In water-cooled nuclear reactors, and in particular in nuclear reactors cooled by pressurized water, fuel assemblies are used that comprise a frame having fuel rods placed therein, each fuel rod being constituted by tubular cladding filled with fuel pellets, e.g. pellets of uranium oxide UO2, and closed at its ends by sealed plugs.
The cladding and the plugs of rods are generally made of zirconium alloy, and the cladding is filled with helium under pressure prior to the rods being sealed in completely leaktight manner.
Inside a nuclear reactor in operation, the cladding of the rods is subjected to mechanical stresses and the cladding can suffer corrosion, such that it is possible for cracks to appear that pass through the cladding in some of the rods in a fuel assembly. The rod is then no longer sealed, and the helium that was contained in the cladding together with the fission gases given off by the fuel can leak out through the cladding, thereby causing radioactive substances to be disseminated into the environment of the fuel assembly.
While the nuclear reactor is shut down, e.g. for reloading or for maintenance and repair, at least some of the fuel assemblies are unloaded from the core, these fuel assemblies being placed in a fuel pool within which certain repair and reconditioning operations are performed on used fuel assemblies.
In particular, during these repair and reconditioning operations on fuel assemblies, leaky fuel rods are taken from the fuel assemblies, with these leaky rods being stored, for example, in temporary manner in a storage structure within the fuel pool.
In theory, the fuel contained in the rods can be recovered in fuel reprocessing facilities in which the fuel material is separated from the worn and cracked cladding and is processed as radioactive waste. Nevertheless, for several years, the regulations in force have tended to ban the transport between power plants and reprocessing facilities of fuel rods that might lead to radioactive materials being disseminated.
It is therefore necessary to provide methods and devices that enable leaky fuel rods to be stored for long durations without involving transport operations between nuclear reactors and reprocessing facilities, or in some circumstances the transport of leaky rods in sealed sheaths.
Proposals have been made in particular to use capsules each comprising a tubular sheath suitable for containing a fuel rod and closed at its ends by plugs that can be secured in leaktight manner to the end portions of the capsule sheath. The plugs secured to the two ends of the capsule also include respective valves in respective channels passing through the plugs, thus enabling a leaky rod that is generally full of water inside the pool to be purged and enabling the inside volume of the capsule to be filled with an inert gas under pressure. After the plugs have been closed in leaktight manner at the ends of the capsules, the leaky rod is confined so that it is no longer in danger of disseminating radioactive substances into the surroundings. The capsules are designed to be suitable for being inserted in support structures. These support structures in which leaky rods have been encapsulated can themselves be transported to a storage site, a reprocessing facility, or a hot laboratory for examination or investigation.
Devices are also known for transporting irradiated fuel material and comprising cases inside a transport container that is fitted with a means for closing the ends of the container, with damper means, and with grip means enabling it to be handled, the cases being of square-section of tubular shape suitable for being closed in leaktight manner by end walls or covers that are welded to the ends of the cases, and in which irradiated fuel materials are placed.
When it is desired to perform long-duration storage or very long-duration warehousing of leaky fuel rods stored in a pool, known encapsulating or transport devices do not make it possible in simple and practical manner to package large quantities of leaky fuel rods.
The term “warehousing” of nuclear materials is used to mean storage that enables the materials to be recovered at any time. Such very long-duration warehousing may be intended to last, for example, for a duration of 300 years or more. Long-duration storage may be intended to last, for example, for a duration of 50 years to 100 years.
During continuous operation of a nuclear reactor, it can become necessary, after the reactor has been operating for a certain length of time, to dispose of a large number of leaky rods that have been removed from fuel assemblies during successive campaigns of nuclear reactor maintenance and reloading.