Patent Number: 
Section: description

FIG. 1 shows an improved 30B cylinder 10 constructed in accordance with the present invention. The cylinder 10 is shown inside the bottom half of a protective shipping package or xe2x80x9coverpackxe2x80x9d 12. The overpack 12 is shown supported in a cradle 8 and with its top half removed and its safety straps open. As is well understood in the art, during shipment to cylinder 10 is filled with up to 5,020 pounds of substantially pure uranium hexafluoride and fully enclosed in the overpack, as shown in FIG. 1A. For the most part the improved 30B cylinder 10 of the present invention is entirely conventional and will be described in detail only in so far as it differs from the prior art conventional cylinder. The conventional 30B cylinder 10 is manufactured according to ANSI N14.1 and ASME Boiler and Pressure Vessel Code, Section VIII, Division 1. Accordingly the conventional 30B cylinder is 81xc2xd inches plus or minus xc2xd inch and has a diameter of 30 inches plus or minus xc2xc inch. The conventional 30B cylinder has a minimum volume of 26 cubic feet. It is preferred that the cylinder be manufactured according to ANSI N14.1-2000 and therefore include the advantages described in U.S. Pat. No. 5,777,343 which stem from the elimination of a weld backing bar. However, the advantages of the present invention may also be obtained with cylinders manufactured to earlier versions of ANSI N14.1 which required weld backing bars. The improved 30B cylinder 10 includes a valve which is protected by a valve protection cover assembly 14 (FIGS. 1 and 2). This cover assembly, not found in conventional 30B cylinders, provides a second barrier to the egress of uranium hexafluoride or, more critically, the ingress of water. The valve protection cover assembly 14 fits within the chime 15 which extends from the domed end or head of the cylinder 10. More particularly, the distal end of the valve protection cover assembly 14 is recessed at least xc2xd inch and preferably 0.75 inches or more from the plane defined by the free edge of the chime. This space allows for deformation of the overpack during the drop test without any contact with the valve protection cover assembly 14. Therefore the cylinder 10 fitted with the cover assembly 14 may be used with standard overpacks such as the overpack 12 shown in FIGS. 1 and 1A. It should be noted that the axial length of the chime 15 is not fixed by ANSI N14.1, but the overall length, the diameter, and the minimum capacity for the cylinder are fixed. The diameter and length are critical dimensions to ensure that a tank fits in a conventional overpack. Until applicants"" invention it had not been recognized that lengthening one chime 15 and shortening the other (unnumbered) to allow a xc2xd to xc2xe or greater inch clearance as discussed above would allow a valve protection cover assembly to survive a 30 foot drop test undamaged, indeed untouched, by the deformation of the overpack, this despite the improved safety and likely resulting reduction in transportation index. The valve protection cover assembly 14 (FIG. 2) includes a cap 16 that is held in place by six bolts 18. Two of the bolts 18 are safety wired, and the wire is sealed to guarantee that the cap 16 has not been tampered with once it is bolted in place. Additional bolts, up to all six, could be safety wired if desired. The valve protection cover assembly 14, as shown in greater detail in FIG. 4, includes a cap 16 and a base 20. The base 20 is an annular disk that surrounds the valve 30. The base 20 is a disk that is welded to the wall 22 of the cylinder 10. Its diameter and thickness are selected so as not to interfere with the standard industry plumbing used to connect with the valve 30 to fill or empty the cylinder 10 of uranium hexafluoride. The base 20 is welded to the wall 22 continuously around its outer and inner-perimeters, and these welds are thoroughly inspected to guarantee their integrity. These welds therefore provide a reliable barrier to prevent any matter from passing under the base 20 and so passing from the outside of the cylinder 10 into the volume where the cap assembly surrounds the valve 30 or vice versa. The base 20 also includes six evenly spaced threaded bores (not shown) with which the bolts 18 cooperate to hold the cap 16 in place. An upper surface 24 of the base 20 includes two regions, an inner region 28 and an outer region 30. The inner region 28 is annular and stands proud of the outer region by about {fraction (1/32)} inches. The inner region 28 is machined flat and provides a working surface against which the cap 16 seals. The necessary surface flatness may be achieved by machining the base 20 either before or after welding the base 20 to the wall 22. The cap 16 is a fabricated steel component which includes a dome 40 and a flange 42. While cap 16 could be machined from a single piece of steel, it is preferred for economy and ease of manufacture to fabricate it from two pieces which are welded together as shown. This weld is thoroughly inspected to guarantee its integrity. The flange 42 mates with the base 20. To this end the flange 42 includes a machined annular surface 44 which seats against the corresponding flat inner surface 28 of the base 20. A pair of O-rings 46 and 48 fit in recesses 50 and 52, respectively, which are formed in the annular surface 44 of the flange 42. The recesses 50 and 52 are circular in plan view, but any endless shape could be used if desired. The recesses 50 and 52 may be formed with a slight undercut as shown in order to retain the O-rings 46 and 48 in place. When the annular surface 44 and the annular surface 28 are seated against each other, the O-rings 46 and 48 are compressed to form an effective seal. This seal is sufficiently complete to achieve a leak rate of less than 10xe2x88x923 ref.