[en] The reactor core is be composed of fuel elements containing both fissile and fertile materials so that the fissile material is in a zone from the horizontal midplane of the core to the one end of the fuel element. The fertile is then in a zone from the horizontal mid-plane to the other end of the fuel element. Half the elements have then fissile zones over the mid-plane of the core and the other half have their zone below the mid-plane. The second claim is that the zone of fertile materials can be situated at the primary end of the elements. (G.B.)