Patent Number: 048250880
Section: description

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 lightweight cask assembly 1 having structural walls formed from a titanium alloy 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 toroidal 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 toroidal 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. Support ring assembly 8b is formed from a titanium alloy and is part of the end plate assembly 15 discussed hereinafter. 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 turnbuckle assemblies 12a, 12b, and 12c, 12d (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 formed from a titanium alloy (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 having 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 titanium, 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 titanium. 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 on 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 radiative. 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, both the outer wall 54 and inner wall 56 is formed from a high strength alloy of titantium, such as Ti-3-Al-2.5-V, or Ti-6-Al-4-V. Such a titanium alloy is approximately three to four times stronger than most stainless steel on a pound-per-pound basis. Moreover, because titanium is about half the density of most stainless steels, this titanium alloy is about 75% to 100% stronger than most stainless steel on a volume-per-volume basis. The end result is that both the outer wall 54 and the inner wall 56 may be made substantially thinner with a material only about one-half as dense as stainless or low alloy steel. Hence the savings in weight are manifest. While other high strength alloys of titanium may be used, Ti-3-Al-2.5-V is preferred for its easy weldability. Disposed between the outer wall 54 and the inner wall 56 is a layer of Boro-Silicone, which is a shielding material formed from particles of boron suspended in a matrix of silicone. This material advantageously absorbs neutrons from neutron-emitting radioactive materials (such as transuranic 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 it 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, a layer of center shielding material 64, the previously mentioned support ring assembly 8b and a lower reinforcing ring 65. In the preferred embodiment, the outer plate 60 is again formed from a titanium alloy such as Ti-3-Al-2.5-V approximately one-eighth inch thick. The inner plate 62, like the inner wall 56, is again formed from Ti-3-Al-2.5-V approximately one inch 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 titanium alloy inner plate 62 is joined around the bottom edge of the inner wall 56 360 degrees via weld joint 66. The top of the outer container 18 includes a reinforcing ring 68 again made of Ti-3-Al-2.5-V. This ring 68 is preferably about two 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-half inch thick and a cylindrical inner wall 74 which is approximately one-eighth of Ti-3-Al-2.5-V. 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 the 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. In both of the shield inserts 22, 23, the bottom edges of the inner and outer walls 72, 74 are welded around a bottom plate 82, while the upper edges of these walls are both welded around an insert reinforcing ring 89. Both bottom plate 82 and ring 89 are formed from Ti-3-Al-2.5V. It should be noted that in the preferred embodiment, the use of a high strength titanium alloy such as Ti-3-Al-2.5V allows the inner wall 74 of each of the shield inserts 22 and 23 to be much thinner than if this wall were made of steel and yet still comply with the U.S. government impact stress criteria. Such thin inner wall minimizes the distance between the shielding material 76 and the radioactive waste disposed inside the insert 22, 23, which in turn minimizes the weight of the shielding material 76 required to meet U.S. government surface radiation requirements. 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 applicants have 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 all these criteria maximizes the capacity of the cask assembly 1 to carry radioactive materials while simultaneously minimizing the weight of the cask. The use of titanium alloy in the outer and inner walls 54, 56, 72, 74 of both the outer container 18 and shield inserts 22, 23 has the further advantage of enhancing the overall thermal conductivity of the cask assembly. Despite the fact that the heat conductivity of titanium is only about half as great as the heat conductivity of conventional structural materials such as 304 stainless steel, the fact that the walls 54, 56, 72 and 74 may be made so much thinner as a result of the higher strength of titanium more than offsets the difference in thermal conductivity. The end result is that the use of titanium not only results in a lighter-weight cask, but a safer cask capable of more effectively dissipating the heat of decay of the radioactive materials contained therein, hence insuring that this heat will not create unwanted pressures with the cask assembly 1. 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 94. 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 and insertion of the threaded plug 102 into the threaded interior of the vent pipe 100. A drain tube 104 is fluidly connected at is 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 closure 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 the 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 degrees 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 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 screw-type 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 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 ledge 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 136a of the shield insert 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, 140 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 circumscribe the inner edges 160a, 160b of the shield insert 23 and the outer container 18. As may best be 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, 167c are uniformly spaced 120 degrees 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 ledges 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, 146b. 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, 174.1a-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, 174.1a-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 is 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.