Patent Number: 051026155
Section: description

DETAILED DESCRIPTION Referring to FIG. 1, a container is provided for storing and transporting radioactive material, such as racks of irradiated fuel bundles 1, comprising a vessel 2 and a cap 3. Referring to FIG. 2, the vessel 2 has an upwardly open cavity 4 for accommodating radioactive material. The particular container illustrated has a vessel 2, cavity 4 and cap 3 of substantially rectangular transverse cross-section. The vessel 2 and cap 3 have rounded longitudinal edges. The vessel 2 has walls with a core 5 of radioactive shielding material enveloped and isolated within a continuous metal lining. The vessel's continuous metal lining comprises: an upwardly open internal liner 6; an external vessel liner 7; and a top vessel plate 8 (a plan view of which is shown in FIG. 5). The internal liner 6 has side and bottom walls, the inner surfaces of which define the cavity 4. The external liner 7 also has side and bottom walls which are spaced outward from the internal liner 6. Referring to FIGS. 2 and 5, the top vessel plate 8 has a central opening 9 defining the cavity 4. An inner portion of the top vessel plate 8 adjacent the central opening 9 is continuously welded to an upper portion of the internal liner 6. An outer portion of the top vessel plate 8 is continuously welded to an upper portion of the external vessel liner 7. The concrete shielding material 5 fills the internal space defined by the internal and external vessel liners 6 and 7 and the top vessel plate 8. The cap 3 covers the top surface of the vessel 2 sealing the cavity 4. The cap 3 also has a core 10 of concrete shielding material enveloped and isolated within a continuous metal lining. The lower outer peripheral edge of the cap 3 is continuously welded to the upper outer peripheral edge of the vessel 2. The continuous metal lining of the cap 3 comprises: a bottom cap plate 11; a top cap plate 12 spaced upward from the bottom plate; and an external cap liner 13. The external cap liner 13 has side walls, the top and bottom portions of which are continuously welded respectively to the outer portions of the top and bottom cap plates 11 and 12. The concrete shielding material fills the internal space defined by the bottom and top cap plates 11 and 12 and the external cap liner 13. The concrete shielding material of the vessel core 5 and cap core 10 is high density concrete having aggregates of magnetite or specularite preferably. Concrete mixtures also may be designed to impart desirable properties during disposal. Concrete is alkaline and therefore inhibits corrosion of steel liners and reinforcing bars. Ground water containing dissolved salts may penetrate the exterior liner. The alkaline concrete buffers such penetration of corrosive ions. The concrete may be reinforced or not depending upon design stresses. The continuous metal lining of the container is preferably of carbon steel plate due to its relatively low cost and wide availability in a variety of grades. The metal lining may be made of stainless steel, copper, titantium or other metal suitable for the corrosive environment anticipated particularly in a disposal site. The outer surfaces of the container may be coated with epoxy paint to facilitate concrete decontamination. The inner surfaces of the internal liner 6 may also be coated with epoxy paint to inhibit corrosion which clouds the water and impairs the loading operator's vision when the vessel 2 is loaded with concrete material underwater. A particular advantage of the invention is its ability to be loaded underwater such that the shielding material is not exposed to contaminated water and the exterior of the container may be easily decontaminated. The vessel 2 includes two diametrically opposing lifting lugs 14 attached to the outer side walls of the vessel 2 for lifting the vessel 2 and container. A spreader beam 18 suspended from an overhead crane may be used engaging the lugs 14 with mating trunnions 30 as shown in FIG. 2. The lugs 14 are anchored in the vessel's concrete core 5 using embedded studs 15 welded to a lug anchoring plate 16 which is itself welded to the external vessel liner 7. The cap 3 includes at least one lifting eyelet 17 attached to the top surface of the cap 3 for lifting the cap 3 with a crane. The lifting eyelets 17 are anchored in the cap's concrete core 10 using embedded studs 19 welded to an eyelet anchoring plate 20 which is itself welded to the top cap plate 12. Referring to FIGS. 2 and 4, in order to accurately and quickly position the cap 3 upon the vessel 2 prior to welding the cap 3 and vessel 2 together, aligning pins 21 and aligning sockets 22 are provided. The aligning pins 21 are connected to the top surface of the vessel as shown by welding to the top vessel plate 8. The aligning sockets 22 are recessed within the lower surface of the cap by welding adjacent a hole in the bottom cap plate 11. The aligning