The Defence Advanced Research Projects Agency intends to assemble a nuclear thermal propulsion (NTP) system in orbit, Aviation Week reported, citing the Pentagon's 2020 budget.
"The program will initially develop the use of additive manufacturing approaches to print NTP fuel elements… In addition, the program will investigate on-orbit assembly techniques (AM) to safely assemble the individual core element subassemblies into a full demonstration system configuration, and will perform a technology demonstration", the budget document says.
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According to the outlet, a high-assay low-enriched uranium (HALEU) propulsion system, which is being developed as a fuel source for next-generation nuclear reactors, will use the fissionable isotope uranium-235 (U-235) with a concentration of more than 5% but less than 20%. This assay of U-235 is much lower than the 90% used in US naval reactors, but also higher than the average 3-5% U-235 used in commercial reactors, allowing for smaller reactors.