Refractory Open-Cell Foam Fuel for High-Efficiency Nuclear Space Propulsion, Phase II

Low-density, high specific stiffness ceramic and metal open-cell foams may be utilized for creation of an innovative fuel element for use in space nuclear reactors. Highly porous structural foam may be developed for high thermal efficiency, high temperature fuel elements for both propulsion and gas-cooled power reactors. Current designs, such as annular rods or pebble beds, cannot operate at extremely high temperatures and thus limit efficiency. In the proposed Phase II project, the foam fuel element material will consist of a tricarbide, U(ZrNbC), composed of uranium carbide that is vapor infiltrated into a foam matrix of niobium carbide and zirconium carbide (in Phase I, tantalum carbide was substituted for uranium carbide). The porous foam structure provides an extended surface area for highly efficient heat transfer and reduces density, reducing hydrogen turbopump power demands and increasing thrust-to-weight ratio. In Phase I, Ultramet teamed with Sandia National Laboratories to demonstrate the feasibility of these highly innovative foam fuel elements. This work included chemical vapor infiltration of the carbides into an open-cell foam skeletal structure by Ultramet and fabrication of a matrix of foam densities for thermal/gas flow testing at Sandia. The results of flow testing, coupled with the results of thermomechanical modeling performed at Sandia, confirmed the high potential of this design following continued development. In Phase II, Ultramet and Sandia propose to develop and demonstrate fabrication of the actual (U-Zr-Nb)C foam fuel material. Ultramet will finalize process optimization using the (Ta-Zr-Nb)C material and provide the processing conditions to Sandia which, in conjunction with the University of Florida Institute of Space Power and Propulsion (INSPI) facility, will produce and test (U-Zr-Nb)C foam in a hot hydrogen environment. Sandia will also perform comprehensive thermomechanical modeling, expanding on the modeling performed in Phase I.