Advanced fuel fusion reactors: towards a zero-waste option

Most of the studies and experiments on nuclear fusion are currently devoted to the Deuterium-Tritium (DT) fuel cycle, the easiest way to reach ignition. Some of the main technological questions of future DT fusion reactors have been identified already. Among those, in particular, the radioactive inventory in such reactors is due, besides tritium, to the neutron-induced radioactivity in the reactor structures. The recent stress on safety by the world community has stimulated the research on other fuel cycles than the DT one, based on ‘advanced’ reactions, such as Deuterium-Helium-3 (DHe). Several studies have addressed the design of DHe reactors: concerning small-size near-term experiments, to begin to explore the possibilities of DHe plasmas, a DT burning plasma experiment at high magnetic field and high plasma densities is particularly attractive. Ignitor is a proposed compact high magnetic field tokamak, aimed at reaching ignition in DT plasmas and at studying them for periods of a few seconds. A design evolution of Ignitor in the direction of a reactor using a DHe fuel cycle has been proposed: a feasibility study of a high-field DHe experiment of larger dimensions and higher fusion power than Ignitor, still based on the core Ignitor technologies, has led to the proposal of the Candor fusion experiment. This paper deals with the radioactive waste issue for fusion reactors, proposing an innovative solution (the “zero-waste” option), which is a clear advantage of fusion power versus fission, in view of its ultimate safety and public acceptance. Even if feasible in theory, a zero-waste option for fusion reactors using the DT fuel cycle will be difficult to obtain. As a further step towards the zero-waste option, the features of fusion reactors based on alternative advanced fuel cycles have been examined, to assess whether that goal could be reached for such devices. Fusion reactors with advanced DHe fuel cycle turn out to have quite outstanding environmental advantages. Activation behaviour of materials after service in a DHe advanced fuel fusion experiment has been investigated. EUROFER, SiC/SiC and V-Cr-Ti materials have shown the possibility of being declassified to non-radioactive material (clearance) after their irradiation in the reactor plasma chamber wall, if a sufficient interim cooling time is allotted. AISI 316L, on the contrary, suffers the presence of Ni and N (alloying elements) and Nb and Mo (impurities).