Abstract:
Substantially pure uranium hexafluoride is shipped in a conventional overpack. A cylinder includes a vessel with a cylindrical sidewall A head permanently affixed to the sidewall closes one end of the vessel. A valve controls the flow of matter into and out of the vessel. A sealing surface connected to the vessel surrounds the valve. A cap covers the valve and a pair of seals is located between the sealing surface and the cap. The volume between the two seals, the cap and the sealing surface defines a test volume. A test port connects the test volume and an exterior surface of the vessel, and fasteners press the cap against the sealing surface. A maximum rate of leakage from within the cap to the atmosphere outside the cap is determined by measuring the leakage rate into the test volume with a leak testing apparatus connected to the test port. Thereafter the cylinder is placed in a conventional overpack. The maximum leakage rate is determined using a test apparatus consisting essentially of a vacuum pump, a pressure gauge, a container of known volume, a valve to control the flow of air from the atmosphere into the container which are all adapted to be connected to the test port at the same time, a valve controlling the flow of gas from the container into the test volume, and a valve controlling the flow of gas from the test volume to the vacuum pump.

Description:
RELATED APPLICATIONS 
     This application is a continuation in part of U.S. Ser. No. 09/840,314, filed Apr. 23, 2001, now U.S. Pat. No. 6,534,776 and titled VESSEL FOR URANIUM HEXAFLUORIDE TRANSPORT. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a vessel for the transportation and storage of substantially pure uranium hexafluoride, to improvements in a vessel for transporting substantially pure uranium hexafluoride, for example a vessel known in the trade as a 30B cylinder, and to a method and apparatus for using such an improved cylinder. 
     Enriched uranium hexafluoride has been shipped in conventional 30B cylinders for many years. Uranium hexafluoride is considered enriched if it includes more than 1% Uranium 235 (U 235 ), and shipments of enriched uranium hexafluoride (up to 5%) must be made in conventional, approved 30B cylinders. If the cylinders are full, they must be shipped in an approved overpack for impact and thermal protection. Such shipments are considered safe if the cylinders are properly packaged and transported. So long as water or other possible moderators of neutrons are kept separate from the uranium hexafluoride itself, a critical event (an uncontrolled nuclear chain reaction) cannot occur. 
     As with all aspects of the nuclear industry within the geographic limits of its authority, the Nuclear Regulatory Commission (NRC) regulates the transport of uranium hexafluoride. Because its authority extends to United States ports and because its regulations are among the most conservative in the world, the NRC&#39;s regulations establish minimum standards for most international shipping of uranium hexafluoride. American National Standards Institute, Inc. published ANSI N14.1, Packaging of Uranium Hexafluoride for Transport, in 1971. This standard was adopted by the NRC&#39;s predecessor and established the approved design of the conventional 30B cylinder. Uranium Hexafluoride for Transport, in 1971. This standard was adopted by the NRC&#39;s predecessor and established the approved design of the conventional 30B cylinder. 
     The conventional 30B cylinder, currently defined by ANSI N14.1-1995, is a steel vessel about 81½ inches long and 30 inches in diameter. It is made from half-inch carbon steel formed into a cylindrical body 54 inches long capped by two roughly semi-ellipsoidal heads. A pair of chimes protects the ends of the vessel. The conventional 30B cylinder has a tare weight of about 1425 lbs. and a volume of about 26 cubic feet. When filled to its maximum permitted capacity of 5020 lbs. with uranium hexafluoride having up to approximately five percent by weight uranium 235 isotope, as little as 15 liters of water could conceivably initiate a critical event. It is therefore vitally important that water be excluded from the cylinder. 
     There are other risks associated with the shipment of uranium hexafluoride. If this chemical is heated to its triple point of 146° F. in the presence of air, gaseous hydrogen fluoride (HF (g) ) can be formed. Such an event is conceivable if the valve on a conventional 30B cylinder breaks during a fire event. Hydrogen fluoride gas is extremely harmful, and its release must be guarded against since death follows almost immediately if it is inhaled. 
