What if spinning molten uranium at thousands of revolutions per minute held the secret to halving the travel time to Mars? That’s what engineers at The Ohio State University are counting on. Their centrifugal nuclear thermal rocket, or CNTR, will try to expand the limits of propulsion physics by substituting the solid fuel components of conventional nuclear thermal propulsion (NTP) with a liquid uranium core a change that has the potential to bring unprecedented efficiency to deep-space travel.
The design extends from decades-old NTP designs, in which a fission reactor heats up a propellant, usually hydrogen, which expands through a nozzle to create thrust. During the 1960s, the Rover and NERVA programs obtained specific impulses near 900 seconds, about twice the optimum of chemical engines. The CNTR design, though, capitalizes on the increased heat transfer of liquid fuel in superheating hydrogen to temperatures close to 5,000 kelvins, possibly extending specific impulse to 1,500–1,800 seconds. This jump would enable spacecraft to go further on less fuel, making possible six-month one-way crewed missions to Mars and quicker robot missions to the outer planets.
In the CNTR, liquid uranium is held in a fast-spinning cylinder. Centrifugal pressure holds the heavy fuel against the outside wall, and hydrogen is bubbled through it, picking up heat before being released. This layout eliminates solid fuel rod thermal limits, but presents daunting engineering hurdles. Startup and shutdown must not induce reactor instabilities. Hydrogen interaction with structural material at very high temperatures can lead to corrosion and embrittlement. Most importantly, uranium vaporization can bleed fissile material into the exhaust, degrading reactivity and cutting performance.
To meet this, Ohio State researchers are investigating dielectoresis employing electric fields to trap and redirect vaporized uranium into the core. Even at a 99 percent recovery rate, losses would affect mission success. Another obstacle is maintaining reactivity under differing thermal conditions. The small CNTR core, with 37 centrifugal fuel elements and 12 control drums, is moderated by zirconium hydride, which can introduce a positive moderator temperature coefficient, a safety problem. The effect is being eliminated by doping the moderator and reflector with a small percentage of erbium-167, enhancing stability with a flat power distribution.
Thermal control is similarly important. Monte Carlo calculations indicate that approximately 95 percent of fission heat is left in the liquid fuel, but over 2 percent is taken up by the moderator, which has to be cooled to avoid hydrogen loss above 1,150 kelvins. Control drum materials, charged with boron carbide, have also got high localized heating from neutron capture reactions, for which special cooling measures have to be adopted.
The CNTR’s neutronic design has been optimized by sensitivity studies varying fuel element spacing, reflector thickness, and poison loading to maximize the effective neutron multiplication factor (k_eff) and reduce power peaking. These variations, along with genetic algorithm optimization, have resulted in modelled performance in excess of 1,500 seconds of specific impulse but only with idealized assumptions demanding greater rotation rates and extra centrifuges.
Operational lifespan adds another degree of complexity. Fission products such as xenon-135 and samarium-149, with large neutron absorption cross sections, can “poison” the reactor, decreasing k_eff. In conservative assumptions where all fission products are still within the core, xenon accumulation upon shutdown can prohibit restart for up to 48 hours, whereas samarium buildup might restrict lifetime to 10 hours of operation. In practice, high temperatures will enable many of the fission products to diffuse into hydrogen bubbles and be carried out through the nozzle, preventing these effects but this must be experimentally confirmed.
Complementary studies at the Oak Ridge National Laboratory have experimented with zirconium carbide coatings to protect fuel and core material from hydrogen invasion, utilizing the INSET 2.0 high-temperature irradiation furnace at the research reactor at Ohio State. The coatings may prove critical to CNTR survivability under harsh thermal and radiation environments.
In funding and collaborations from NASA, as well as from BWX Technologies, Ohio State aims to advance CNTR design to a mature state in five years. It would be available for mission architectures new to us from direct injection to the outer planets to resource utilization in the asteroid belt and reduce crew exposure to cosmic radiation. “We need to keep space nuclear propulsion as a consistent priority in the future, so that technology can have time to mature. It’s a huge benefit that we can’t afford to miss out on.”
