Abstract:
A canister is provided for storing, transporting, and/or disposing of spent nuclear fuel. The canister includes a canister shell, a top shield plug disposed within the canister, and a leak-tight closure arrangement. The closure arrangement includes a shear ring which forms a containment boundary of the canister, and which is welded to the canister shell and top shield plug. An outer seal plate, forming an outer seal, is disposed above the shear ring and is welded to the shield plug and the canister.

Description:
CONTRACTUAL ORIGIN OF THE INVENTION 
     The United States Government has rights in this invention under a contract with the Department of Energy. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to canisters for storing, transporting and/or disposing of spent nuclear fuel and, more particularly, to an improved closure mechanism for such canisters, and a method of ensuring leaktight closure of such canisters. 
     2. Related Art 
     Spent nuclear fuel is placed into canisters for storage and transportation, and in some instances, for permanent disposal in a geologic repository. As shown in FIG. 1, a typical canister, generally denoted  10 , comprises a right-circular cylinder  12  with a bottom plate  14 . As can best be seen in FIG. 2, the canister closure at the top of the cylinder  12  includes an outer lid  16  and an inner lid  18 , with the canister  10  further including a shield plug  20  for the spent nuclear fuel, denoted SF. The two lids  16  and  18  are field welded, by welds indicated at  22  and  24 , after the spent fuel SF and shield plug  20  are installed in the canister  10 . It is noted that the spent fuel requirements of Title 10 of the Code of Federal Regulations, Part 72(10 C.F.R. §72) requires redundant seals such as two welded lids. It should also be pointed out that some conventional canister designs weld the shield plug directly to the canister in order to eliminate the need for an inner lid. 
     ANSI N14.5, American National Standard for Radioactive Materials-Leakage Tests on Packages for Shipment, specifies the methods for demonstrating that Type B radioactive material transportation packages comply with the containment requirements of 10 C.F.R. §71. ANSI N14.5 is also the standard applied to spent fuel storage systems and defines the word “leaktight” as a leakage rate no greater than 1×10 −7  standard cubic centimeter per second (std cm 3 /s). If it can be demonstrated that a package is leaktight, the package can be stored or shipped without consideration of the package contents. On the other hand, leak rates which are greater than leaktight have to be evaluated against the contents of the package to demonstrate acceptability. ANSI 14.5 also identifies the sensitivity range for a helium mass spectrometer “sniffer” test as 1×10 −3  to 1×10 −6  std cm 3 /s and the sensitivity range for a helium mass spectrometer “envelope” test as 1×10 −3  std cm 3 /s to 1×10 −8  std cm 3 /s. 
     The typical fuel canister is leak tested using the helium mass spectrometer sniffer test. In this test, after the inner lid is welded to the canister, the canister is filled with helium and the weld joint is tested. The actual sniffer test simply consists of using a probe which is connected to a mass spectrometer and which is held near the weld to sample the ambient air for helium. Once the inner lid penetration is sealed, the void between the inner and outer lids is filled with helium and the sniffer leak test is repeated for the outer lid in the same manner. Such testing indicates that a typical canister has a leak rate of no greater than about 1×10 −5  std cm 3 /s. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the invention, a canister is provided for storing, transporting, or disposing of spent nuclear fuel, the canister comprising a canister shell, a top shield plug disposed within the canister, and a leak-tight closure arrangement, the closure arrangement comprising: a shear ring forming a containment boundary of the canister, and weld means for welding the shear ring to the canister shell and to the top shield plug. 
     Advantageously, the shear ring comprises a plurality of pieces welded together, although the shear ring can also be of one piece construction. 
     Preferably, an outer seal plate is disposed above said shear ring and welded to the shield plug and the canister. 
     Advantageously, the shield plug includes bolt holes, drilled in an outer surface thereof, for attaching a lifter thereto. 
     In a beneficial implementation, the mating surfaces of the shear ring and the canister shell are tapered. 
