Refractory Open-Cell Foam Fuel Matrix for High-Efficiency Nuclear Space Propulsion Systems

Low-density, high specific stiffness ceramic and metal open-cell foam materials may be utilized for creation of an innovative fuel matrix 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 this project, the foam fuel material will ultimately consist of a tricarbide, UZrNbC, composed of enriched uranium that is vapor infiltrated into a foam matrix of niobium carbide (NbC) and zirconium carbide (ZrC), although tantalum carbide (TaC) will be substituted for uranium carbide (UC) during initial development. The porous 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. The foam matrix can have 80-90% open porosity for maximum convection, and the ligament dimensions can be tailored to provide either good or poor thermal conduction as needed. Recent work on porous materials has demonstrated effective heat transfer coefficients with helium near 26,000 W/m2?K. NbC and ZrC foams have several important advantages over other high temperature materials, including low density, lack of degradation in hydrogen at 3000 K (where they also retain structural integrity), and minimal neutron cross-section. The dispersed fissile material is an integral part of a high-conductivity matrix. Thus, a much lower temperature difference can exist between the fuel and the propellant. Extremely high outlet temperatures can be achieved, yielding the high specific impulse required for interplanetary exploration. Ultramet will team with Sandia National Laboratories to demonstrate the feasibility of integrating porous foam fuel elements into a new reactor concept for compact, high-performance space reactors.