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Completed TRL 2 (started at 2, targeting 3)
Project Objective
The current work intends to evaluate the performance of different cooling channel spacings in nuclear fuels while exposed to hot hydrogen gas. In current Space Nuclear Propulsion concepts, hydrogen gas is the coolant and propellant in the system; thus, nuclear fuel and other reactor materials must be compatible with hydrogen at temperatures above 2500K.
Project Description
Previous tests on nuclear fuels (ceramic/metallic) at high temperatures have demonstrated the formation of macrocracks between cooling channels. The crack formation on refractory carbide fuel can be attributed to the residual stresses developed during ramp up and cool down. An increased spacing between cooling channels can potentially withstand residual stresses during testing in hot hydrogen gas.
ZrC was down selected as surrogate for nuclear fuel (U,Zr)C. A low-density (67.75% TD) group of ZrC samples was fabricated to match the density of modern approaches in additive manufacturing/3D printing of the same material [1]. A high-density group of samples was produced with spark plasma sintering and achieved densities in the vicinity of 97.1% TD. Subsequently, ultra sonic drilling was employed to incorporate cooling channels with distances of 1.3, 2, and 3 mm (Figure 1). The experimental approach intended to expose all samples for 30 minutes to prototypical temperature and hydrogen environment inside a nuclear thermal engine.
Project Results and Conclusions
All samples (low- and high-density groups) were exposed to hot hydrogen at 6 standard liters per minute for 30 minutes at 2600K, and the samples did not display any cracking upon completion of the test. The high-density group displayed comparable mass losses at the 30-minute interval to those of refractory carbide materials under similar test conditions [2]. A high-density sample with the least cooling (1.3 mm) channel spacing was exposed to hot hydrogen for 3.5 hours until it developed a crack across the cooling channels.
Initial burn out of impurities and free carbon leads to highest mass loss rate during first 30 minutes. Subsequent test intervals display reduced mass losses as the sample stabilizes under elevated temperatures. Scanning Electron microscopy was performed on sample on all samples before and after the test. Microscopy analyses revealed thermal etching on grain structure of the tested samples.
The high-density group reported negligible density changes after 30 minutes of testing. A 4.65% increase in density was reported on the low-density group and is attributed to further consolidation during the test. The sample density decreased from 97.10%TD to 94.17%TD after the 3.5 hours of hot hydrogen exposure. The residual stress influence on cooling channel spacing is material dependent, and further testing is planned with uranium bearing compounds to better understand the mechanical performance of refractory carbide fuel.
References:
Results from this work provide preliminary data on the performance of ZrC as a surrogate for (U,Zr)C nuclear fuel. Samples with cooling channel distances of 1.3, 2 and 3 mm survived 30 minutes of hot hydrogen exposure. A sample with the shortest cooling channel distance of 1.3 mm was exposed to 3.5 hours when crack formation began. These results aim to demonstrate the stress limitations of ultra-high temperature ceramics and nuclear fuels at the temperature and hydrogen environment of a nuclear thermal engine. Further testing with fuel material is intended to be performed to gain a better understanding of the mechanical performance of this material in prototypical SNP environments. Space Nuclear Propulsion (SNP) research is an area of interest for NASA’s Science and Technology Mission Directorate (STMD) Strategic Framework.
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