Conditioning means containing the radioactivity and immobilising the radioactive atoms. The atoms have to be immobilised until they are no longer radioactive, or if that is not possible, until they have nearly reached that state. The capacity of the package to withstand damage from the radiation naturally depends on the activity of the material being processed.
The activity is greatest with spent fuel from nuclear power plants. When a reactor is in operation, the radioactivity is contained by the zirconium cladding, which consists of tubes that hold the fuel pellets. This cladding must be the primary containment barrier once the fuel has been unloaded. To reinforce the barrier, the irradiated fuel assemblies are placed in special containers known as canisters or casks.
If the irradiated fuel is reprocessed, it is the residue of highly radioactive material considered as waste that is contained. This radioactive residue is incorporated into a material known as the 'matrix', selected for its ability to withstand damage caused by radiation. The matrix must be able to incorporate a sufficient quantity of radioactive atoms. It must be able to resist being dissolved in water for thousands of years.
Special types of glass are currently used as the matrices for high activity waste at the La Hague plant. Glass is an amorphous material with an extremely flexible structure that absorbs the effect of the radiation. Glass is a very effective conditioning matrix for all the different types of fission products.
As part of research into waste, a number of ceramics have been developed that are more expensive but even more durable. These ceramics – mineral compounds processed at a very high temperature – have the property of being able to incorporate into their crystal structure atoms with good chemical compatibility. Long-lived radioactive atoms could therefore be completely immobilised in these structures, as bespoke matrices.
Protection could thus be offered for very long periods of time against elements such as iodine-129 and caesium-135, which are naturally fairly mobile and difficult to destroy by transmutation.
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