Improved vessel for uranium hexafluoride transport

ES 2 335 649 T3

DESCRIPTION

Improved container for the transport of uranium hexafluoride.

Background of the invention

The present invention relates to a container for the transport and storage of uranium hexafluoride, and in particular to improvements in a container known in the art as cylinder 30B.

Hexafluoride in enriched uranium has been transported in conventional 30B cylinders for many years. Uranium hexafluoride is considered enriched if it includes more than 1% Uranium 235 (U₂₃₅), and the transport of the enriched uranium hexafluoride (up to 5% by weight, inclusive) has to be done in authorized conventional 30B cylinders. Such cylinders filled with uranium hexafluoride must be transported in an approved overpack for thermal and impact protection. Such transports are considered safe if the cylinders are properly packaged and transported. As long as water or other possible neutron moderators are kept separate from uranium hexafluoride itself, a critical event (an uncontrolled nuclear chain reaction) cannot occur.

As in all aspects of the nuclear industry within the geographical limits of its authority, the Nuclear Regulatory Commission (NRC) regulates the transport of uranium hexafluoride. Because its authority extends to ports in the United States, and because its standards are among the most conservative in the world, the NRC's standards establish minimum standards for most international shipments of uranium hexafluoride. The American National Standards Institute, Inc., published in 1971 ANSI N14.1, Packaging of Uranium Hexafluoride for Transport. This standard was adopted by the predecessor of the NRC, and established the authorized design of the conventional 30B cylinder.

The ANSI N14.1 standard specifies the types of materials for which your authorized cylinders are suitable. Specifically, “footnote a” to Table 1 of Section 5.5, Section 5.5, Packaging Requirements, Standard UF6 Cylinders, of the ANSI N14.1 standard contemplates that a conventional 30B cylinder can be used to transport uranium hexafluoride. containing less than 0.5% impurities. For the purposes of the present application, a mixture consisting of at least 99.5% by weight of uranium hexafluoride and the remainder of other materials is referred to as "substantially pure" uranium hexafluoride.

The conventional 30B cylinder, currently defined by the ANSI N14.1-1995 standard, is a steel container approximately 2 m long and 76 cm in diameter. It is made of 1.27 cm carbon steel that has been shaped into a 1.37 m long cylindrical body topped by two approximately semi-elliptical cylinder heads. A couple of edges protect the ends of the container. The conventional 30B cylinder has a tare weight of approximately 646 kg and a volume of at least 0.74 m³. When filled to its maximum allowable capacity of 2,280 kg with uranium hexafluoride endowed with up to 5 percent by weight of the uranium 235 isotope, it would be conceivable that as little as 15 liters of water could initiate a critical event. Therefore, it is extremely important that water is excluded from the cylinder.

There are other risks associated with the transport of uranium hexafluoride. If this chemical is heated to its triple point of 63 ° C in the presence of air, hydrogen fluoride gas (HF (g)) may form. Such an event is conceivable if the valve of a conventional cylinder 30B breaks during the occurrence of a fire. Hydrogen fluoride gas is extremely harmful, and precautions must be taken against its release, since, if inhaled, the almost immediate result is death.

Two openings are made in the conventional cylinder 30b. The openings are located at approximately diagonally opposite locations on opposite cylinder heads. An opening accommodates a valve that is used routinely to fill and empty the uranium hexafluoride vat. The other opening is a plug used for periodic inspection, hydrostatic verification and cleaning of the tank. This valve and this plug form the only barriers to the entry of water in the conventional cylinder 30B.

