Preventing Nuclear Risks in Outer Space

Space is becoming an increasingly competitive domain and nuclear technologies, including both nuclear propulsion and energy production, could play a key role in advancing a variety of these missions.

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Over 50 years after President Kennedy’s landmark speech calling for an American mission to the Moon, space is becoming an increasingly competitive domain and nuclear technologies, including both nuclear propulsion and energy production, could play a key role in advancing a variety of these missions. This adoption of nuclear technology isn’t new, however, during the Space Race, both the United States and the Soviet Union used Highly Enriched Uranium (HEU) fuels for their space missions. For example, in 1965, the U.S.-launched SNAP-10A reactor used HEU fuel and the Soviet Union launched 35 space reactors using HEU from 1967 to 1988. One of the reasons using HEU was generally preferred was its ability to power lighter and smaller reactors, but its continued use in space missions raise important questions for the nonproliferation community moving forward.

Today, many countries are interested in missions to the Moon and beyond. In 2018, for example, Russia announced its plan to establish a lunar colony by 2040 and in January 2019, China succeeded in landing its first lunar lander. Following China’s successful mission, President Trump announced plans to return to the Moon by 2024 and then traveling to Mars. This 2024-goal is four years earlier than NASA’s previous target of 2028. In September, 2019, India launched the Chandrayaan-2 to the Moon. Moreover, the U.S. and Japan agreed to expand its cooperation in human space exploration. Today, many countries are rapidly improving their space capabilities to achieve a variety of goals.

For U.S. space exploration, two important technologies that could support missions include nuclear power for long-duration stays on the Moon and nuclear propulsion to shorten the travel time to Mars. However, using nuclear technology in outer space could raise concerns in the nonproliferation community if countries choose to use highly enriched uranium for space missions, as HEU is a weapons-grade material the United States and others have long sought to restrict. By using HEU itself, the United States is tacitly encouraging other states to pursue HEU applications which could lead to unintended consequences. Currently, there is no international norm or nuclear watchdog specifically for the usage of HEU in outer space.

This paper describes current nuclear space projects and examines their impacts on nuclear nonproliferation. Reviewing international and U.S. stakeholders’ activities reveal how important these projects could be for space competition moving forward.


The United States and China are rapidly improving their space capabilities to achieve a variety of goals. In January 2019, the Chinese Chang’e-4 mission became the first lunar lander to land on the far side of the Moon, and the lunar rover deployed on that mission, Yutu-2, continues to explore the region. China also plans to build a Moon-based research station within about 10 years.

A few months after these successes in space, President Trump announced the United States would return to the Moon and eventually send humans to Mars. For this mission, the National Aeronautics and Space Administration (NASA) has two key projects involving nuclear materials: the Kilopower project and Nuclear Thermal Propulsion project. The Kilopower project is exploring nuclear energy reactors to support human exploration missions on the Moon and Mars. The Nuclear Thermal Propulsion project is an option to propel human exploration missions to Mars and other deep space destinations. These space projects involve both the United States and its international partners.


As many countries have ambitious plans to reach the Moon, NASA’s efforts to support human exploration missions to the Moon and Mars has become increasingly important. President Trump has urged NASA to put boots on the Moon by 2024. This desire to make space exploration a priority increases pressure for NASA to further develop its nuclear technology which begins to raise concerns in the nuclear nonproliferation community.

Since 2015, NASA has funded the Los Alamos National Laboratory to build what is called the Kilopower fast reactor which could be fueled with either low enriched uranium (LEU) or HEU. HEU is a weapons-grade nuclear material that can be used to make a nuclear weapon. Unlike HEU, however, LEU cannot be used to make an improvised nuclear device. While NASA has not made a final decision of which fuel to use for this reactor, because HEU fuel has an advantage that allows space engineers to build systems for outer space with low complexity and a much lower volume/mass reactor than LEU, Los Alamos has already successfully tested the HEU design in 2018.

However, in the current international environment, the proliferation of HEU has become a major concern for the nuclear nonproliferation community and hence a policy issue for two reasons. First, is the United States government concerned that allowing NASA to use HEU in space would contradict the U.S. position as a global leader against the general use of HEU in the international community, and second, should the international community establish a clear norm against using nuclear materials in outer space? 

The U.S. Nuclear Regulatory Commission (NRC) regulates “Category I” HEU nuclear fuel and does so with the highest physical protection since it’s a key material in nuclear weapons. Because of this, the U.S. government takes extraordinary measures to secure HEU against theft or diversion and to reduce the inventories of HEU in the world. According to Assistant Secretary of State Christopher Ford, almost 100 HEU facilities around the world have converted to LEU or been shutdown thanks to the Global Threat Reduction Initiative and about 4.5 metric tons of HEU has been repatriated to the United States and Russia. Therefore, the U.S. would be pursuing counterintuitive objectives in nuclear non-proliferation if NASA continues to use HEU for outer space. 

