[en] A fast reactor is proposed whose core is divided into two zones. The first zone in the direction of coolant flow contains fuel elements of metallic fuel, eg., uranium, ie., higher density fuel. The second zone contains ceramic fuel of lower density, eg., plutonium fuel. The first zone reaches 0.3 to 0.7 of the entire core height. The first zone fuel elements have greater cross section. In a second variant, the cross section and the material of fuel elements are the same along the entire core height but the fuel in the first zone has higher enrichment than that in the second zone. These fast reactor core configurations result in a higher breeding ratio which significantly improves the reactor economy and reduces the doubling time. (Z.M.)