Terrestrial Nuclear Innovation Poised to Expand to Outer Space
Due to the imperatives of climate mitigation, as well as lingering safety concerns about current nuclear reactor designs, companies and countries around the world are pursuing new nuclear energy technologies. Generally termed advanced reactors, these new designs use new methods to fission the atom to produce energy in ways that can be safer, cheaper, and produce less waste than existing reactors. In my role at the Nuclear Innovation Alliance, I work on efforts to catalyze commercialization of these reactors so they can contribute to climate mitigation and regulatory modernization so that we can develop effective policies for these new technologies (1). Doing so requires interdisciplinary analysis that combines technical factors, technology development, economic use cases, and policy design. One of the most promising areas of nuclear innovation, and one of the most challenging for which to design governance systems, are transportable reactors (2). These are reactors that can move from location to location, either on a ship, aircraft, truck, train, or even spacecraft. In the last several years, the private and public sectors have had increasing interest in using nuclear energy for space applications, including on the Moon, raising questions about commercialization and governance.
Nuclear energy has long been an enabling technology for space exploration, including for historic lunar missions such as Apollo. The renewed use of nuclear energy on the Moon raises security concerns, environmental protection, and operations (see below). Recognizing the potential governance challenges of lunar nuclear energy is a prerequisite to developing policy models to address concerns. Existing space law and emerging policy approaches to lunar governance can provide a sufficient basis for near-term use of nuclear energy on the Moon, but beyond that we will need additional frameworks. In this piece I identify and provide an overview of those longer-term concerns with nuclear power on the Moon, highlighting potential policy pathways to inform the international community.
Why nuclear energy?
Nuclear energy has played a critical role in space exploration and other government space activities. The simplest types of space nuclear energy are radioisotope power sources, which provide heat and sometimes electricity to a spacecraft from the radioactive decay of specific radioisotopes (3). These systems have supported science experiments during the Apollo missions, the Soviet Union’s Lunokhod lunar rovers, Martian rovers, and interstellar missions such as Pioneer, Voyager, and New Horizons (4). As these systems rely on the predictable decay of isotopes and use solid state systems, they can provide small amounts of power reliably for decades or even centuries, depending on the specific radioisotopes used. However, their small power output generally limits them to robotic missions (5).
During the Cold War, the United States and Soviet Union also investigated the use of nuclear reactors in space to provide electricity and potentially propulsion (6). Ultimately, the U.S. launched one space reactor for research purposes into low earth orbit while the Soviet Union launched several dozen for military applications (7). Both countries also pursued nuclear thermal propulsion, also known as nuclear rockets, but never flight tested any (8).
When considering energy options for space operations, mission planners are generally limited to three types of energy: chemical, solar, and nuclear. Chemical energy is primarily used for space launch as it provides limited energy per mass for long-term missions. Solar enables in-space generation of power and, coupled with energy storage, supports almost all long-duration missions in Earth orbit. However, the Moon poses a challenging environment for solar and storage as its day–night cycle lasts 28 days, requiring up to 14 Earth days of energy storage to survive the lunar night. The interest in the Peaks of Eternal Light is driven primarily by their ability to provide solar power near indefinitely. Comparably, nuclear energy has a high energy density and is not dependent on sunlight, making it an ideal energy source to survive the lunar night, as well as operate in areas without access to solar power like cold traps or lava tubes (9). This makes nuclear especially appealing for space mining applications as a typical commercial microreactor can provide more than 1 megawatt of electricity and 3 megawatts of heat, ideal for extraction of lunar ice (10).
As countries and companies look to expand economic and scientific activities to the Moon and beyond, the performance advantages of nuclear energy underlie renewed interest in the technology. The United States, Russia, and China are all actively developing space nuclear technologies. U.S. activities and policy are the most transparent, including the ground testing of the Kilopower space reactor in 2018 and a plan for a lunar surface test of a commercially provided fission reactor by 2027 (11). The U.S. also has two separate projects to build nuclear rockets, one led by NASA for Mars exploration and another by the Department of Defense intended for military applications in cislunar space (12). Each of these activities builds on commercial innovations in terrestrial advanced nuclear technologies and the development pathways could lead to commercial availability of space nuclear energy systems from multiple companies by 2030.
Meanwhile, Russia is building on the historic Soviet legacy in space nuclear systems and is pursuing development of a space reactor and a nuclear rocket (13). China has indicated that it is pursuing development of a nuclear rocket to support its space mining ambitions (14). Russia and China’s emerging collaboration on lunar activities builds upon initial space nuclear collaboration as Russia provided the radioisotope power system for China’s Chang’e 4 mission (15). Intended for deep space missions, the European Space Agency is also investigating the use of the long-lived radioisotope Americium-241, which indicates potential future interest in lunar nuclear technologies (16).
Governance for nuclear energy on the Moon
Although not initially intended for space nuclear systems, the Outer Space Treaty (OST) and associated space treaties provides the primary governance framework for government and commercial space nuclear activities (17). Under these treaties, each state that launches a space object to orbit is considered a launching state and retains jurisdiction and responsibility for that object (18). The OST and space liability treaty mean that a state has strict liability for damages to Earth from space nuclear activities, as well as liability for space damages resulting from negligence (19). The OST prohibits the stationing of weapons of mass destruction in orbit or on celestial bodies – while nuclear energy sources are not considered such weapons, the inherently dual use nature of nuclear fuels could raise concerns (20).
