The method of cladding removal depends on the fuel type:
Magnox fuel (metallic): The magnesium alloy cladding is removed by mechanical stripping using a die. The Magnox can is slit and peeled away from the uranium metal fuel rod. At the French UP1 facility at La Hague, cladding was instead removed by selective dissolution in nitric acid.
AGR and LWR fuel (oxide): A chop-leach method is used. Fuel assemblies are first dismantled and repackaged. A large hydraulic shear then cuts the fuel pins into short lengths of approximately 5 cm. These pieces fall into baskets immersed in hot 7M nitric acid at approximately 90 degrees C. The acid dissolves the oxide fuel from the stainless steel or Zircaloy cladding, which remains as insoluble “hulls”. These hulls are removed and classified as intermediate level waste (ILW).
At THORP, the dissolution is a batch process taking several hours per batch. The neutron poison gadolinium nitrate is added to the dissolver to prevent criticality. At La Hague, a rotating continuous dissolver is used instead.
Key Point: Not all the fuel is leached from the hulls — approximately 0.1% of the fuel remains with the cladding and must be dealt with separately as ILW.