During transportation, a cylinder 30B is housed in a protective shipping container or "overpack." Overpacking protects the cylinder inside from accidental impacts and insulates the cylinder to reduce the likelihood of leakage in the event of a fire or other accidental overheating event. The overpack and cylinder 30B are routinely transported by ocean going vessels, as well as rail and road transport. When the cylinder arrives at a treatment plant, it is removed from the overpack and a standardized piping system is connected to the valve. The ANSI N14.1 standard specifies the exact location of the valve, as well as its orientation, so that the treatment plant intakes are properly aligned and connected to the valve. Even if small, a change in the position or orientation of the valve may make it impossible to safely connect the cylinder to the plant taps. Once the cylinder 30B is connected to the pipes of the treatment plant, it is heated in an autoclave to evaporate and thus remove the uranium hexafluoride for further treatment.

Overpacks are regulated by government agencies. The US Department of Transportation (DOT) has issued a standard regulation, DOT 21 PF1, which defines an overpack. This regulation is published in 49

ES 2 335 649 T3

CFR 178.358. The Secretary of Transportation allows certain variations of this design in Certification USA / 4909 / AF, Revision 15. Overpacks manufactured in accordance with this regulation or its permitted variations are called “standard packaging”. In addition, the NRC has issued standards that define what is called "high-performance packaging." These containers are authorized by the NRC if they meet the performance standards defined in the regulations. Performance specifications are published in 49 CFR 173.401-476. A common feature of both DOT and NRC regulations is that the overpack must be designed to fit a conventional 30B cylinder, as defined by ANSI N14.1.

Overpacks and 30B cylinders are jointly verified, as required by the NRC, prior to authorization for use in the transportation of uranium hexafluoride. A standard test that must be passed is the "30-foot drop test." In this test, the cylinder 30B and the overpack are dropped from a height of 9 m onto a fixed concrete platform. The container is oriented so that the cylinder valve is facing straight down, worst case scenario. To pass this test, no part of the overpack can touch the valve or anything belonging to the valve, and the valve must remain firmly closed. If this test and the other required tests are passed, cylinder 30B is cleared to be the contents of the overpack. Enriched Uranium Hexafluoride can only be shipped in a 30B cylinder in an overpack for which that cylinder is the authorized content.

Regulations require periodic verification of 30B cylinders regardless of overpacking. Specifically, the dOt has adopted the ANSI N14.1 standard which, in turn, requires periodic verification of the 30B cylinders. This verification includes a hydrostatic test every five years. Before this test, the cylinder is cleaned. It is then filled with water and pressurized to check for leaks. This test checks the integrity of the structure, including the various welds. The test is expensive, in part because it creates 0.74 m³ of radioactive wastewater that must be disposed of as low-intensity radioactive waste.

In addition, the NRC regulates how densely conventional 30B cylinders can be stacked in overpacks on cargo ships or other means of transportation. It does this by giving each ship or mode of transport a total "transport index" of 200. Each cylinder 30B has a transport index of five, so that a ship not carrying another nuclear cargo can carry a total of forty ( 40) conventional 30B cylinders. (200 ± 5 = 40). This safety limit denies charterers of conventional 30B cylinders in standard overpacks the savings that bulk shipments could achieve, especially in light of the availability of chartered vessels dedicated to radioactive materials. However, this standard is necessary because, although hydrostatic testing guarantees structural integrity and although overpacking provides thermal and impact protection, there is no sure way to guarantee that the valve will continue to maintain its tightness 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 periodic hydrostatic checks and could guarantee the integrity of its valve. Any improvement to the conventional cylinder 30B must recognize the substantial investment in the equipment that is used to handle the existing cylinders 30B, including both the pipes and the existing overpacks. This requires that the essential dimensions of the cylinder and the location and orientation of the valve do not change.

US-A-4 197 467 discloses a multi-walled steel container for transporting spent rods of nuclear reactor fuel. The container has outer, middle and inner cylindrical sidewalls made of steel. Cooling water is contained between the outer and middle side walls. Protective layers of lead and uranium are provided between the intermediate and interior walls. A solid steel stock is permanently attached to the side walls at one end of the container. The cylinder head includes a double-walled steel cover that is bolted to the cylinder head in a releasable manner, and is removable to allow fuel rods to be bolted in and out of the container. The cover has a valve for introducing purge air into the container, and a valve for releasing air from the container. The valves are covered by individual caps pressed against the respective sealing surfaces by means of fixing (bolts). The cover itself is covered by an outer cover bolted to the cylinder head.