     Two openings are formed in the conventional 30B cylinder. The openings are located at approximately diagonally opposite locations on opposite heads. One opening accommodates a valve which is used routinely for filling and emptying the tank of uranium hexafluoride. The other opening is a plug used for periodic inspection, hydrostatic testing, and cleaning of the tank. This valve and this plug form the only barriers to water entry into the conventional 30B cylinder. 
     During shipment a 30B cylinder is housed in a protective shipping package or “overpack.” The overpack protects the cylinder within from accidental impacts and insulates the cylinder to reduce the chance that it will leak if there is a fire or other accidental overheating event. The overpack and 30B cylinder are routinely shipped by ocean-going vessels as well as by rail and road transport. When the cylinder arrives at a processing plant, it is removed from the overpack and standardized piping is connected to the valve. The 30B is then heated in an autoclave to evaporate and so remove the uranium hexafluoride. 
     Overpacks are regulated by governmental agencies. The U.S. Department of Transportation (DOT) has issued a standard specification, DOT 21 PF1, which defines an overpack. That regulation is published at 49 CFR 178.358. The Department of Transportation allows certain variations of this design in Certificate USA/4909/AF, Revision 15. Overpacks made to this specification or its permitted variations are termed “specification packages”. In addition, the NRC has issued regulations which define so-called “performance packages”. These packages are approved by the NRC if they meet the performance standards set forth in the regulations. The performance specifications are published at 49 CFR 173.401-476. One common feature of both the DOT and the NRC regulations is that the overpack must be designed to fit a conventional 30B cylinder as defined by ANSI N14.1 
     Regulations require periodic testing of 30B cylinders. Specifically, the DOT has adopted ANSI N14.1 which in turn requires periodic testing of 30B cylinders. This testing includes a hydrostatic test every five years. Before this test, the cylinder is cleaned. Then it is filled with water and pressurized to inspect for possible leaks. This test checks the integrity of the structure including the various welds. This test is expensive, in part because it creates 26 cubic feet of radioactive waste water which must be disposed as low-level radioactive waste. 
     Further, the NRC regulates how densely conventional 30B cylinders in overpacks may be packed on cargo ships or other conveyances. It does this by allowing each ship or conveyance a total “transportation index” of 200. Each conventional 30B cylinder has a transportation index of five, so a ship carrying no other nuclear cargo can carry a total of forty (40) conventional 30B cylinders. (200÷5=40.) This safety limit denies shippers of conventional 30B cylinders the economy that volume shipments could achieve especially in light of the availability of dedicated charter vessels for radioactive materials. However, this regulation is necessary because even though the hydrostatic test assures structural integrity and the overpack provides thermal and impact protection, there is no sure way to guarantee that the valve will remain watertight using the current 30B design. As noted above, even a small amount of water could conceivably initiate a critical event. 
     It would be a substantial improvement if a cylinder could be devised that did not require hydrostatic testing and which could guarantee the integrity of its valve. Any improvement to the conventional 30B cylinder must recognize the substantial investment in equipment which is used to handle the existing 30B cylinders, including both the piping and the existing overpacks. This requires that the essential dimensions of the cylinder and the locations of the valve and plug not change. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a vessel for the shipment of uranium hexafluoride includes a cylindrical wall closed by pair of approximately semi-ellipsoidal ends welded to form a sealed container. A service valve is located in one end. The valve is covered by a removable, watertight valve protection cover assembly. The vessel also includes a test port by means of which the integrity of the valve protection cover assembly may be tested after the cylinder has been filled with uranium hexafluoride and the valve protection assembly has been installed. The valve protection assembly 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 made according to the present invention has a double barrier to prevent ingress of water or egress of uranium hex fluoride. The valve, a first barrier, is enclosed by a cover assembly which forms the second barrier. The double barrier has resulted in a transportation index of 0. In effect, then, adding the second barrier allows the improved 30B cylinders to be shipped in bulk with safety acceptable to the NRC, resulting in substantial savings to the industry. 