-cm3/sec, when tested according, for example, to the soap bubble test described in A.5.7 of ANSI N14.5-1997, Leakage Tests on Packaging for Shipment. Under this test a xe2x80x9creference cubic centimeter cubed per secondxe2x80x9d is defined as a volume of one cubic centimeter of dry air per second at one atmosphere absolute pressure and 25xc2x0 C. A seal which has the above leak rate or less is considered essentially impermeable for purposes of this application. While conventional O-rings 46 and 48 are preferred for ease of manufacture, other resilient sealing elements including cast-in-place rubbers or resilient polymers such as urethane are also possible. Such alternative materials and manufacturing techniques need only provide a sufficiently leak resistant seal to be satisfactory, and they are included within the meaning of the term xe2x80x9cresilient seal elementsxe2x80x9d used in this application. The flange 42 includes an annular outer region 58, recessed from the plane of annular surface 44. The outer region 58 is aligned with the outer region 30 of the base 20 . The two outer regions 30 and 58 define a gap 60 between them when the cap 16 is in place on the base 20. The flange 42 has six holes (not shown) through the outer region 58 for the bolts 18. These holes aligned with corresponding threaded passages in the base 20. When the cap 16 is put in place and the bolts 18 tightened to a predetermined torque, the outer region 58 of the flange 42 is stressed, assuring a predetermined, constant load on the O-rings 46 and 48 and the mating annular surfaces 24 and 44. While forming the gap 60 is preferred because it allows the flange 42 to flex slightly, any design that allows a sufficiently tight seal between the base 20 and the cap 16 is acceptable. The valve protection cover assembly 14 includes a means for testing the integrity of the seal between the cap 16 and the base 20. This test facility includes a test port 60, which leads through internal passages 62, 64, and 66 to test channel 68. The test channel 68 is a semicircular recess (in vertical cross-section) in the annular surface 44 of the flange 42. The recess 68 extends in a complete circle spaced between the recesses 50 and 52. The flange 42 includes a bore 70 (FIGS. 1 and 4) diametrically opposite the test port 60. This bore cooperates with a pin 72 which projects up from the outer region 28 of the base 20. When the cylinder 10 is in its normal, horizontal position, the pin 72 is at the 12 o""clock position and helps the worker accurately position the cap and place the bolts 18 in their holes. Once the cap 16 is in place and the bolts 18 tightened appropriately, the integrity of the seal around about may be tested. This is done by connecting the test port to a calibrated source of fluid under pressure or vacuum. The fluid reaches the test channel 68, and if the joint is secure, the fluid can go no farther. If a leak occurs, then the test equipment shows a drop in pressure or vacuum, and the O-ring seals can be inspected and replaced or other repairs made as necessary. Once the testing is complete, a plug 70 is used to seal off the test port. There are a variety of test procedures available, and these are set out in ANSI N14.5-1977. These tests assure leakage rate equal to or less than 1xc3x9710xe2x88x923 ref-cm3/sec. Although the testing facility is shown as a port, passages, and channel machined in the flange 42 of the cap 16, it is also possible to machine these elements into the base 20. If this is done, the test channel is formed in the surface 28 of the base 20 so that it is located between the places where the O-rings contact the base 20 and is connected to a test port by suitable passages. Similarly, the O-rings 46 and 48 could be mounted in grooves formed in the base. However, the construction shown in the Figures is preferred because it is easier to maintain and because the O-rings 46 and 48 and the test channel 68 are less likely to be damaged when connecting conduits the valve 30. While the bolts 18 are used to draw the cap 16 tight against the base 20, other fastenings are possible. For example a threaded connection between the base could be used with the necessary O-ring seals and test port channel formed in a screw-on cap. Alternatively, the base 20 could have external threats on its outer peripheral surface and a nut like that used in a plumber""s union could be used to pull the cap down against the base. Thus it is clear that the present invention provides a vessel 10 for the shipment of uranium hexafluoride includes a cylindrical wall closed by pair of approximately semi-ellipsoidal heads 22 welded to form a sealed container. A service valve 30 is located in one end. The valve 30 is covered by a removable, watertight valve protection cover assembly 14. The vessel also includes a test port 60 by means of which the integrity of the valve protection cover assembly may be tested after the cylinder 10 has been filled with uranium hexafluoride and the valve protection assembly 14 has been installed. The valve protection assembly 14 is shaped so that it fits within the envelope of the standard 30B cylinders, and so fits within the overpacks already approved by the NRC and owned by shippers of uranium hexafluoride. The vessel 10 made according to the present invention has a double barrier to prevent ingress of water or egress of uranium hexafluoride. The valve 30, a first barrier, is enclosed by a cover assembly 14 which forms the second barrier. The double barrier is expected to permit the transportation index of 0. In effect, then, adding the second barrier will allow the improved 30B cylinders to be shipped in bulk with safety acceptable to the NRC, resulting in substantial savings to the industry.