded interior of the 
vent pipe 100. A drain tube 104 is fluidly connected at its upper end to 
the bottom of the valve seat 97 by way of a fitting 106. In the preferred 
embodiment, the drain tube 104 is formed from stainless steel, and is 
housed in a side groove 108 provided along the inner surface of the wall 
56 of the outer container 18. As is most easily seen in FIG. 4B, the lower 
open end 109 of the drain tube 104 is disposed in a bottom groove 110 
which extends through the shallowly conical floor 112 of the outer 
container 18. 
In operation, the vent, purge, and drain assembly may be used to vent the 
interior of the outer container 18 by removing the vent plug 102 from the 
vent pipe 100, screwing an appropriate fitting (not shown) into the 
threaded vent pipe 100 in order to channel gases to a mass spectrometer, 
and simply screwing the conical end 96 of the drain plug 94 out of sealing 
engagement with the valve seat 97. If drainage is desired, the drain plug 
94 is again removed. A suction pump is connected to the drain pipe 92 in 
order to pull out, via drain tube 104, any liquids which may have 
collected in the bottom groove 110 of the conical floor 112 of the outer 
container 18. Gas purging is preferably accomplished after draining by 
removing the vent plug 102, and connecting a source of inert gas to the 
drain pipe 92. The partial vacuum within the container 18 that was created 
by the suction pump encourages inert gas to flow down through the drain 
tube 104. Although not specifically shown, the interior of the drain plug 
98 may be provided with one or more rupture discs to provide for emergency 
pressure relief in the event that the cask assembly 1 is exposed to a 
source of intense thermal radiation, such as a fire, over a protracted 
period of time. 
The closures 13 used in connection with the cask 1 may be either screw-type 
double-lidded closures 115a, 115b (illustrated in FIG. 3), or breech-lock 
double-lidded closures 117a, 117b (illustrated in FIG. 5). 
With reference now to FIG. 3, each of the screw-type closures 115a, 115b 
includes an outer lid 120a, 120b, and an inner lid 122a, 122b. The inner 
lid 122a, 122b in turn includes an outer edge 124a, 124b which is seatable 
over a ledge 126a, 126b provided around the opening 128a, 128b of the 
shield insert 22 or the outer container 18 respectively. A gasket 130a, 
130b circumscribes the outer edge 124a, 124b of each of the inner lids 
122a, 122b of the two closures 115a, 115b. In the preferred embodiment, 
these gaskets 130a, 130b are formed of Viton because of its excellent 
sealing characteristics and relatively high temperature limit (392.degree. 
F.) compared to other elastomers. The gasket 130a, 130b of each of the 
inner lids 122a and 122b is preferably received and held within an annular 
recess (not shown) that circumscribes the outer edge 124a, 124b of each 
lid. Each of these gaskets 130a, 130b is capable of effecting a 
fluid-tight 360. seal between the outer edge 124a, 124b of each of the 
inner lids 122a, 122b and the ledges 126a, 126b. To facilitate the 
insertion of shield insert 22 into the container 18, it is important to 
note that the opening 128b of the container 18 is at least as wide as the 
interior of the container 18 at all points. 
Each of the outer lids 120a, 120b of the screwtype closures 115a, 115b 
includes a threaded outer edge 134a, 134b which is engageable within a 
threaded inner edge 136a, 136b that circumscribes the openings 128a, 128b 
of the shield insert 22 and the outer container 18 respectively. Swivel 
hooks 137a, 137b (indicated in phantom) may be detachably mounted to the 
centers of the outer lids 120a, 120b to facilitate the closure operation. 
Finally, both of the outer lids 120a, 120b of the screw-type closures 
115a, 115b includes a plurality of sealing bolts 138a-h, 139a-h, 
threadedly engaged in bores extending all the way through the outer lids 
120a, 120b for a purpose which will become apparent shortly. 
To seal the cask assembly 1, inner lid 122a is lowered over ledge 126a of 
the shield insert 22 so that the gasket 130 is disposed between the outer 
edge 124a of the inner lid 122a and ledge 126a. The detachably mountable 
swivel hook 137 is mounted onto the center of the outer lid 120a. The 
outer lid 120a is then hoisted over the threaded inner edge 136a of the 
shield insert 22. The threaded outer edge 134a of the outer lid 120a is 
then screwed into the threaded inner edge 136a to the maximum extent 
possible. The axial length of the screw threads 134a and 136a are 
dimensioned so that, after the outer lid 120a is screwed into the opening 
128a to the maximum extent possible, a gap will exist between the inner 
surface of the outer lid 120a and the outer surface of the inner lid 122a. 
