Refractory Open-Cell Foam Fuel Matrix for High-Efficiency Nuclear Thermal Propulsion
- Friday, 01 August 2014
A fuel form for fission applications is functional at extremely high temperatures with minimal erosion or fission product losses.
Marshall Space Flight Center, Alabama
A tricarbide foam fuel material has been developed that can operate at temperatures near 3,000 °C, without substantial hydrogen erosion, while providing highly efficient heat transfer to the coolant or propellant. A tricarbide foam fuel matrix of zirconium carbide (ZrC), niobium carbide (NbC), and uranium carbide (UC) has been successfully deposited and hydrogen tested. It shows that high-temperature, high-porosity foams can be produced that resist hydrogen corrosion and prevent the diffusion of fission products from the matrix. Chemical vapor deposition (CVD) technology was applied to nuclear materials systems that may be used in thermal propulsion and very high-temperature gas reactors.
The foam fuel concept is a new fuel form for fission applications. In addition to operating at extremely high temperatures, and with minimal fission product loss, this foam fuel can survive a complete loss of coolant. This is a large gain in the area of safety for the reactor system. This design enables high-temperature reactor operations for hydrogen production, and for nuclear-thermal propulsion. The tricarbide foam fuel is mechanically robust and very efficient given its extremely high surface area, high melting point, low thermal stresses, and much reduced pressure drop compared to conventional fuel types. This innovation is an engineered material in which the porosity, size, and thermal conductivity of the ligaments can be controlled independently to meet specific requirements.
Extensive design and analysis work indicates that the foam fuel matrix can be used in a compact, high-power-density core with great efficiency. Advances in multiphysics computational fluid dynamics modeling of the foam structure on the micro scale yielded optimal foam properties concerning porosity, ligament size, and ligament thermal conductivity. High-power-density fuels can be maintained well below the melting point at reasonable flow velocities and power densities necessary to produce the high thrust required for nuclear-thermal propulsion.
The processing for fabrication of both a layered and a solid eutectic mixture of surrogate tricarbide (TaZrNb)C in foam form has been demonstrated. The ability to deposit the three carbides simultaneously, and adjust their individual concentrations to approach the desired levels, was one of the primary goals of the process development effort. The overall composition and uniformity still needs optimization relative to the precise control needed for tricarbide fuel, but the fact that all three materials were depositing, and as a dense structure, is quite positive.