Spent nuclear fuel (SNF) may be considered either as waste (SFW), which will eventually be packaged and disposed of [25], or reprocessed to recover uranium and plutonium followed by conditioning of residue in the form of high level waste (HLW) containing mainly fission and activation products, and so-called minor actinides (Np, Am, Cm) [11, 25] . There is no specific treatment step of SFW except that fuel elements are stored at the reactor site for some time to allow their intense radioactivity to decay and associ­ated heat to decrease. SFW elements can then be moved to longer term storage facilities (dry or wet), before deep geological disposal. If not declared as waste, SNF elements could be shipped to a reprocessing plant, but only after a suitable storage (cooling) period. The decay/cooling storage period at reactor sites usually varies from three to five years or even longer; afterwards, the spent fuel can be transferred to ‘away from reactor’ storage for up to 50 years or more, depending on the national policies with regard to reprocessing or disposal.

HLW formed after reprocessing of SNF contains fewer long-lived acti­nides than SNF due to extraction of plutonium. The world industrial repro­cessing practice (as used, for example, at the La Hague Reprocessing Plant in France) demonstrates that the volume of HLW after conditioning is less than the total volume of SNF assemblies [26] . Both LLW and ILW gener­ated by reprocessing are treated by methods described in this chapter. HLW contains some long-lived fission products such as Tc-99 and I-129, and minor actinides (Np, Am, Cm). An additional technological procedure — partitioning — can be introduced into reprocessing technology for extraction of minor actinides to reduce the HLW radiotoxicity. The extracted minor actinides could then be transmuted by fission using fast neutrons. The par­titioning and transmutation (P&T) approach can reduce the radiotoxicity of SNF by a factor of 100 or more [27] resulting in less dangerous waste for disposal.