Fuels and targets for the transmutation of HLLL radioactive waste

The long-lived radionuclides contained in spent fuel assemblies are responsible for the persistence of radioactivity over thousands, even millions of years. The main elements contributing to the radiotoxicity of spent fuel are plutonium (1% of spent fuel), the "minor" actinides americium, curium and neptunium (0.07% of spent fuel) and fission products (4% of spent fuel).

Plutonium, considered a resource rather than a waste in France, is already the subject of an industrial recycling strategy implemented in PWRs, using MOX fuel. For the other long-lived radionuclides present in waste, research is being carried out at international level into solutions for separating them from spent fuel and transmuting them into short-lived or inert elements in nuclear reactors, as an alternative to geological storage in a glass matrix.

Research carried out in France until 2006 under the December 30, 1991 law on the management of high-level, long-lived radioactive waste, and continued today under the June 28, 2006 law on the sustainable management of radioactive materials and waste, shows that not all types of nuclear reactor offer the same transmutation potential, and that not all radionuclides can be transmuted efficiently. Thus, the best transmutation performances are obtained in fast neutron spectrum reactors, and transmutation is only reasonably applicable to minor actinides (mainly americium, neptunium and curium). In the case of fission products, their contribution to the radiotoxicity inventory decreases after several hundred years, which, combined with the technical difficulties involved in their implementation, greatly reduces the value of transmutation.

Transmutation of minor actinides is necessarily accompanied by their prior separation, individually or in groups, from spent fuel. Separation and transmutation are therefore the two inseparable stages on which research has been intensively pursued since 1991, and which this article aims to review.