Patent Number: 053176111
Section: summary

BACKGROUND 1. Field of the Invention This invention relates to fuel elements and assemblies thereof for nuclear thermal engines. 2. Background of the Invention Nuclear thermal engines utilize fissionable material to heat a propellant, typically hydrogen, which is discharged through a nozzle to generate thrust. A design objective in such engines is to achieve a high thrust-to-weight and specific impulse. This requires a core design featuring a compact configuration with high heat transfer area, a high coefficient of heat transfer and optimum flow rate and flow velocity. An early design for nuclear thermal engines used prismatic fuel elements in the form of hexagonal rods stacked side by side with a pattern of longitudinal bores in each rod through which propellant flowed axially. Such fuel elements were subject to stress induced cracking between the bores as a result of the very high thermal stresses occurring during operation. An experimental particle bed fuel element utilizes small fuel beads packed in an annular support. Propellant flows radially inward through a porous peripheral wall, the particle bed and a porous annular inner wall, and then flows axially out through a central passage. These particle bed fuel elements can develop hot spots causing melting of the fuel beads. Thus, while the particle bed fuel elements have a high heat transfer capability, it is difficult to control propellant flow through them and they have poor mechanical stability. Another proposed design utilizes flat annular plates with radial grooves or holes for propellant flow radially inward to a central discharge passage. However, the very high temperature gradient, for example 100.degree. K at the periphery and 3000.degree. K at the central discharge passage, produces severe stress cracking in the flat, annular plates. A suggestion has been made to utilize fueled truncated conical shells seated on one another with grooves in the contacting confronting shell surfaces for coolant flow. See, also, U.S. Pat. No. 3,150,054 to Fox, which shows a truncated conical reactor design. There is a need therefore for improved fuel elements and assemblies for nuclear thermal engines. SUMMARY OF THE INVENTION It is the primary object of the invention to provide an improved fuel element and assembly for nuclear thermal engines, and in particular for nuclear thermal rocket propulsion engines. It is a more particular object of the invention to provide such improved fuel elements and assemblies having high heat transfer area, high power density without hot spots, and in which the flow is readily controllable. It is also an object of the invention to provide such improved fuel elements and assemblies which are modular so that different engines with different thrust ratings can be easily fabricated. It is another object of the invention to provide such improved fuel elements and assemblies which can be constructed with existing technology. These and other objects are realized in fuel elements of the type having a truncated conical shell tapering inward from a base, typically at an angle of between about 30.degree. to 60.degree. and preferably about 45.degree. to 60.degree.. Conical shell fuel elements embodying the present invention have an unfueled annular lip extends radially outward at the base of the truncated conical shell, and has radial passages through which propellant flows inwardly. The annular lips of multiple fuel elements stack one on top of another to form a stack of fuel elements with frusto-conical flow passages between adjacent elements Angularly spaced, radially extending ribs on one surface, preferably the outer surface, of the truncated conical shells of the fuel elements form propellant flow channels in the frusto-conical flow passages between elements While not sized to contact the adjacent fuel element in the stack, the ribs maintain the spacing between elements distorted by the heat. The truncated open ends of the shells are free to expand and contract independently without creating significant stresses. The stack of truncated conical shell fuel elements is supported in a fuel assembly which comprises a cylindrical housing with a central inlet opening at one end and a central outlet at the opposite end. The stack of fuel elements is mounted in the housing with an annular flow distribution channel between the periphery of the stack and the housing. A central baffle spaced from the inlet opening deflects propellant entering through the inlet opening to the annular flow distribution channel through which it flows axially and then radially inward through the passages in the lips, over the truncated conical shells through the channels formed by the ribs, and then axially out through the centers of the truncated conical shells and the outlet opening. An annular spring maintains alignment of the stack of fuel elements and takes up all axial tolerances in the stack of fuel elements. When mounted at the inlet end, this annular spring has openings through which propellant passes axially into the annular flow distribution channel.