SBIR-STTR Award

A new carbon-based fuel is introduced with outstanding potential to eliminate the loss of uranium, minimize the loss of carbon, and retain fission products for many hours of operation in hydrogen environment at temperatures in excess of 3,200K. The proposed fuel is a Cer-Cer made of mixed uranium-refractory carbide particles such as (U, Zr)C or (U, Zr, Nb)C dispersed in a refractory carbide matrix such as ZrC. For efficient operation in NTR applications for Isp of 1000 sec. or more, a fuel temperature of 3000 K or greater is necessary. Various fuel materials have been tested for NTR applications with most based on carbide fuel technology because of their improved thermal properties enabling the design of very small, high power density cores. Fuel designs from dispersed microspheres in graphite, to composite mixed carbides with graphite, to solid solution mixed carbides have been tested. Fuels bearing graphite are not tenable because of the high reactivity of free carbon with the hot hydrogen propellant. Solid solution, mixed carbides are most often brittle but otherwise perform well under the high temperature flowing hot hydrogen environment. The life limiting phenomenon for their use in NTR applications is the loss of uranium due to vaporization from the fuel surface at temperatures in excess of 2800 K. Though the proposed Cer-Cer fuel is relatively at lower level of technology maturity, its unique potential for elimination of uranium loss and retention of fission fragments at very high operational temperatures would amply justify the proposed research program.

In this proposal a new carbide-based fuel is introduced with outstanding potential to eliminate the loss of uranium, minimizes the loss of uranium, and retains fission products for many hours of operation in hydrogen environment at temperatures in excess of 3,200K. The proposed fuel is a ceramic-ceramic (CerCer) composite of mixed uranium-refractory carbides such as (U, Zr)C or (U, Zr, Nb)C in a matrix of refractory carbides that mostly include transition metal carbides such as ZrC, NbC, TaC, and HfC. Due to its low neutron absorption cross-section, ZrC is the primary refractory carbide of choice. Replacing ZrC with higher temperature refractory carbides such as TaC and HfC could further improves the high temperature performance of CerCer fuels. However, higher neutron absorption cross-section penalty for Ta and Hf could potentially offset the performance enhancement gain. Due to complete containment and encapsulation of mixed uranium carbide in zirconium carbide matrix, the proposed CerCer fuel could be conveniently fabricated to different geometrical shapes such as solid block prismatic, twisted ribbon, pebbles, wafer, or square lattice honeycomb. Considering the operational parameters for the NT/BP systems, it is reasonable to argue that the proposed CerCer fuel concept could set the upper material performance limits while providing more flexibility in the geometrical design of the fuel.