An Axially Heterogeneous Sodium-Cooled Fast Reactor Designed to Transmute Minor Actinides

An axially heterogeneous sodium-cooled fast reactor design is developed for converting minor actinide waste isotopes into plutonium fuel. The reactor design incorporates zirconium hydride moderating rods in an axial blanket above the active core. The blanket design traps the active core's axial leakage for the purpose of transmuting ²⁴¹Am into ²³⁸Pu. This ²³⁸Pu is then co-recycled with the spent driver fuel to make new driver fuel. Because ²³⁸Pu is significantly more fissionable than ²⁴¹Am in a fast neutron spectrum, the fissile worth of the initial minor actinide material is upgraded by its preconditioning via transmutation in the axial targets. Because the ²⁴¹Am neutron capture worth is significantly greater in a moderated epithermal spectrum than the fast spectrum, the axial targets serve as a neutron trap that recovers some of the axial leakage lost by the active core.

A low transuranic conversion ratio is achieved by a degree of core flattening that increases axial leakage. Unlike a traditional "pancake" design, neutron leakage is recovered by the axial target/blanket system. This heterogeneous core design is constrained to have sodium void and Doppler reactivity worth similar to that of an equivalent homogeneous design. Contrary to a homogeneous design, concentrating minor actinides (MAs) in an axial blanket mitigates the problem of above-threshold multiplication during sodium voiding. Because minor actinides are irradiated only once in the axial target region, elemental partitioning of the minor actinides from plutonium is not required. This fact enables the use of metal targets with pyroprocessing. After reprocessing, the target's newly bred ²³⁸Pu and remaining unburned MAs become the feedstock for the next batch of driver fuel.