The nuclear reactor that NASA plans to launch to the Moon’s surface later this decade to
power a manned mission would not require weapons-grade, highly enriched uranium (HEU) fuel,
according to a government study released yesterday that contradicts previous assertions. The report,
Analysis of Alternative Core Designs for Fission Surface Power Capability Demonstration Mission,was
released to the Nuclear Proliferation Prevention Project at the University of Texas at Austin, in
to a Freedom of Information Act request.

As recently as 2018, U.S. officials had claimed that bomb-grade fuel was necessary to reduce the weight
of space power reactors and had
tested such a reactor at a national laboratory. By contrast, the new
report reveals that using low-enriched uranium (LEU) fuel, which is unsuitable for nuclear weapons,
would not increase the total weight of the reactor system if
a “moderator” were used to slow down the
neutrons to facilitate nuclear fission.

The report compares the weight of reactor systems including fuel, moderator, and radiation shield. It
finds that two alternative LEU designs have similar weight ranges as the HEU baseline design, and the
lightest estimate is actually for one of the LEU versions. The report says the two proposed moderators,
yttrium hydride (YH) and zirconium hydride (ZrH), still require some research and development
e is time because NASA’s deadline for launching a power reactor is not until 2027.

Before being finalized this year, the report was distributed within the U.S. government in February 2020
as a “final draft.” This may explain why the U.S. government’s Space Policy Directive6, in December
2020, effectively banned bomb-grade uranium fuel in space reactors by declaring that,
“The use of HEU
in space nuclear power and propulsion systems should be limited to applications for which the mission
would not be viable with other nuclear fuels or non-
nuclear power sources.” Last month, the
Department of Energy
confirmed that space nuclear power reactors must comply with Space Policy

The U.S. government’s rapid evolution from viewing bomb-grade uranium fuel as essential for space
power in 2018, to effectively banning it in 2020, is analyzed in NPPP’s April 2021 presentation to
a national laboratory. In October 2019, NPPP hosted a
symposium on space reactors and proliferation,
including participation by officials from NASA and Congress. The national-security importance of
avoiding HEU in space reactors also was analyzed in a 2020
project by UT-Austin graduate students. 

CONTACT: Prof. Alan J. Kuperman