Patent Number: 060581549
Section: summary

FIELD OF THE INVENTION The present invention relates to a leaktight closure apparatus using seals for a multi-use containment unit, for example for irradiated nuclear fuel assemblies or high-activity waste. DESCRIPTION OF RELATED ART A technique for shipping and storing irradiated fuel assemblies is being developed. Briefly, the technique consists of: introducing a thin-walled metal box, one of whose ends opens fully, into a thick-walled metal external packing that can be hermetically closed. Said unit acts to confine the radioactive material while providing satisfactory mechanical resistance. The thick external packing provides adequate radioactive shielding and most of the mechanical resistance so that the entire package consisting of the external packing, the containment unit and the radioactive contents can be transported, i.e. meets the regulatory specifications governing serious accidents such as dropping, PA1 introducing the container assembly, i.e. the external packing and the unit, into the pool used for storing irradiated nuclear fuels for shipping, PA1 loading said irradiated fuel assemblies into the unit, PA1 positioning the cover of the unit to ensure that the radioactive material is confined and then draining the unit, PA1 drying the inside of the unit, possible outside the pool, and hermetically closing the resulting package. PA1 at least two removable solid metal disks that are entirely located inside said tubular body, bearing directly or via a support disk or collar, on a shoulder constructed in the upper section of all or part of the periphery of the inner wall of the tubular body, each of said solid metal disks comprising sealing means located at their periphery and co-operating with the inner wall of the tubular body, PA1 gripping apparatus also located inside the tubular body above said solid metal disks and acting on them to create a leaktight seal. Said package may then be shipped to its destination in complete safety. The containment unit is then removed from its external packing and stored in the required location. The multi-use containment unit is also known in English as a "Multi-Purpose Canister" (MPC) and the external packing is known as an "Over-Pack" (OP). The open end of the unit is usually hermetically sealed, either by means of a welded cap or, reversibly, using seals gripped between a cap and a collar fastened to the open end of said unit. FIG. 1 shows this type of reversible closure by means of seals. FIG. 1 shows the thin wall (1) of the multi-purpose canister (MPC). Reference (2) is the internal cavity of the unit into which the irradiated nuclear fuel assemblies are introduced, and (3) is the collar fastened to the upper open end of the unit. (4) is the cap comprising two seals (5) in their housings that ensure a leaktight seal once the cap has been closed by means of bolts (6). In this type of closing apparatus it will be seen that after introducing the unit into the overpack, the collar (3) rests on the upper end of the overpack (7). Therefore operations to insert or remove the reversible seal unit into or from the overpack can only be carried out via said open end of the overpack. This means that the operations must be carried out either in a shielded cell or similar apparatus, or in a pool to provide the radiological protection that the multi-purpose canister is incapable of providing. In order to facilitate insertion and extraction of the multi-purpose canister into and out of the over-pack in the vertical position and avoid the use of an intermediate shielded apparatus between the over-pack and the required storage site, it is advantageous to use an over-pack that fully opens at its lower as well as its upper end. Thus, once the bottom of the over-pack has been removed and the package has been positioned immediately above the storage site, the unit containing the irradiated fuel assemblies may be removed via the lower end of said over-pack by sliding it out. It will be clear that the presence of a collar on the unit prevents this type of handling in which the unit is inserted into the over-pack via the top end and removed via the bottom end. In order to carry out these handling operations it is therefore essential that outer surface of the unit is smooth and fitted with no protruding collar. The applicant has therefore aimed at developing this type of unit that has both a reliable reversible leaktight closure, but that is also easy to produce and has no features that project from the outer cylindrical wall. SUMMARY OF THE INVENTION The invention is a removable sealed closing apparatus for a metal multi-purpose canister (MPC) for irradiated nuclear fuel assemblies, composed mainly of a tubular or cylindrical body, normally circular in cross-section, having a single major axis and a lower end that is closed by a welded base and an upper end that is completely open. Said unit is designed to be entirely enclosed in a thick-walled over-pack, said leaktight closure apparatus comprising: The solid disks are thus stacked on top of one another and, as will be seen below, the leaktight seal is particularly created between the sealing means located at the periphery of the solid metal disks and the vertical wall of the tubular body. The support disk may also be a collar. Furthermore, the lowest solid disk comprising sealing means may bear directly on said shoulder without interposition of a support disk or collar. As will be seen in detail below, the sealing means normally comprise O-rings that are compressed by means of the gripping apparatus, which may be of a known type. The tubular body of the multi-purpose canister generally has thin walls that may be between 1 and 10 cm. More usually, however, they are between 2 and 5 cm thick, but despite this the apparatus of the invention ensures the leaktightness of the unit over time. In contrast, the walls of the over-pack are of the order of 10 to 30 cm thick, but more usually between 15 and 25 cm. The diameter of the multi-purpose canister is normally between 0.5 and 2.5 meters, most commonly between 1.5 and 2 meters, and its length, depending on the fuel assemblies it is designed to contain, varies between 3 and 5 meters. The solid disks and the bearing disk or collar are normally at least 15 mm thick. This thickness is related to the thickness of the seal and designed to avoid the risk of distortion by the gripping action ensuring the leaktight seal. The thickness of the disks is normally less than 50 mm to avoid adding any disadvantageous extra weight to the canister and to leave more space available for the contents. The closure apparatus of the invention may also comprise apparatuses for testing leaktightness. For this purpose the invention uses at least two solid metal disks fitted with sealing means; leaktightness can therefore be tested and therefore guaranteed. This type of closure apparatus composed of at least two components (i.e. a metal disk and a gripping apparatus) may be penetrated by an aperture comprising its own sealing system comprising, for example, two seals whose leaktightness can be tested.