Summary of the invention

The present invention provides a single-walled steel container for the transport of substantially pure uranium hexafluoride, as set forth in claim 1.

Preferably, the uranium hexafluoride transport container includes a cylindrical wall closed by a pair of approximately semi-ellipsoidal heads to form an airtight container. At one end is a service valve. The valve is covered by a removable waterproof valve protection cap. The container also includes a test connection by which the integrity of the valve protective cap can be verified after the cylinder has been filled with uranium hexafluoride and the valve guard assembly installed. The protective valve cap is formed to fit inside the standard 30B cylinder envelope, thus fitting into the overpacks already authorized by the NRC and used by uranium hexafluoride charterers.

ES 2 335 649 T3

The container made according to the present invention has a double barrier to prevent the ingress of water or the egress of uranium hexafluoride. The valve, a first barrier, is enclosed by a cap assembly that forms the second barrier. The double barrier is expected to allow a transport index of 0. So, in effect, the addition of the second barrier will allow the improved 30B cylinders to be shipped in bulk in conventional overpacks with safety acceptable to the NRC, resulting in substantial savings to the industry.

Brief description of the drawings

Figure 1 shows an improved cylinder 30B constructed in accordance with the present invention and held in an open protective shipping container or "overpack" which, in turn, rests on a frame;

Figure 1A shows an overpack for a cylinder 30B fully closed and in a frame;

Figure 2 is a front view of the cylinder of Figure 1;

Figure 3 is a view looking in the direction of arrows 4-4 in Figure 2 and partially in cross section; and Figure 4 is an enlarged view of a portion of Figure 3 showing a valve guard assembly on the valve.

Description of the preferred embodiments

Figure 1 shows an improved cylinder 10 (container) 30B constructed in accordance with the present invention. The cylinder 10 is shown within the lower half of a protective transport or "overpack" container 12. The overpack 12 is shown supported on a frame 8 and with its upper half removed and its security straps open. As is well understood in the art, cylinder 10 is filled with up to 2280 kg of substantially pure uranium hexafluoride during shipping and is completely enclosed in the overpack, as shown in Figure 1A.

For the most part, the improved cylinder 30B 10 of the present invention is completely conventional and will be described in detail only where it differs from the conventional cylinder of the prior art. The conventional 30B cylinder 10 is manufactured in accordance with the ANSI N14.1 standard and the ASME Boiler and Pressure Vessel Code, Section VIII, Division 1. Consequently, the conventional 30B cylinder is 2070 mm plus or minus 13 mm long and has a diameter of 762 mm plus or minus 6 mm. The conventional 30B cylinder has a minimum volume of 0.74 m³. It is preferred that the cylinder is manufactured to the ANSI N14.1-2000 standard and therefore includes the advantages described in US Patent 5,777,343, arising from the elimination of a welded back bar. However, the advantages of the present invention can also be obtained with cylinders manufactured according to previous versions of the ANSI N14.1 standard that required welded back bars.

The improved cylinder 30B 10 includes a valve 30 that is protected by a valve protective cover assembly 14. This cover assembly, which is not found on conventional 30B cylinders, provides a second barrier against uranium hexafluoride leakage or, more critically, water entry. Valve guard cover assembly 14 fits within ridge 15, which extends from cylinder head or domed end of cylinder 10. More particularly, the distal end of valve guard cover assembly 14 is recessed at least 12.7 mm and preferably 19 mm or more from the plane defined by the free edge of the ridge. This space allows for deformation of the overpack during the drop test without any contact with the valve guard cover assembly 14. Therefore, the cylinder 10 provided with the valve protection cover assembly 14 can be used with standard overpacks, such as the overpack 12 shown in Figures 1 and 1A.

