Joining of Tungsten Cermet Nuclear Fuel

Nuclear Thermal Propulsion (NTP) has been identified as a critical technology needed for human missions to Mars and beyond due to its increased specific impulse (Isp) as compared to traditional chemical propulsion systems. Recently, the Game Changing Development (GCD) Program, which is a partnership between NASA, DOE, and industry, was initiated to evaluate the feasibility of a low enriched uranium (LEU) NTP system. A critical aspect of NTP is to develop a robust, stable fuel. One of the fuel configurations currently being evaluated is a W-UO2 cermet. Fabrication of full-size cermet elements (>20?) has proven to be difficult. As a result, the use of cermet segments to produce a full-size fuel element is of interest. However, techniques for joining the segments are needed. During Phase I, diffusion bonding techniques were developed for producing fuel elements from cermet segments. Microscopic examination and preliminary properties testing showed excellent joints were formed. For example, quantitative tensile testing of W samples produced at 1500C HIP with a Nb interfacial coating showed the failures were in the bulk W and not at the Nb-W interfaces. Therefore, the strength of the joints were greater than the strength of the bulk W material. Using the most promising fabrication methods, a 6.3' long simulated cermet fuel element comprised of twenty-five 0.25' thick segments was produced to demonstrate proof-of-concept. During the Phase II investigation, the HIP diffusion bonding process will be optimized for making W cermet based fuel elements. This will be accomplished by performing a process parameter-characterization-properties study. The optimized fabrication methods will then be used to make prototype fuel elements with W claddings and subscale fuel elements for delivery to NASA for hot hydrogen testing.