The fuel in F-1 is metallic uranium with the natural content of the 235 U isotope (0.72%), graphite as a moderator, and cadmium rods to control the neutron flux. Structurally, it is a spherical structure with a diameter of about 6 meters, made of loose graphite bricks. The graphite stack has holes in which fuel and control rods are placed, as well as research and control equipment.