     In accordance with a further aspect of the invention, a method is provided for producing a leaktight closure for a canister comprising a canister shell and a top shield plug, the method comprising: welding a shear ring to the canister shell and to the top shield plug, supplying a test gas to the canister, welding an outer seal plate to the canister so as to seal the canister and create a space between the seal plate and the shield plug, sampling the air between the shield plug and the seal plate to test internal sealing of the canister, supplying a test gas to the space between the seal plate and shield plug, and testing the outer seal plate for leakage. 
     Supplying a test gas to the canister preferably comprises removing a pipe plug in the canister, filling the canister with helium and reinstalling the pipe plug after the filling step. 
     In an advantageous implementation, a leak test adapter is installed on the seal plate after welding of the seal plate and a mass spectrometer is connected to the adapter to sample the air between the shield plug and the seal plate. Preferably, supplying of the test gas to the space between seal plate and shield plug comprises filling the space with helium, and the method further comprises removing the leak test adapter, and using a seal plug to seal the outer seal plate. 
     Further features and advantages of the present invention will be set forth in, or apparent from, the detailed description of preferred embodiments thereof which follows. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 and 2, which were described above, are a cutaway perspective view and a detail of the perspective view, respectively, of a prior art canister; 
     FIG. 3 is an exploded perspective view of the canister closure mechanism of the invention; 
     FIG. 4 is a cross sectional view of a fully assembled canister incorporating the canister closure mechanism of the invention; 
     FIGS. 5 to  7  are cross sectional views, showing steps in the assembly and testing of the canister of FIG. 4; and 
     FIG. 8 is a cross sectional view showing a detail of the canister and illustrating an alternative embodiment of the shear ring of FIGS. 3 to  7 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 3, the basic components of the leaktight closure mechanism of the invention include a three-piece segmented shear ring  30 , including pieces  30   a,    30   b  and  30   c.  It will be understood that a one-piece, spliced shear ring or a two-piece shear ring could also be used. In the illustrated embodiment, the container containment boundary for the canister cylinder  32  is formed by welding the three segments together and welding the resultant shear ring  30  to the canister shell  32  and to the top shield plug assembly including shield plug  34 . This is shown in FIG. 4 wherein, as illustrated, shear ring  30  is received in an annular recess  32   a  in the inner wall of shell  32  and is welded by a weld  31  to shell  32  and by a weld  33  to shield plug  34 . As is also shown in FIG. 4, an outer seal plate  36  is welded by respective welds  35  and  37  to the shield plug  34  and the shell  32 , respectively. Outer seal plate  36  provides the redundant seal required by 10 C.F.R. §72. 
     As shown in FIG. 4, the canister  10  also includes canister leak testing components which are located on the circumference of the shield plug  34  and the seal plate  36 . The components, which are conventional, include an L-shaped hole  38  connected to a vertical channel  40  in the shield plug  34  which communicates with the interior of the canister  10 , a pipe plug  42  disposed in the vertical leg  38   a  of hole  38  and seal plug  44  which seals off a larger diameter opening  46  which is connected to pipe leg  38   a.  In addition, outer seal plate  36  includes an outer seal plate boss  48  in which a pipe plug  50  is received and a seal plug  52  for sealing opening  54  in seal plate  36 . An intermediate diameter opening  56  is disposed between, and provides communication between, upper opening  54  and the smaller diameter opening in which pipe plug  50  is received. 
     Once the shear ring seal welds  31  and  33  are completed, a leak test adapter  58  of the kind disclosed in U.S. Pat. No. 5,548,992 (Hallett et al) is installed in the shield plug penetration, as shown in FIG.  5 . In general, adaptor  58  includes a stem member  58   a,  which is received in a cylindrical body  58   b,  operated by handle  58   c  and sealed by o-rings  58   d,  and which is used, inter alia, to remove pipe plugs such as plug  42  and thus open a connection to a helium supply or mass spectrometer, indicated at  59 , through a branch connector  58   e.  Reference is made to the Hallett et al patent, which is hereby incorporated by reference, for more details with respect to adaptor  58 . The adaptor  58  is used in FIG. 5 to remove the pipe plug  42  (as illustrated), evacuate the canister  10 , and reinstall the pipe plug  42  once the canister  10  is filled with helium. 