Most importantly, there is neither an international norm nor organization to prohibit and monitor weapon-usable nuclear material in outer space. In 2019, the Trump administration put out new U.S. guidance on the approval process for launching spacecraft with nuclear components or systems, but the document does not specifically address international norms nor the prohibition of weapon-usable nuclear material in outer space. The International Atomic Energy Agency (IAEA), the nuclear watchdog of the United Nations (UN,) regularly monitors nuclear materials under the comprehensive safeguards agreements but unlike on the earth, they will not be able to conduct regular inspections in outer space. There is also a UN resolution known as the “Principles Relevant to the Use of Nuclear Power Sources In Outer Space” which contains vague language regarding the use of nuclear power sources in outer space and was adopted by the UN Committee on the Peaceful Uses of Outer Space in 1992 and subsequently endorsed by UN General Assembly after the accident of Soviet Cosmos-954. Since then, however, there have been no additional revisions to these principles and even the U.S. Ad-Hoc working group found several technical flaws in the resolution’s language, including “Nuclear reactors shall use only highly enriched uranium 235 as fuel” which excludes flexibility in a country’s choice of fuels such as low enriched uranium. Moreover, there is a treaty to ban the stationing and installing of weapons of mass destruction in outer space or on celestial bodies, but this treaty doesn’t include prohibiting weapon-usable materials. Thus, none of these prohibits or adequately addresses the use of nuclear weapon-usable materials in space, especially in the case of nuclear propulsion.


With regard to the Kilopower project, NASA and the Department of Energy are currently doing trade studies on a 10 kWe reactor for a surface nuclear reactor design. That final report was scheduled to be completed in the Spring of 2020, and it is considering the use of both LEU and HEU fuels.

Even before the results of these trade studies are published, however, members of congress have been calling for the prioritization of LEU for use in space in a variety of ways. Representative Bill Foster encouraged NASA to use LEU for its space reactor instead of HEU through an amendment introduced for the appropriations bill in 2019. Additionally, the NASA Authorization Act of 2019 introduced by Senator Ted Cruz passed in the Senate Committee on Commerce, Science, and Transportation requires that NASA prioritize the use of LEU for any space nuclear power systems to address nuclear nonproliferation concerns. Lastly, the Space Subcommittee of the House Committee on Science, Space and Technology passed their NASA authorization bill and said NASA should prioritize usage of LEU where feasible.

Furthermore, Rep. Foster voiced concerns that using HEU in space would send the wrong message about the development and use of such fuels in other countries including commercial enterprises. In particular in the U.S., SpaceX and Blue Origin are aiming to launch commercial spacecraft to the Moon and Mars. As commercial enterprises develop interest in leading the development, launch, and technologies for space, the U.S. must emphasize the risk associated with the use of nuclear weapon-usable materials in space. If NASA decides to use HEU as a fuel source, the use of weapons- grade nuclear materials could become a global standard in outer space missions.

Because of the interest many countries and private enterprises have for space missions and the accelerated adoption of advanced space technologies, intergovernmental organizations could be key to establishing new norms in outer space to meet this current challenge and address nuclear non-proliferation concerns. More than 10 years ago, there was an attempt by intergovernmental organizations to develop safety standards for national purposes in outer space in 2009, the UN Committee on the Peaceful Uses of Outer Space, in partnership with the IAEA, released the IAEA Safety Framework for Nuclear Power Sources Applications in Outer Space. Since then, however, there has been no active discussion on this matter and no international forum to date has created any substantial norms or regulations on the use of weapons- grade nuclear materials in space exploration, including none that addresses usage in commercial projects. Furthermore, it is unclear which intergovernmental organization (the IAEA, UN or a new body) should handle this matter. 


Both countries and commercial enterprises are competing in space. In order to achieve the goals of reaching the Moon and traveling to Mars, the technology these actors choose will be vital in determining whether or not these missions succeed. However, NASA’s 2018 test using HEU, a weapons –grade material, remains a major concern in the nuclear nonproliferation community since the United States has long led an effort to restrict these materials around the world. In the wake of Representative Bill Foster’s statement on the concerns of HEU, both chambers of the U.S. Congress have called for the prioritization of LEU for NASA’s space projects.

There is no international rule prohibiting using weapons-grade nuclear materials in outer space. The international nuclear watchdog (the IAEA) will not be able to effectively inspect outer space. Establishing international norms on how to inspect nuclear materials and other related technological developments for use in outer space are urgent matters. If NASA continues on its current trajectory of using HEU in space, the United States may establish a dangerous precedent.

Since NASA has not made a final decision on whether it will use LEU fuel or HEU fuel, there is still time to consider if we seriously need HEU for space projects. On the earth, there are norms and regulations to reduce the risks of nuclear proliferation; in outer space, there are fewer rules governing nuclear materials and yet the dangers posed by proliferation still exist.

The views expressed are those of the author and do not necessarily reflect the official policy or position of the Embassy of Japan or Japanese government.

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