It should be noted that the axial length of the edge 15 is not set by the ANSI N14.1 standard, but the overall length, diameter, and minimum cylinder capacity are. Diameter and length are critical dimensions to ensure that a tank will fit in a conventional overpack. It was not recognized until the present applicants' invention that lengthening one edge 15 and shortening the other (not numbered) to allow a clearance of 12.7 to 19 mm or greater, as set forth above, would allow an assembly of valve protection cover would survive without damage a 9 meter drop test, certainly intact, due to the deformation of the overpack, this despite the improved safety and the probable reduction in the transport index.

The valve guard cover assembly 14 (Figure 2) includes a cap 16 that is held in place by six bolts 18. Two of the bolts 18 are provided with brake wire, and the wire is sealed to ensure that the cap 16 has not been tampered with once it has been screwed into place. If desired, additional bolts could be fitted with brake wire, up to a total of six.

The valve guard cover assembly 14, as shown in greater detail in Figure 4, includes a cap 16 and a base 20. The base 20 is an annular disc that surrounds the valve 30. The base 20 is A disc

ES 2 335 649 T3 that is welded to the wall 22 of cylinder 10. Its diameter and thickness are selected so that it does not interfere with the industry standard pipes used for connection with valve 30 to fill or empty cylinder 10 of uranium hexafluoride.

Base 20 is welded to wall 22 continuously around its outer and inner perimeters, and these welds are thoroughly inspected to ensure their integrity. Therefore, these welds provide a reliable barrier to prevent any matter from passing under the base 20 and thus passing from the outside of the cylinder 10 into the volume in which the cover assembly surrounds the valve 30 or vice versa. Base 20 also includes six equally spaced threaded holes (not shown) with which bolts 18 cooperate to hold cap 16 in place.

An upper surface 24 of base 20 includes two regions: an inner region 28 and an outer region 32. The inner region 28 is annular and protrudes slightly from the outer region by about 0.8mm. Inner region 28 is machined flat and provides a work surface against which cap 16 achieves sealing. The necessary flatness of the surface can be achieved by machining the base 20 either before or after welding the base 20 to the wall 22.

Cap 16 is a component fabricated from steel that includes a dome 40 and flange 42. Although cap 16 could be machined from a single piece of steel, it is preferred to fabricate it, for economy and ease of fabrication, from two pieces. which are welded together as shown. This weld is meticulously inspected to ensure its integrity.

Flange 42 mates with base 20. To this end flange 42 includes a machined annular surface 44 that seats against corresponding interior surface 28 of base 20. A pair of O-rings 46 and 48 fit into recesses 50 and 52 respectively, which are formed in annular surface 44 of flange 42. Recesses 50 and 52 are circular in plan view, but any endless shape could be used if desired. Recesses 50 and 52 may be formed with a slight skew, as shown, to hold O-rings 46 and 48 in place. When annular surface 44 and annular surface 28 are seated together, the o-rings 46 and 48 are compressed to achieve an effective seal. This tightness is complete enough to achieve a leak rate of less than 10³ cm_ref.³/ sec when tested according to, for example, the soap bubble test described in A.5.7 of the ANSI standard N14.5-1997, Leakage Tests on Packaging for Shipment. In this test, a "reference cubic centimeter cubed per second" is defined as a volume of one cubic centimeter of dry air per second at an absolute pressure of 101.325 kPa and 25 ° C. A gasket having the above leakage rate or less is considered essentially waterproof for the purposes of this application.

Although conventional O-rings 46 and 48 are preferred for ease of manufacture, other resilient elements are also possible, including cast-in-place rubbers or resilient polymers such as urethane. Such alternative materials and such manufacturing techniques need only provide a seal that is sufficiently resistant to leakage to be satisfactory, and are included within the meaning of the term "resilient sealing elements" used in the present application.