Critically, the OST ties space activities into broader international law, meaning that state’s obligations under the Nuclear Proliferation Treaty (NPT) extend to outer space as well. Under the NPT, states are obligated to prevent the spread of nuclear weapons, from activities related to nuclear energy (21). Applying the NPT to space may limit international trade in space nuclear power systems by invoking national export control laws. It also creates national obligations to track and monitor the use of nuclear materials in a launching state’s space objects.
With these primary treaties in mind, there are several major governance issues that are likely to emerge and may need new solutions. Beyond statutory law, there is limited soft law guidance that could influence national space nuclear activities. Accordingly, as countries and companies look to deploying space nuclear power systems to the Moon, iterative development of national and international law is needed to establish sustainable governance.
a. Government versus Commercial Ownership and Operation
The OST makes a state responsible for overseeing and authorizing space activities of private entities subject to its jurisdiction (22). This means that national oversight of private space nuclear activities is likely required. Each state will need to develop specific rules for launch, operations, and end-of-life activities for private entities using space nuclear technologies. Some obligations under international space law, such as liability, may or may not be channeled to the private entity. With uncertainty regarding liability, commercial insurance may not be readily available, requiring self-insurance or liability exposure. Others, such as non-proliferation, may shape the nature of government oversight over private activities.
b. Safety of launch and space travel
Perhaps the most controversial aspect of any lunar mission using space nuclear power is launching it from Earth to get there. Several radioisotope systems have suffered from launch failures historically, potentially releasing radiological contamination into the environment (23). Any time spent in Earth orbit before heading to the Moon could also raise risks of accidental reentry, as happened when a Soviet reactor crashed in Canada during the Cold War (24). Accordingly, any national framework for use of space nuclear power systems will need to address launch safety and operations in Earth orbit. Notably, despite having a lower power output, radioisotope systems generally have greater radioactivity at launch than fission reactors, which will not begin operations until they are in space. Reactors will need to be designed to prevent “prompt criticality,” meaning rapid and uncontrolled fission, in any conceivable accident conditions. To provide regulatory certainty, the Trump Administration issued a memorandum that provided clear launch pathways for space nuclear systems on both government and private missions (25). However, greater clarity through a legislated regulatory pathway may be necessary.
c. Decommissioning and waste storage
At the end of their useful lives, space nuclear power systems require decommissioning of the reactor and storage of nuclear waste (26). Historically, this has been done in-situ for radioisotope systems on the Moon and Mars. These systems generally present low safety risks during operations and the relatively short half-lives of useful radioisotopes means that fuel for such systems will decay to extinction within several decades or centuries. Historical space reactors required more active solutions, namely ejection into graveyard orbits around Earth. Decommissioning of reactors and storing waste on the Moon may be more challenging. The OST prohibits harmful contamination of celestial bodies and in-situ storage of radioactive materials could arguably violate this provision (27). It could also present safety and operational concerns as areas that are initially developed are more likely to see long-term use.
Non-proliferation is a cross-cutting issue with all space nuclear activities. The use of highly-enriched uranium for the Kilopower test reactor in the U.S. has been controversial because it goes against U.S. policy to discourage highly-enriched uranium for any civil activity, even NASA (28). Commercial entities are much more likely to pursue low-enriched uranium designs in response. During operations, reactors produce plutonium which could, with technically intensive reprocessing, be usable as a weapons material. The ground-based supply chain for space nuclear systems, such as radioisotopes, could raise security risks or other proliferation concerns. Perhaps most worryingly, the long distances and jurisdictional nature of space means that any space nuclear systems are operating well beyond the territorial jurisdiction of the launching state. Enforcement of nominal jurisdiction may be a challenge, especially if such mechanisms as “flags of convenience” emerge for lunar activities (29). The potential sensitivity of proliferation concerns related to lunar operations are already emerging. In response to the U.S. issuing a new space policy to support nuclear technologies, China’s Global Times cited concerns that the actions are inherently of a military nature and could lead to nuclear weapons production on the Moon (30).
Addressing the concerns
Although these issues are challenging, emerging governance approaches to the Moon can provide an operational framework for early space nuclear activities.
As an initial matter, the principle of subsidiarity and associated polycentricity can guide regulation of specific governance concerns (31). Some aspects, like operations, can be handled by the reactor operator with limited oversight by government entities. Others, like management of non-proliferation, are more global in scope. Different types of nuclear energy (like RTGs or reactors) or different use cases could also be best handled with different policy regimes. Due to the specific structure of international space and nuclear law, subsidiarity in this context will shape the relationship between local mission operators, national oversight, and international influence. Addressing pressing near-term challenges, like dust plumes from launches on the lunar surface, could shape the forums and venues for future space nuclear energy policymaking.
Internationally, Transparency and Confidence Building Measures can address concerns about security, proliferation, and related OST obligations (32). Including details about space nuclear systems in mission registration, such as nuclear materials and operational plans, can mitigate worries that lunar nuclear-powered operations are military in nature. Monitoring of space nuclear systems can demonstrate that no diversion of materials for weapons production is occurring. Terrestrially, the International Atomic Energy Agency works with the international community to ensure that materials for commercial nuclear energy are not diverted through safeguards and other methods. Similar but adapted systems may be necessary for space reactors. Understanding reactor designs and regulation can ameliorate concerns about environmental risks from any potential accidents. Details about decommissioning and waste storage plans can ensure that future space explorers are aware of site-specific risks from in-situ stored materials. In the case of radioisotope systems, understanding the radioisotopes involved can enable space actors to understand decay rates and associated risks.
The development and use of well-defined safety zones, such as those envisioned in the Artemis Accords, can regulate relationships between local space actors (33). Operational activities as well as decommissioning and waste storage can inform the extent and duration of proposed safety zones.
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