The management of the high-level radioactive waste (HLW) generated by the current fleet of light water reactors (LWR) is one of the most important issues that needs to be addressed. Due to the long-lasting radiotoxicity of the HLW this material needs to be isolated for thousands of years if directly disposed into a deep geological repository. Although minor actinides (MA)(neptunium, americium and curium) constitute a small fraction of the HLW, they are largely responsible for the long-lasting radiological toxicity and heat produced by the waste (Figure 1). For example, the heat generated by 241Am strongly influences the size of the repository to dispose HLW. Therefore, the destruction of these transuranic elements into stable or shorter-lived isotopes (transmutation) by further irradiation is of great benefit as it can reduce the long-term radiotoxicity of the waste and extend the repository capacity.
Under neutron irradiation MA can undergo neutron capture or fission reaction depending on the energy of the neutrons. Neutron capture is generally avoided as only produces heavier actinides, whereas fission reactions produce fission products with shorter half-lifes and lower radiotoxicity. In LWR, with neutron energies lower than 0.1 eV, MA mainly undergo neutron capture. In contrast, in Fast Reactors fission reactions are promoted by a factor of 2-6 depending of the isotope. Despite this improvement, MA fission rates can be between 15-30%, meaning that the fuel needs to be recycled a few times to ensure high performance levels. However, due to the impact of MA in the reactor core safety their concentration needs to be limited to 5% in an homogeneous cycle.
One dedicated system for the transmutation of MA is the accelerator driven system (ADS). Contrary to the fuel of the Fast Reactor, the fuel of the ADS can contain 30 wt% MA, 20 wt% Pu and 50 wt% ZrN as dilution material. It has been suggested that an 800 MWth ADS is capable of transmuting 500 kg of MA in 600 effective full power days, equivalent to the MA produced in 10 units of LWR with 1GWe.