Once this has been accomplished, the securing bolts 138a-h are each 
screwed completely through their respective bores in the outer lid 120a so 
that they come into engagement with the inner lid 122a, thereby pressing 
the gasket 130a and into sealing engagement between the ledge 126a and the 
outer edge 124a of the lid 122a. The particulars of this last step will 
become more apparent with the description of the operation of the 
breech-lock double-lidded closures 117a, 117b described hereinafter. To 
complete the closure of the cask assembly 1, the outer screw-type closure 
115b is mounted over the opening 128b of the outer container 18 in 
precisely the same fashion as described with respect to the opening 128a 
of the shield insert 22. 
With reference now to FIGS. 5, 6A, and 6B, the breech-lock double-lidded 
closure 117a, 117b also includes a pair of outer lids 140a, 140b which 
overlie a pair of inner lids 142a, 142b respectively. Each of the inner 
lids 142a, 142b likewise includes an outer edge 144a, 144b which seats 
over a ledge 146a, 146b that circumscribes the opening 148a, 148b of the 
shielding insert 23 and outer container 18, respectively. Each of the 
outer edges 144a, 144b is circumscribed by a gasket 150a, 150b for 
effecting a seal between the edges 144a, 144b and their respective ledges 
146a, 146b. Like opening 128b, opening 148b is at least as wide as the 
interior of the outer container 18. 
Thus far, the structure of the breech-lock double-lidded closures 117a, 
117b has been essentially identical with the previously described 
structure of the screw-type double-lidded closures 115a, 115b. However, in 
lieu of the previously described screw threads 134a, 134b, the outer edges 
154a, 154b of each of the outer lids 140a, 140b are circumscribed by a 
plurality of uniformly spaced arcuate notches 156a, 156b which define a 
plurality of arcuate flanges 158a, 158b. Similarly, the inner edges 160a, 
160b which circumscribe each of the openings 148a, 148b of the shield 
insert 23 and outer container 18, respectively, include notches 162a, 162b 
which also define arcuate flanges 164a, 164b. The flanges 158a, 158b which 
circumscribe each of the outer lids 140a, 140b are dimensioned so that 
they are insertable through the arcuate notches 162a, 162b which 
circumscribe the inner edges 160a, 160b of the shield insert 23 and the 
outer container 18. As may best been seen in FIG. 6A and 6C, such 
dimensioning allows the flanges 164a, 164b of each of the outer lids 140a, 
140b, to be inserted through the notches 162a, 162b of each of the 
openings 148a, 148b and rotated a few degrees to a securely locked 
position wherein the arcuate flanges 158a, 158b of the outer lids 140a, 
140b are overlapped and captured by the arcuate flanges 164a, 164b that 
circumscribe the inner edges 160a, 160b. It should be further noted that 
the axial length L1 (illustrated in FIG. 6B) of the interlocking flanges 
158a, 158b and 164a, 164b is sufficiently short to leave a small gap L2 
between the inner surface of the outer lids 140a, 140b and the outer 
surface of the inner lids 142a, 142b. The provision of such a small 
distance L2 between the outer and inner lids allows the outer lids 140a, 
140b to be rotated a few degrees into interlocking relationship with their 
respective notched inner edges 160a, 160b without transmitting any rotary 
motion to the inner lids 142a, 142b which could cause the inner lid 
gaskets 150a, 150b to scrape or wipe across their respective ledges 146a, 
146b. 
Connected around the outer edges of the outer lids 140a, 140b are three 
suspension pin assemblies 166a, 166b, and 166c and 167a, 167b, and 167c 
(not shown) respectively. Each of these suspension pin assemblies 166a, 
166b, 166c and 167a, 167b, 167c are uniformly spaced 120.degree. apart on 
the edges of their respective outer lids 140a, 140b. As the structure of 
each suspension pin assembly is the same, only a suspension pin assembly 
166a will be described. 