     The second barrier is a valve protection cover assembly. One important aspect of using such a valve protection cover assembly is some way to test that it does not allow water into its interior space. The valve cover includes a waterproof domed cavity surrounded by a flange that is bolted against a mating flange on the cylinder. The walls are inherently totally water proof; the only place for water to leak in is between the flanges. To seal the flanges against water, a pair of concentric, resilient seals is provided between the flanges. To assure that the flanges are correctly sealed, a test port is located between the seals. The present invention provides a method and apparatus, which when used in conjunction with an appropriate vessel makes it possible to ship radioactive materials with the assurance that water will not leak between the flanges. This is accomplished using an extremely sensitive vacuum based leak detection system which can test for leakage around the seals. Once the seals have passed the leak test, the test port can be sealed with a suitable plug. This test method and apparatus are described for use with the improved 30B cylinder described herein. However it may also be used with other cylinders for the transport of substantially pure uranium hexafluoride such as 48 inch diameter cylinders. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an improved 30B cylinder constructed according to the present invention and held in an open protective shipping package or “overpack” which in turn rests in a cradle; 
     FIG. 1A shows an overpack for a 30B cylinder fully closed and in a cradle; 
     FIG. 2 is an end view of the cylinder of FIG. 1; 
     FIG. 3 is a view looking in the direction of arrows  3 — 3  FIG.  2  and partially in cross section; 
     FIG. 4 is an enlarged view of a portion of FIG. 3 showing a valve protection assembly over the valve; and 
     FIG. 5 is a schematic illustration of an apparatus used to test the seal between the valve protection assembly and a flange on the improved 30B cylinder. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     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 “overpack”  12 . The over pack  12  is shown supported in a cradle  13  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 uranium hexafluoride and fully enclosed in the overpack, as shown in FIG.  1 A. 
     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 81½ inches plus or minus ½ inch and has a diameter of 30 inches plus or minus ¼ inch. The conventional 30B cylinder has a volume of at least 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 of the cylinder  10 . 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. 
     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 10 −3  ref.-cm3/sec, when tested according, for example, to the soap bubble test described in A.5.7 of ANSI N 14.5-1997, Leakage Tests on Packaging for Shipment. Under this test a “reference cubic centimeter cubed per second” is defined as a volume of one cubic centimeter of dry air per second at one atmosphere absolute pressure and 25° C. 
     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. In addition, any of the seals may be lubricated with a conventional lubricant to provide a better seal. 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 “resilient seal elements” 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 align 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  61 , 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  61 . 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&#39;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 of less than 1×10 −3  ref-cm 3 3/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&#39;s union could be used to pull the cap down against the base. 
     FIG. 5 is a schematic diagram of a test apparatus  78  constructed in accordance with the teachings of the present invention. The apparatus is shown connected to the test port  61  in the flange with the valve cover assembly  14  fastened to the machined surface  44  on the cylinder  10 . The apparatus  78  can be used to prove a seal up to 1×10 −5  ref. cm 3 /sec. The apparatus  78  includes an exhaust valve  80 , a reference volume  82 , a reference volume valve  84 , a vacuum pump valve  86 , a vacuum pump  88 , and a pressure sensor  90  which includes a pressure display  92 , all connected by various conduits and X fitting  96  to the test port  61 . 
     The first step is to determine the volume enclosed by valve  84 , valve  86 , the conduits  94 ,  102  and  103  and the volume between the seals  50  and  52 . This is termed the Test Volume, V test . To this end the reference volume  62  is filled with air at atmospheric pressure by closing the valve  84  and opening valve  80 . Thereafter exhaust valve  80  is closed, and the atmospheric pressure, P atmo ., is recorded. 
