The transformation of uranium or mixed uranium/plutonium (U/Pu) oxides through fission and/or neutron capture reactions in pressurized (PWR) or boiling (BWR) water nuclear reactors generates more than 400 radionu­clides (representing 40 elements of the periodic table) with differing physi­cal and chemical properties and making spent nuclear fuels extremely radioactive. The fissile and/or fertile properties of the major actinides, U and Pu, heighten the interest of recycling them (they are moreover the major contributors to the long-term radiotoxicity of spent nuclear fuel). The PUREX process not only separates valuable U and Pu from the waste by taking advantage of the extracting properties of tributyl-phosphate (TBP), it also allows nuclear waste to be conditioned in a safe, inert glass matrix while generating a minimum of secondary technological waste. Spent nuclear fuel reprocessing thus significantly reduces the mass, volume, toxic­ity, and thermal activity of the ultimate nuclear waste to be disposed of in deep geological repositories (Schulz et al., 1990). Furthermore, recycling purified Pu in MOX fuel, which in turn can be reprocessed after fission, makes the option of closing the fuel cycle ever more possible.

Further possible innovative routes have been identified beyond this first but nevertheless essential step towards developing sustainable nuclear energy, preserving natural fossil resources and minimizing the impact on the human environment. Future nuclear fuel cycles will undeniably require recycling of not only U and Pu but of all the valuable and hazardous radio­nuclides, such as long-lived fission products and minor actinides (MA: nep­tunium, Np, americium, Am, and curium, Cm). Although they account for only a thousandth of the mass of the major actinides, the MA remain, after several centuries, the major contributors to the radiotoxicity of spent nuclear fuel (provided Pu has been eliminated). The current P&T research policy is not driven strictly by the problem of waste repository safety since it has been demonstrated that, under certain reductive geological condi­tions, the glass matrix alteration would be very slow and the release of LLRN from the ultimate waste negligible (although variable according to the geological repository conditions). The issue is rather that of limiting the potential radiotoxicity of nuclear waste.

Nevertheless, the potential noxiousness of Am is different from those of Cm and Np, since Am is the major long-term radiotoxic contributor before Cm and Np. After a century of decay of the thermal fission products and Cm isotopes, Am-241 is the largest contributor to the heat release of nuclear waste. On the other hand, the problems encountered when recycling LLRN are not equivalent: recycling Am appears less detrimental than for Cm because of the presence of Cm-244, a radioactive, thermal, and neutron — emitting isotope that worsens the difficulties of fabricating Cm-based fuels or targets.

A few hundred years after nuclear waste disposal, the fission products would contribute to only a negligible fraction of its radiotoxicity. Although computational studies carried out on the chemical behaviour of deep geo­logical repositories revealed that some fission products, such as iodine-129, caesium-135, and technetium-99, could be released to the biosphere after several hundred thousand years, because of their high solubility in water (OECD/NEA, 1999), recent studies of nuclear waste storage scenarios have confirmed the mobility of iodine, but invalidated those of caesium and technetium, especially in clay repositories. Nonetheless, the strategy adopted in France — where two waste management acts were enacted in 1991 and 2006 to organize French research programs on P&T and help prepare the construction of both an industrial workshop for manufacturing MA-based fuels and an actinide burner reactor by 2020 (Warin, 2007) — has focused on the development of separation processes for iodine, caesium, technetium, neptunium, americium, and curium. This policy was further followed in Europe, where EURATOM has funded many collaborative projects since the 1990s (Dozol et al., 1997, Madic and Hudson, 1998, Dozol et al., 2000a, Madic et al., 2000, Dozol et al., 2004, Madic et al., 2004). It still consists in:

• Taking advantage of the potentialities of the PUREX process to separate most of the iodine and technetium and at least 99% of the neptunium by optimizing the operating conditions: increasing the feed acidity actually forces the oxidation of Np(V) into its +VI valence state, which is better extracted by TBP. The remaining Np is expected to be quantitatively recovered in the following Am and Cm recovery step.

• Devising and developing complementary separation steps, making use of highly selective molecules (since TBP is only suitable for extract­ing elements at valence states above +III) to recover caesium and more than 99.9% of americium and curium downstream from the PUREX process.

Calculations actually proved that the time required for the radiotoxicity of a spent nuclear fuel to decrease below that of natural U (extracted from the mine to manufacture the fuel) is estimated to exceed 100 000 years. This period shortens to 10 000 years if Pu is recycled, and shrinks to only three centuries if MA are also recycled (leaving less than 0.1% of the latter radio­nuclides in the nuclear glass waste). However, for physical reasons of trans­mutation efficiency in reactors, the decontamination factors of Am and Cm with regard to the lanthanides (Ln, known as neutron-absorbing elements unfavourable to MA transmutation) were set so as to limit to 5 wt.% the fraction of Ln in the final ‘Am + Cm’ product.