Patent Number: 
Section: description

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 30a, 30b and 30c. 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 32a 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. xc2xa772. 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 38a of hole 38 and seal plug 44 which seals off a larger diameter opening 46 which is connected to pipe leg 38a. 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 58a, which is received in a cylindrical body 58b, operated by handle 58c and sealed by o-rings 58d, 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 58e. 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 1xc3x9710xe2x88x927 std cm3/s. This is an improvement over the current state of the art sniffer test which is limited to demonstrating leaks no greater than about 1xc3x9710xe2x88x925 std cm3/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 1xc3x9710xe2x88x925 std cm3/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 30m and 32m 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 xe2x80x9cdefense-in-depthxe2x80x9d 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.