Production and Hot Hydrogen Testing of Molybdenum Matrix Cermet Fuels for Nuclear Thermal Propulsion

NASA’s future goals require development of technologies which can expand human presence beyond low earth orbit (LEO) and into the solar system “[to] advance exploration, science, innovation, benefits to humanity, and international collaboration" [1]. As a part of fulfilling this objective, NASA plans to conduct manned deep space missions, in the 2030s. Through the use of a hydrogen propellant, nuclear thermal propulsion (NTP) is a non-chemical propulsion technology capable of high specific impulse (850 - 900 s) and reasonably thrust (100 – 1,100 kN) allowing for reduced trip times for crewed missions to beyond LEO. Neutronic analyses have predicted that low enriched uranium (LEU) fueled nuclear thermal rockets (NTRs) with <20% ²³⁵U enrichment can be designed based upon legacy fuel systems and allow for comparable performance to high enriched uranium (HEU) alternatives [2]. LEU engine designs are expected to significantly reduce the high maintenance cost and perceived political hurdles of developing NTP systems traditionally associated with HEU fuel systems. The successful development of a LEU engine requires the affordable production and qualification of a fuel form which allows for operation in excess of 2500 K, resists interaction with the hydrogen propellant, and has the nuclear properties to enable engine criticality with reduced uranium enrichment.

Through this CIF project, molybdenum (Mo) matrix cermets will be produced via an innovative direct current consolidation technique and hot hydrogen tested using surrogate materials in place of the uranium dioxide (UO₂) particles. The objective of this CIF is to identify the maximum use temperature and characterize fuel system behavior under steady state and thermal cycling conditions for low absorbing Mo matrix fuels as an alternative to tungsten (W). Maximum use temperature can be utilized by engine designers and demonstrates the suitability of Mo type matrix fuel for NTP applications as an alternative to the current baseline of W matrix fuels. Development of alternative NTP fuel matrix candidates supports the affordable development of LEU NTP by mitigating anticipated risks associated with W-184 enrichment required by the baseline NTP engine concept. The end goal is to use CIF funding to demonstrate the feasibility of this low TRL technology to potential stakeholders (i.e. NTP, Kilopower, etc).

Previously developed to support the U.S. nuclear engine for rocket vehicle application (NERVA) program, ceramic metallic (cermet) fuel systems are composed of uranium dioxide (UO₂) ceramic nuclear fuel dispersed within a structural metal matrix. Tungsten (W) is commonly used as a matrix material because its high melting temperature leads to the potential for the highest operating temperatures. However, W requires significant enrichment of W-184 in order for the engine to attain criticality due to the high thermal neutron absorption cross section of other W isotopes. Molybdenum (Mo) is the preferred metal matrix alternative to W due to its relatively high melting temperature, low neutron absorption properties, and compatibility with UO₂ and the hydrogen propellant throughout the range of operating temperatures required for an NTP engine. The reduced ductile to brittle transition temperature of Mo compared to W is also desirable to allow for increased ease of fuel fabrication and allow for a more ductile response in the midband regime, where models have shown cermet fuels are most susceptible to failure. To enable a LEU cermet engine while reducing the required amount of enriched W-184, Mo and W-Mo alloys may be suitable replacements for W at the colder regions of the engine core, including both near the inlet and midband region. Additional anticipated benefits of Mo cermets include a larger pre-existing irradiation test database than W-alloys, which can be used to validate multiphysics modelling and simulation of NTP fuels and guide production efforts. 

Mo cermet fuels production has been previously investigated for space power applications. However, existing test data for Mo cermets is limited and maximum use temperature of Mo for NTP applications is not well understood. For example, Lenz and Mundinger tested Mo cermets for thermal cycling conditions with peak temperatures of 2350°C in a hydrogen atmosphere and reported greater UO₂ fuel mass loss than W cermets despite being a more ductile matrix. Homan reported Mo should not be used in NTP fuel systems for temperatures exceeding 2200°C due to low vapor pressure, which could result in reduced operating temperature of W-Mo alloys or localized cladding stresses and potential fuel failure during temperature transients.

The purpose of this CIF is to investigate Mo cermet behavior and maximum operating temperature potential, utilizing advanced cermet consolidation techniques followed by subscale hot hydrogen testing. Fuel material samples (20 mm diameter; 6 mm thick) will be fabricated via direct current sintering using blended Mo-ZrO₂ powders. This work is the first-of-its-kind in the area of direct current consolidation of Mo matrix cermet fuels. Testing will be completed using MSFC’s compact fuel element environmental test (CFEET) facility to allow for test temperatures in excess of 2000 K in the presence of hot flowing hydrogen. Test data will provide a baseline understanding of the fuel system’s thermodynamic stability and identification of failure mechanisms to provide risk assessment data for engine designers. Qualification of fuel lifetime-temperature relationships (fuel endurance) under steady state and thermal cycling conditions will be established. In order to accomplish this task, Mo cermets will be produced using direct current sintering with zirconium dioxide surrogate fuel particles. Since W cermets have been previously produced by MSFC, powder coating optimization and consolidation is expected to be accomplished quickly within the beginning of the project. Hot hydrogen testing will not allow for an assessment of all responses under true prototypic conditions, such as high pressure hydrogen, stress gradients incurred during operation, or irradiation response, to be attained, but will nevertheless provide key feasibility data regarding potential maximum operating temperature. The ultimate goal of this activity is to determine the maximum use temperature of Mo cermets demonstrate feasibility. Following year efforts could include demonstration of Mo-UO₂ and W-UO₂ cermet production and testing to compare performance of cermet metal matrix candidates for a range of operating temperatures expected of the engine.

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