With reference now to FIG. 6C, suspension pin assembly 166a includes a 
suspension pin 168 which is slideably movable along an annular groove 170 
provided around the circumference of each of the inner lids 142a, 142b. A 
simple straight-leg bracket 172 connects the suspension pin 168 to the 
bottom edge of its respective outer lid. 
In operation, the suspension pin assemblies 166a, 166b, 166c and 167a, 
167b, 167c, serve two functions. First, the three suspension pin 
assemblies attached around the edges of the two outer lids 140a and 140b 
mechanically connect and thus unitize the inner and outer lids of each of 
the breech-lock closures 117a, 117b so that both the inner and the outer 
lids of each of the closures 177a and 117b may be conveniently lifted and 
lowered over its respective opening 148a, 148b in a single convenient 
operation. Secondly, the pin-and-groove interconnection between the inner 
and the outer lids of each of the two breech-lock type closures 117a and 
117b allows the outer lids 140a and 140b to be rotated the extent 
necessary to secure them to the notched outer edges 160a, 160b of their 
respective containers without imparting any significant amount of torque 
to their respective inner lids 142a, 142b. This advantageous mechanical 
action in turn prevents the gaskets 150a and 150b from being wiped or 
otherwise scraped across their respective ledges 146a, 164b. In the 
preferred embodiment, the width of the groove 170 is deliberately made to 
be substantially larger than the width of the pin 168 so that the pin 168 
may avoid any contact with the groove 170 when the outer lids 140a, 140b 
are rotated into interlocking relationship with their respective 
containers 23 and 18. 
With reference again to FIG. 6A and 6C, each of the outer lids 140a, 140b 
includes eight sealing bolts 174a-h, 174.1a-h equidistantly disposed 
around its circumference. Each of these sealing bolts 174a-h, 1741a-h is 
receivable within a bore 175 best seen in FIG. 6C. 
Each of these bores 175 includes a bottom-threaded portion 176 which is 
engageable with the threads 176.1 of its respective bolt 174a-h, 174.1a-h 
as well as a centrally disposed, non-threaded housing portion 177. At its 
upper portion the bore 175 includes an annular retaining shoulder 178 
which closely circumscribes the shank 179 of its respective bolt 174a-h, 
1741a-h. The retaining shoulder 178 insures that none of the sealing bolts 
174a-h, 174.1a-h will inadvertently fall out of its respective bore 175 in 
the outer lid 140a, 140b. In operation, each of the sealing bolts 174a-h 
is screwed upwardly into its respective bore 175 until its distal end 
179.1 is recessed within the threaded portion 176 of the bore 175. After 
the outer lid 140a or 140b has been secured into the notched inner edge 
160a or 160b of its respective container 23 or 18, the sealing bolts 
174a-h are screwed down into the position illustrated in FIG. 6C until 
their distal ends 179.1 forcefully apply a downward-direction force around 
the outer edges 144a, 144b of their respective inner lids 142a, 142b. Such 
a force presses the gaskets 150a and 150b into sealing engagement against 
their respective ledges 146a, 146b. It should be noted that the same bolt 
and bore configuration as heretofore described is utilized in the 
screw-type double-lidded closures 115a, 115b. 
To insure that the outer lids 140a and 140b will not become inadvertently 
rotated out of locking engagement with their respective vessels 23 or 18, 
a locking bracket 180 is provided in the position illustrated in FIG. 6A 
and 6B in each of the outer lids 140a, 140b after they are rotated shut. 
Each locking bracket 180 includes a lock leg 182 which is slid through 
mutually registering notches 156a, 156b, and 162a, 162b after the outer 
lids 140a and 140b have been rotated into locking engagement with the 
inner edges 160a, 160b of either the shielding insert 23 or the outer 
container 18. In the case of outer lid 140b, the mounting leg 184 is 
secured by means of locking nuts 186a, 186b. In the case of outer lid 
140a, the mounting leg 184 is captured in place by inner lid 142b which 
abuts against it. Although not specifically shown in any of the drawings, 
each of the outer lids 120a, 120 b of the screw-type double-lidded 
closures 115a, 115b is similarly secured. However, instead of a locking 
bracket 180, a locking screw (not shown) is screwed down through the outer 
edges of each of the outer lids 120a, 120b and into a recess precut in 
each of the inner lids 122a, 122b.