11.64. After the cooling period, the spent-fuel assemblies would be shipped in strong metal casks to a reprocessing plant where the fission products are removed and the uranium and plutonium are recovered. The method commonly used for this purpose is based on extraction by an organic solvent, and this requires the fuel material to be dissolved in nitric acid to form a solution of nitrates. A simplified flow sheet of the repro­cessing operations is given in Fig. 11.4. Each step is quite complex, but for the present purpose a brief overview will be adequate.

11.65. In the first (or “head-end”) stage, the fuel rod assemblies, either with or without removal of hardware, are chopped into sections from which the spent material is leached with hot nitric acid. This process is commonly referred to as chop-leach. The hulls of zircaloy (or other) cladding and

hardware that remain are subjected to a hot nitric acid soak to remove essentially all of the uranium and transuranic elements, i. e., elements of higher atomic number than uranium. The hardware and hulls form what are called TRU (for transuranium) wastes or alpha wastes, because they contain traces of alpha-emitting transuranium elements. These wastes have been buried temporarily in the past, but more permanent underground disposal is planned for the future.