Nuclear tech in space: What’s on the horizon?

November 4, 2020, 12:12PMNuclear News

Illustration of a Mars transit habitat and nuclear electric propulsion system. Image: NASA

NASA aims to develop nuclear technologies for two space applications: propulsion and surface power. Both can make planned NASA missions to the moon more agile and more ambitious, and both are being developed with future crewed missions to Mars in mind. Like advanced reactors here on Earth, space nuclear technologies have an accelerated timeline for deployment in this decade.

Space nuclear propulsion and extraterrestrial surface power are getting funding and attention. New industry solicitations are expected this month, and a range of proposed reactor technologies could meet NASA’s specifications for nuclear thermal propulsion (NTP). Nuclear electric propulsion could increase the feasibility of crewed missions to Mars with a shorter transit time, a broader launch window and more flexibility to abort missions, reduced astronaut exposure to space radiation and other hazards, expanded payload mass capabilities, and reduced cost.

What we’re watching: NASA’s current work aligns with Space Policy Directive-1, issued by President Trump in 2017. In this election year, it’s worth noting that the agency’s goals may be shifted, changed, or retained by the president who is inaugurated in January.

  • Two weeks ago NASA and the Department of Energy signed a memorandum of understanding that called for three working groups, including a group on space nuclear power and propulsion, to report within six weeks on the development of a multibillion-dollar plan for research, development, and testing of nuclear propulsion systems and a lunar base power supply and grid systems.
  • NASA anticipates that the DOE will release two industry solicitations this month. A “Fission Surface Power System Design Solicitation” would follow a request for information announced in July and developed by NASA, the DOE, and Idaho National Laboratory. Targeting launch readiness by the end of 2026, fission surface power design goals include a power output of not less than 10 kW at the interface end of a 1-kilometer cable, and a flight system mass of 2,000–3,500 kg.
  • A separate “Nuclear Thermal Propulsion Industry Solicitation” would follow both a draft request for proposals (RFP) issued in August and a recently concluded 11-month Flight Demonstration Study that saw four companies submit reactor designs.
  • The National Academies of Sciences was asked to conduct a Space Nuclear Propulsion Technologies study to “identify primary technical and programmatic challenges, merits, and risks for developing and demonstrating space nuclear propulsion technologies of interest to future exploration missions [. . . and] also determine the key milestones and a top-level development and demonstration roadmap for each technology.” The NAS committee leading the study has concluded a series of 13 meetings that began in May 2020 and is to release a report in the spring of 2021.

More on propulsion: Nuclear power could propel a spacecraft using thermal or electric propulsion, and the near-term option is NTP. Through a flight demonstration, NASA wants to verify NTP capabilities and demonstrate regulatory processes. The agency isn’t ready to commit to a particular design, but it is looking for a high-temperature, 500-MW thermal reactor that could heat hydrogen propellant and offer a specific impulse of 900 sec.

Specific impulse is a ratio—the change in momentum per unit mass of propellant—that indicates engine efficiency. Generally, the lighter the fuel or the hotter an engine, the greater the specific impulse. The specific impulse of a chemical rocket that combusts liquid hydrogen and liquid oxygen is 450 seconds, half the 900 second target for nuclear-powered rockets. The increase in specific impulse from NTP comes from the lower weight of the hydrogen exhaust, which is easier to accelerate than the water vapor from a chemical engine.

A nuclear electric propulsion system, on the other hand, would use its nuclear fuel to produce electricity, and then generate thrust by ionizing inert gas propellants (such as xenon and krypton) and accelerating the ions using a combination of electric and magnetic fields or an electrostatic field.

Industry input: NASA recruited Analytical Mechanics Associates (AMA) to manage a Flight Demonstration Study with defined performance parameters that solicited NTP reactor designs from industry.

The eleven-month study officially concluded in August, and results have been transmitted to NASA. Four companies—BWXT, General Atomics, Ultra Safe Nuclear Corporation–Technology (USNC-Tech), and X-energy—submitted designs. An independent review team assessed the strengths, weaknesses, opportunities, and risks of each design, and AMA said in early October that all four companies met the required specifications. Both General Atomics and USNC-Tech issued press releases about their designs.

HALEU, not HEU: DOE/NASA industry solicitations have stipulated that reactors for propulsion would use high-assay low-enriched uranium (HALEU) TRISO fuel. Opting for HALEU (enriched to below 20 percent U-235) over the highly enriched uranium that NASA used for its own Kilopower reactor design means that the fuel wouldn’t be classed as special nuclear material and opens projects to more collaboration from industry and university partners.

NASA and the Department of Defense’s Strategic Capabilities Office (DOD-SCO) both want a fuel source for special purpose reactors, including NTP systems and the mobile microreactors the DOD is seeking through Project Pele. BWX Technologies announced in July that its Nuclear Operations Group had signed a $26-million, 20-month contract, awarded by INL and funded by the DOD and NASA, to expand and upgrade its TRISO fuel manufacturing line. Fabrication and testing is underway, with the first fuel production planned for early in fiscal year 2022.

Testing is being conducted at three facilities. The Compact Fuel Element Environmental Test System and the Nuclear Thermal Rocket Element Environmental Simulator at NASA’s Marshall Space Flight Center would heat fuel elements to prototypic NTP temperatures less than or equal to 2850 K using induction heating. At the Transient Reactor Test (TREAT) Facility at INL, small fuel specimens can be heated to prototypic NTP temperatures using nuclear fission.

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