     Referring to FIG. 6, after these operations are completed, the seal plug  44  is, as illustrated, welded to the shield plug  34 . 
     Referring to FIG. 7, in a further step, after the outer seal plate  36  is welded to the shield plug  34  (by weld  35 ) and to the shell  32  (by weld  37 ), the leak test adapter  58  is installed in the outer seal plate boss  48 , as illustrated. Once the leak test adapter  58  is installed, the adapter  58  is connected to a mass spectrometer (such as that indicated generally at  59  in FIG. 5) which is used to sample the air between the shield plug  34  and the outer seal plate  36 . This process is referred to as a helium mass spectrometer envelope leak test and can be used to demonstrate that the inner seal is leak tight (i.e., has leakage rate less than or equal to 1×10 −7  std cm 3 /s. This is an improvement over the current state of the art sniffer test which is limited to demonstrating leaks no greater than about 1×10 −5  std cm 3 /s. 
     Once the inner seal is tested, the void or space between the shield plug  34  and the outer seal plate  36  is filled with helium, the pipe plug  50  is installed, the leak test adapter  58  is removed, and the seal plug  52  is welded to the penetration or opening of the outer seal plate  36 . A sniff test is then performed on the outer seal plate  36  to demonstrate a leak rate of no greater than about 1×10 −5  std cm 3 /s. 
     The weld shear ring arrangement of the invention does not require specific alignment of the shield plug  34  and the various weld joints are backed by the shear ring  30 , shield plug  34 , and canister shell  32 . The weld joint geometry can be sized to be structurally adequate, while affording the required clearances needed to install the shear ring  30 . Preliminary testing has indicated that preferential weld distortion eliminates these clearances, thereby resulting in metal-to-metal contact between the shield plug  34  and shear ring  30  and between the shear ring  30  and the canister shell  32 . This is an improvement over the current state of the art which relies on the closure welds  22  and  24  for lifting. The metal-to-metal contact between the shield plug  34  and shear ring  30  and canister shell  32  results in the shear ring  30  being the load bearing member and the welds  31  and  33  being classified as seal welds. 
     In an alternative embodiment illustrated in FIG. 8, the mating surfaces  30   m  and  32   m  between the shear ring  30  and the canister shell  32  are sloped or tapered to ensure metal-to-metal contact between these components prior to welding. 
     To permit lifting with the thick shield plug  34  and to provide a redundant seal, the outer seal plate  36 , as indicated above, comprises a ring which is welded, by welds  35  and  37  respectively, to the shield plug  34  and canister shell  32 . Lifting with the shield plug  34  (rather than an outer lid  16  which is the state of the art method) is preferred because the plug  34  is very rigid and reduces the bending moment which is applied to the canister shell  32 . Lifting is accomplished by attaching safety hoist rings (not shown) or a grapple adapter (not shown) to the shield plug  34  using bolt holes drilled in the outer surface of the plug  34 . One such bolt hole, denoted  60 , is indicated in FIG.  6 . The force required to lift the canister is transmitted from the lift attachments, through the shield plug  34 , to the shear ring  30  which contacts or bears on the canister shell  32 . Some of the lifting load is transmitted to the seal welds  31  and  33  but the primary load is through the shear ring  30 . The shear ring  30  could lift the canister without the two seal welds  31  and  33  and thus, the weld shear ring provides a “defense-in-depth” approach and improved safety for lifting the spent fuel canister. 
     Although the invention has been described above in relation to preferred embodiments thereof, it will be understood by those skilled in the art that variation and modifications can be effected in these preferred embodiments without departing from the scope and spirit of the invention.