Patent Number: 043671969
Section: claims

1. A neutronic reactor comprising a core consisting of a plurality of elongated, parallel fuel elements, perpendicular to a midplane of the reactor, containing thermal-neutron-fissionable material and provided with an inlet at one end and an outlet at the other end for coolant, and a plurality of elongated control elements, parallel to each other and to the fuel elements, each consisting of a housing, and a group of control rods which are separately translatable within their housing, each of said groups containing at least one control rod which is substantially shorter than the fuel elements and is located so that its midpoint is downstream, with respect to the flow of coolant through the fuel elements, from the said midplane of the reactor, the remainder of the control rods within a group being of substantially the same length as the fuel elements, said fuel elements and control elements being symmetrically disposed within a body of moderator material. 2. A neutronic reactor according to claim 1 wherein the control elements are arranged in concentric rings about a central control element. 3. A neutronic reactor according to claim 2 wherein at least half the control rods within each control element are constructed of an aluminum-lithium alloy in such a way that they will not absorb all incident neutrons and at least one of the control rods is constructed of cadmium in such a way that it will absorb all incident neutrons. 4. A neutronic reactor according to claim 3 wherein each control element contains a control rod at its center and a plurality of control rods annularly disposed thereabout and the central rod and one of the outer rods are the said substantially shorter control rods. 5. A neutronic reactor according to claim 4 wherein each control element contains two cadmium control rods of the same length as the fuel elements, three lithium-aluminum control rods of the same length as the fuel elements, and two short lithium-aluminum control rods. 6. A neutronic reactor according to claim 5 wherein the fuel elements are vertically disposed and the flow of water past the fuel elements is from top to bottom and the substantially shorter control rods are located with their midplanes located below the midplanes of the fuel elements. 7. A neutronic reactor comprising a core consisting of a plurality of elongated, parallel fuel elements containing thermal-neutron-fissionable material, and a plurality of elongated control elements, parallel to each other and to the fuel elements, each consisting of a housing and a group of control rods separately translatable within their housing, said control elements being symmetrically arranged in concentric rings about a central control element within a body of a moderator material, all control elements within each concentric ring being connected in gangs, means for operating all control rods occupying the same position in each control element within a ring simultaneously and for sequentially withdrawing the said gangs of control rods from the reactor.