     Next the vacuum pump valve  86  is opened if it is not already open, and the vacuum pump  88  is then turned on. This draws air and any other gas from the test port  61 , through conduit  94 , X fitting  96 , through conduit  103 , vacuum pump valve  86  and conduit  98 . It also evacuates the conduit  100  between the X fitting  96  and the reference volume valve  84  and from the conduit  102  that connects the X fitting with the pressure sensor  90  and from the internal regions of the pressure sensor. This vacuum is pulled until it reaches a sufficiently low pressure and maintains that low pressure. This initial process tests the O rings  46  and  48  for gross leakage and also causes them to outgas. The pressure to be maintained depends on the degree of leak resistance to be achieved. In the 30B cylinder shown in the drawings it is considered desirable to sustain a pressure of less than 10 millitorr since this will prove that the leak rate is at most 1×10 −5  ref. cm 3 /sec. This may take several hours because of out gassing. Other cylinders such as 48 inch cylinders may require a different level of acceptable leak rate. 
     Once the pressure has stabilized, for example, at less than 10 millitorr, the vacuum pump valve  86  is closed. If the rate of change of the pressure is more than a desired amount, the test should be stopped and the O rings  46  and  48  should be inspected and the source of the leak eliminated. If the desired low pressure is maintained at a sufficiently steady state, the pressure is noted as P test . In the 30B cylinder a leakage rate of less than 1 millitorr per second is applied. This number is derived from the acceptable leak rate. For the 30B cylinder  10  shown in the Figures, the acceptable leak rate has been set at 1×10 −5  ref. cm 3 /sec., although it could be set at a different value if one wished to obtain a lesser or greater degree of seal. 
     Once the pressure in the test volume has stabilized at or below 10 millitorr, the apparatus  78  is ready to determine the volume V test . This is done by opening reference volume valve  84 . As the reference volume valve  84  is opened, the air in the reference volume  82  expands to fill the test volume, V test . This expansion is essentially adiabatic. The expansion of the air in the reference volume causes the pressure in the test volume to increase. The new pressure is recorded as P mix . 
     The initial volume of the reference volume  82  and the conduits  104 ,  105 , and  100  between the valve  80  and the valve  84 , V cal ., is known by measuring it in any convenient manner when the apparatus  78  is manufactured. 
     The volume V test  is given by Boyle&#39;s Law:            V     ca                   l   .         ×       (       P     a                   tm   .         -     P   mix       )       (       P   mix     -     P   test       )         =     V   test                            
     The next step is to calculate the minimum time interval, the pass test time, for a pressure change of 1 millitorr. This value depends of the test volume, V test , and the maximum acceptable leak rate. For the leak rate of 1×10 −5  ref. cm3/sec. where V test  is measured in cubic centimeters, the following equation is used: 
     
       
         Pass test time in seconds=0.132 ×V   test    
       
     
     The conversion factor (0.132) takes into account the various dimensions that must be converted from one set of units to another and the acceptable leak rate. 
     With these preliminary calculations performed, the actual test for leak rate can be performed. The reference volume valve  84  is closed, and the vacuum pump valve  86  is opened. The vacuum pump is turned on until the pressure drops below 10 millitorr at which time the vacuum pump valve  86  is closed. Once the pressure begins to rise, a timer is started to determine how long it takes for the pressure in V test  to rise one millitorr. This time is compared to the test time in seconds determined in the preceding equation. If the actual time is greater than the pass test time, then the outgassing plus any leakage is less than the acceptable rate, and the valve protection cover has at most the acceptable leak rate, in which case the cylinder, for example cylinder  10 , is considered to have passed the test. Once the cover has passed the test, the conduit  94  is disconnected from port  61  and the port is closed with a conventional plug (not shown). 
     Thus it is clear that the present invention provides a vessel  10  for the shipment of uranium hexafluoride that includes a cylindrical wall closed by pair of approximately hemispherical ends  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  61  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 or other nuclear material 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. 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. 
     Use of the vessel  10  in conjunction with the test apparatus  78  and method disclosed assures that the valve protection cover assembly  14  not only protects the valve  30  from mechanical damage, but also assures that a selected leak rate is not exceeded leak rate. The double barrier in conjunction with the method of testing the seals has resulted in a transportation index of 0 for 30B cylinders constructed and tested according to the present invention.