Flange 42 includes an annular outer region 58, recessed from the plane of annular surface 44. Outer region 58 is aligned with outer region 32 of base 20. The two outer regions 32 and 58 define a gap 60 between them when cap 16 is in place on base 20. flange 42 has six holes (not shown) passing through outer region 58 for bolts 18. These holes align with corresponding threaded passages in base 20. When cap 16 is put in place and bolts 18 are tightened with a predetermined torque, outer region 58 of flange 42 is stressed, ensuring a constant predetermined load on o-rings 46 and 48 and annular surfaces pairs 24 and 44. Although the formation of gap 60 is preferred because it allows flange 42 to bend slightly, any design that allows a sufficiently strong seal between base 20 and cap 16 is acceptable.

The valve protection cover assembly 14 includes a means for checking the integrity of the gasket between the cover 16 and the base 20. This verification device includes a test connection 61 leading through internal conduits 62, 64 and 66 to test channel 68. Test channel 68 is a semi-circular recess (in vertical cross section) in annular surface 44 of flange 42. Recess 68 extends in a complete circle spaced between recesses 50 and 52.

Flange 42 includes a hole 70 (Figures 1 and 4) diametrically opposite to test connection 61. This hole cooperates with a pin 72 projecting upwardly from outer region 28 of base 20. When cylinder 10 is in In its normal horizontal position, the pin 62 is at the 12 o'clock position, helping the operator to accurately position the cover and drive the bolts 18 into their holes.

Once the cap 16 is in place and the bolts 18 are properly tightened, the integrity of the gasket around the cap can be verified. This is done by connecting the test connection to a calibrated source of fluid under pressure or under vacuum. The fluid reaches the test channel 68, and if the joint is secure, the fluid cannot go any further. If a leak occurs, then the verification kit shows a drop in pressure or vacuum, and the O-rings can be inspected and replaced, or other repairs can be made as needed. After the verification is complete, a plug 71 is used to seal the test connection 61.

ES 2 335 649 T3

Various test procedures are available, and these are outlined in the ANSI N14.5-1977 standard. These tests guarantee a leak rate equal to or less than 1 x 10³ cm_ref.³/ sec.

Although the verification device is shown as a connection, conduit, and channel machined into flange 42 of cover 16, it is also possible to machine these elements into base 20. If this is done, the test channel is formed on the surface. 28 of the base 20 to be located between the locations where the O-rings contact the base 20 and is connected to a test connection via suitable conduit. 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 test channel 68 are less likely to be damaged when the lines connect to valve 30.

Although bolts 18 are used to tighten cap 16 against base 20, other fasteners are possible. For example, a threaded connection could be used between the base with the necessary O-rings, and the connecting channel formed in the screw cap. Alternatively, the base 20 could have external threads on its outer peripheral surface and a nut such as that used in plumbing fittings could be used to pull the cover against the base.

Thus, it is clear that the present invention provides a container 10 for the transport of uranium hexafluoride that includes a cylindrical wall closed by a pair of approximately semi-ellipsoidal butts 22 welded together to form an airtight container. At one end is a service valve 30. Valve 30 is covered by a removable and watertight valve protection cover assembly 14. The container also includes a test connection 61 by means of which the integrity of the valve guard cover assembly can be verified after the cylinder 10 has been filled with uranium hexafluoride and the valve guard assembly 14 has been installed. The valve guard assembly 14 is formed to fit within the standard 30B cylinder enclosure, and therefore fits within the overpacks already authorized by the NRC and owned by the uranium hexafluoride charterers.

Container 10 manufactured in accordance with the present invention has a double barrier to prevent ingress of water or egress of uranium hexafluoride. Valve 30, a first barrier, is enclosed by a cover assembly 14 that forms the second barrier. The double barrier is expected to allow a transport index of 0. So, in effect, the addition of 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.