An active zone of a reactor, a fuel element and a fuel assembly for creating said active zone are intended for use in fast reactors with a nitride fuel and a liquid-metal coolant primarily in the form of molten lead and alloys thereof. The problem addressed by the invention consists in creating an active zone with a void reactivity effect and with effective power flattening along the radius of the active zone. The active zone comprises a homogeneous uranium-plutonium nitride fuel, the mass fraction of which in the active zone is a minimum 0.305, and consists of 3 parts - a central part, an intermediate part and a peripheral part which form fuel assemblies comprising fuel elements with geometrically identical shells but with a differing height of the fuel column in the central part, the intermediate part and the peripheral part of the active zone, wherein the radial distribution of the fuel across the volume of the active zone is characterized by a stepped shape in a longitudinal section. The radius of the central part of the active zone is from 0.4 to 0.5 of the effective radius of the active zone, while the height of the fuel column in the fuel elements of the fuel assemblies in the central part is from 0.5 to 0.8 of the height of the fuel column in the fuel elements arranged in the fuel assemblies in the peripheral part of the active zone. The heights of the fuel columns forming a stepped intermediate part (with at least one step) for diameters ranging from 0.5 to 0.85 of the effective diameter of the active zone are selected within the range from 0.55 to 0.9 of the height of the fuel column in fuel elements arranged in the fuel assemblies in the peripheral part of the active zone.