A new concept for a core in a high-conversion reactor is presented. This core consists of tight lattice fuel elements enriched with plutonium. These fuel elements are made up of layers of higher enrichment (mixed-oxide fuel) and natural uranium; therefore, the core is composed of alternate layers of high- and low-enrichment fuels. The reactor with this core was nicknamed "Big Mac."

Due to spectral effects and leakage into and out of the different zones, it was found that this reactor has a high conversion ratio >0.9 with negative void coefficient, a relatively long fuel cycle, with 6% average fissile plutonium enrichments. Several interesting spectral effects were found and analyzed.