Spent nuclear fuel assemblies are not the only materials removed from reactors that must be placed in interim storage. After reprocessing spent fuel to recover the uranium and plutonium contained in it, the highly radioactive waste produced by the reprocessing operation must be managed. This is the case in France, where a second type of interim storage is used for such materials: dry storage.
Reprocessing residues consisting of fission products and minor actinides have been vitrified and placed in containers. These vitrified waste containers inherit the radioactivity already present in the spent fuel, other than that emitted by plutonium. They were originally produced and stored in Marcoule, but are now stored in ventilated shafts at the plant in La Hague. Heat is removed by either natural or mechanical convection.
Before the vitrified waste packages are stored in this way, the fuel assemblies had already been stored in pools prior to processing. As the name "interim storage" suggests, this solution is in principle temporary. However, it is the current end point for high-level waste pending decisions regarding their final destination, which will almost certainly be disposal in a geological repository.
At the La Hague plant, the shafts are located under the floor of a large hall featuring multiple "manhole covers", as the shaft cover plates have been aptly described by the Chairman of the national assessment commission (Commission Nationale d’Evaluation - CNE). The waste is located under the floor. Most of the radioactivity produced by reactors since the start of the French nuclear programme lies under this floor. Despite the "radioactive inferno" beneath their feet, the photograph shows operators going about their business without requiring too many obvious precautions. This shows that effective protection against even high-level radioactivity can be afforded by a good thickness of material able to absorb the most penetrating rays.
Vitrified waste from the earliest French reactor process – the natural uranium-graphite-gas (Uranium Naturel Graphite Gaz - UNGG) process, subsequently discontinued in favour of the current pressurised water (PWR) reactors – is stored at the Marcoule facility. The high-level waste from this process has been vitrified. Solutions containing fission products and minor actinides have calcined and embedded in a glass matrix by a processing line in the vitrification shop (Atelier de Vitrification de Marcoule - AVM) since 1978. Spent fuel processing at Marcoule ceased in 1997. The vitrified waste containers are stored in pits at the Marcoule site.
For how long will it be possible to continue using these interim storage facilities? This is one of the themes of current waste research. According to the national assessment commission (Commission Nationale d’Evaluation - CNE), "current industrial interim storage of high-level waste, as practiced in La Hague, appears to comply with all reasonable foreseeable requirements". This practice may be extended for an indeterminate period – possibly 70 or 100 years – subject to construction of an additional wing at the La Hague facility.
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