sockets 22 correspond to and mate with the aligning pins 21 to position and align the cap 3 upon the vessel 2. The container is particularly suited to be loaded with concrete material while immersed in the water of a short term storage pool. In addition to the continuous metal linings which envelope and isolate the concrete radioactive shielding cores, and the epoxy paint coatings described above, underwater loading is further facilitated by the provision of a drain, a vent and a gasket ring. Since the contaminated water, surrounding the radioactive material within the loaded cavity 4, adds to the risk of radioactive leakage and to the corrosion of the metal fuel racks 1, it is desirable to drain the contaminated water from the cavity 4 and to vacuum dry the cavity 4 and its contents prior to transporting the container to long term storage. A gasket ring 23 is positioned between the top surface of the vessel 2, embedded within a groove in the vessel top plate 8, and the lower surface of the cap 3, such that the gasket ring 23 engages the bottom cap plate 11. Referring to FIG. 5, the gasket ring 23 is positioned adjacent the central opening 9, in the top vessel plate 8, which defines the cavity 4. The gasket ring 23 is used as a temporary seal during the draining and drying of the cavity 4 to seal the cavity 4. Referring to FIGS. 2 and 3, a drain pipe 24 communicates between a lower portion of the cavity 4 and the exterior of the container. Drain control means, comprising first and second drain plugs 25 and 26, are housed within an enlarged outer portion of the drain pipe 24, for sealing the drain pipe 24 and for enabling fluid to pass between the cavity 4 and the exterior of the container. Referring to FIGS. 1 and 2, a vent pipe 27 communicates between an upper portion of the cavity 4 and the exterior of the container. In the particular embodiment shown the vent pipe 27 is embedded within the cap 3. Venting control means, comprising first and second vent plugs 28 and 29, are housed within an enlarged outer portion of the vent pipe 27, for sealing the vent pipe 27 and for enabling fluid to pass between the cavity 4 and the exterior of the container. The following sequence of operations is carried out in order to load the container with radioactive material and seal the container. The radioactive material, such as for example irradiated spent fuel bundles 1 are initially stored underwater in racks in the short term storage pools of a nuclear power station. When the radioactive material is to be transferred to another site for long term storage, a vessel 2 with its first and second drain plugs 25 and 26 installed is placed underwater in the short term storage pool by an overhead handling crane above the pool. The crane is fitted with a lifting beam 18 and the lifting beam trunnions are engaged with the lifting lugs 14 of the vessel 2 as shown in FIG. 2. The crane is then used to lift the racks of radioactive material and place them within the cavity 4 of the vessel 2 at all times maintaining the radioactive material underwater. The loaded vessel 2 is then lifted from the pool and placed on a platform adjacent the pool. The cap 2 is lifted by its lifting eyelets and placed upon the top surface of the vessel 3. The weight of the cap 3 compresses the gasket ring 23 to temporarily seal the cavity 4. Aligning pins 21 and mating aligning sockets guide the cap 3 into proper alignment upon the vessel 2. The outer surfaces of the container have previously been coated with epoxy paint to facilitate concrete decontamination. A mixture of water and cleaning solution (such as Alxonox/Alcojet*) is used with long handled brushes to decontaminate the outer surfaces. A chemical cleaner may also be used to further decontaminate the container's outer surfaces. Vacuum pumps and conduits are then attached to the vent pipe 27 and drain pipe 24 after removal of the vent plugs 28 and 29 and drain plugs 25 and 26. The water from within the cavity 4 is drained away and returned to the pool. The vacuum pump is used to vacuum dry the cavity 4 and its contents via the vent pipe 27 and drain pipe 24 to prevent corrosion of the internal liner 6 and the racks supporting the irradiated fuel bundles. After drying the drain plugs 25 and 26 are replaced and the cavity is back filled with helium gas via the vent pipe 27. The vent plugs 28 and 29 are then replaced and the container is leak tested. Referring to FIG. 4, the lower outer peripheral edge of the cap 3 is continuously welded to the upper outer peripheral edge of the vessel 2. The weld shown in FIG. 4 is a full penetration butt weld preferably deposited by semiautomatic welding equipment. The cap 3 to vessel 2 weld is inspected by non-destructive methods. Upon completion of welding the decontaminated container is lifted by the lifting beam 18 and crane and placed upon a flatbed truck or rail car to be transported to its long term storage site. *Trade-mark