Patent Number: 047599111
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawing, it is seen that the invention is generally referred to by the numeral 10. Nuclear fuel element 10 is comprised of a plurality of nested rigid porous cylinders 12. Cylinders 12 are suitably sized in progressively decreasing circumference to allow positioning of each of the cylinders 12 within the cylinder of the next largest size. This results in a single fuel element wherein all of the cylinders 12 are in coaxial alignment. Cylinders 12 may be formed by methods known in the art such as foaming a material to form open lattices, wrapping fuel fibers, or weaving a base material. In the preferred embodiment, the pore size is in the range of from 0.5 to 5.0 millimeters. The pore size can also be varied from one cylinder to the next such as having smaller pores at the outside cylinder. This helps to achieve an optimal balance between full density, heat transfer surface area, gas velocity, and pressure drop for particular applications. The nuclear fuel distributed on each cylinder 12 may be any of the known fuels in the art such as uranium or plutonium oxides or nitrides, uranium dicarbide (UC.sub.2), uranium carbide (UC), plutonium carbide (PuC), or americium carbide (AmC). Application of the nuclear fuel to cylinders 12 may be accomplished by coating or impregnating the cylinder base material with the desired fuel. An open pore foam material which can serve as the skeleton on which the fissile material may be deposited is reticulated vitreous carbon (RVC). RVC has an exceptionally high void volume (97%), high surface area combined with self-supporting rigidity, low resistance to fluid flow, resistance to very high temperatures in non-oxidizing environments, and is available in a wide range of porosity grades. Protective coatings such as carbon or zirconium carbide for the nuclear fuel may be accomplished by means known in the art such as vapor deposition coating after the fuel has been distributed on each of cylinders 12. In the preferred embodiment, the power density of fuel element 10 is enhanced by providing varying quanities of nuclear fuel from the larger or outermost to the smaller or innermost of cylinders 12. Although the total amount of fuel in each of cylinders 12 differs from that of adjacent cylinders, the fuel is evenly distributed on each cylinder. With this fuel distribution, each cylinder is allowed to operate at its maximum power level within the heat transfer constraints existing at its radial location. The process for preparing the fuel element is as follows. First, select a reticulated vitreous carbon skeleton of appropriate pore and ligament size. Second, deposit the selected fissile material on the skeleton by vapor deposition coating. Third, subject the coated skeleton to high temperature to cause the fissile material to change to its carbide form. Fourth, deposit a protective carbon barrier layer by chemical vapor deposition. Fifth, deposit a final protective layer of zirconium carbide by chemical vapor deposition. Control of the axial and azimuthal flow distribution can be achieved by appropriately varying the coating thickness of the protective layers at the inlet side of the outermost cylinder to restrict the flow passages where lower flow is desired. In operation, coolant gas flows radially through and then axially out of fuel element 10 as indicated by the arrows in the drawing. Movement of the coolant gas through fuel element 10 transfers heat generated by the nuclear fuel to the gas. The heated gas is then directed to a suitable energy conversion system and subsequently vented to space or else the gas is cooled and the process is repeated on a continual basis. Alternately, the heated gas may be directed through a nozzle for rocket propulsion applications. As an alternate embodiment, the protective coatings described above may be deleted. Also, the fuel element may be formed directly from uranium metal foam by the process described in U.S. Pat. No. 4,560,621, then reacted with carbon to form uranium carbide, and then coated with protective layers as described above. Because many varying and differing embodiments may be made within the scope of the inventive concept herein taught and because many modifications may be made in the embodiment herein detailed in accordance with the descriptive requirement of the law, it is to be understood that the details herein are to be interpreted as illustrative and not in a limiting sense. These differing embodiments may include radial coolant flow in the outward direction and may include longitudinal flow through transverse layers or flat disk-like foam fuel members of varying fissile material content to achieve the optimal power distribution. The differing embodiment may also include annular configurations with internal neutron moderators. The differing embodiments may also include tapered or conical fuel cylinders with the outlet end flared outward to accommodate higher exit gas velocities.