Refractory Metal Coated Uranium Nitride Fuel for Nuclear Thermal Propulsion

NTP is a critical technology needed for human missions to Mars due to its high specific impulse (Isp). To reduce cost and potential burdensome security and handling requirements, low enriched uranium (LEU) fuel is desired. The high uranium density of uranium nitride (UN) over uranium oxide (UO2) favors the use of UN for the LEU option. However, similar to UO2, techniques are needed to produce refractory metal coatings on the UN particles to allow fabrication of the cermet fuel element and to protect the UN from the hydrogen propellant. During this investigation, techniques for producing spherical UN particles along with methods for producing refractory metal coatings on the spherical nitride particles were developed. To facilitate development of the spheroidization process and subsequent refractory metal coating of the particles, UN surrogate materials were used. Characterization of these materials showed the as-received nitride powders were comprised of angular particles, which is the morphology of the current UN powders. To produce spherical particles for subsequent coating, plasma processing techniques were used. Analysis of the plasma treated nitride particles demonstrated the ability to produce spherical nitride powder with significant improvements in flowability. Using these powders, the ability to refractory metal coat individual nitride particles was demonstrated, and characterization confirmed continuous, uniform coats were produced. During Phase II, the techniques will be optimized and scaled for producing kilograms of powder per run. Coated particles will be produced and used to make cermet based fuel segments for testing at NASA. At the conclusion of the Phase II effort, refractory metal coated nitride powder will be delivered to NASA for producing an NTR Element Environment Simulator (NTREES) size element. To commercialize the materials and techniques, Plasma Processes will collaborate with BWXT and NASA during Phase II. NTP is a critical technology needed for human missions to Mars due to its high specific impulse (Isp).  To reduce cost and potential burdensome security and handling requirements, low enriched uranium (LEU) fuel is desired.  The high uranium density of uranium nitride (UN) over uranium oxide (UO2) favors the use of UN for the LEU option.  However, similar to UO2, techniques are needed to produce refractory metal coatings on the UN particles to allow fabrication of the cermet fuel element and to protect the UN from the hydrogen propellant. During this effort, techniques for producing spherical UN particles along with methods for producing refractory metal coatings on the nitride particles were developed.  Characterization showed spherical nitride powders can be produced using plasma processing and significant improvements in flowability were achieved. PVD techniques were then used to produced tungsten coatings on the powders and analysis showed complete encapsulation of the nitride particles with W, i.e., 0.5-3µm thick continuous W coatings were achieved. During Phase II, the spheroidization and coating techniques will be optimized and scaled for producing kilograms of powder.  UN surrogate materials will be procured and screened to different sizes. To minimize satellite formation, fine particles from the feedstock will be removed. Plasma processing parameters will be optimized to maximize spheroidization and minimize vaporization. Nitrogen plasmas will be evaluated to maintain stoichiometry and for tailoring the composition of the plasma spheroidized nitride powders. PVD process for producing uniform W and Mo coatings on nitride powders will be optimized. Refractory metal coatings will be evaluated to prevent hydrogen reaction with the underlying nitride particles at high temperature. Powders will be consolidated to produce samples for testing at NASA. Materials in the as-received, plasma processed, refractory metal coated, and consolidated conditions will be characterized to determine the relationships between the different processing parameters on morphology, coating quality, composition, and microstructures. Optimized processing methods will produce refractory metal coated nitride powders for delivery to NASA for producing an NTREES size cermet element. The materials and processing techniques for NTP and other advanced applications will be commercialized with BWXT, NASA, and other customers.

The proposed technology supports NASA’s GCD Program and directly benefits Nuclear Thermal Propulsion (NTP) and Nuclear Electric Propulsion (NEP).  Space nuclear power and propulsion are game changing technologies for space exploration.  Potential NASA missions include rapid robotic exploration missions throughout the solar system and piloted missions to Mars and other destinations such as near earth asteroids. Commercial sectors that will benefit from this technology include medical, power generation, electronics, defense, aerospace, chemicals, and corrosion protection.  Targeted commercial applications include refractory metals for rocket nozzles, crucibles, heat pipes, propulsion components, sputtering targets, turbines, rocket engines, nuclear power components, and powder for additive manufacture.

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