Optimization of fusion-fission hybrid reactor fuel composition

One of the perceived barriers for the sustainability of nuclear energy is the issue of the high-level waste in the form of spent fuel. This issue affects the public image of the industry as well as the rentability of a nuclear operation. Strategies aimed at the reduction and utilization of spent fuel are hence desirable to reduce the radiotoxicity of the waste and reduce the volume requirement for a geological repository. There are proven technologies that can extract U and Pu isotopes suitable to be burned on thermal reactors, but the minor actinides and the nonfissile isotopes of Pu represent a problem, since they are the main contributors to the radiotoxicity of spent fuel by being decay chain heads. Their early elimination significantly reduces the activity of the spent fuel over time. Fusion-fission hybrids are being considered as an alternative for actinide burning and transmutation taking advantage of the energetic neutrons generated in the fusion reactor, in order to burn actinides present in a fission blanket around the fusion neutron source. In the present paper, operating scenarios for the Compact Fusion Neutron Source (CFNS) fed by reprocessed PWR spent fuel are considered. The goal of the simulations is to optimize the combination of coolant, matrix and fuel composition such that the maximization of both the actinide burning and the energy multiplication in the system is achieved, with the fission blanket generating excess energy after feeding the fusion neutron source; the systems is also subject to the constrain of being a subcritical assembly for safety considerations. The neutronic behavior for different combinations of gas coolants (Li, CO2 and He) was modeled using the MCNPX code using the materials and the geometry already defined for CFNS, and using typical spent fuel minor actinides concentration and composition.