Alumni nuclear thermal propulsion system schematic

The European Space Agency commissioned a study in 2024 on European nuclear thermal propulsion that would allow for faster missions to the Moon and Mars than currently possible. The Alumni study was carried out by the French Atomic Energy and Alternative Energies Agency (CEA), ArianeGroup, and Framatome Space, together with ESA. The Alumni study concluded on the merits of nuclear propulsion in space and proposed a new nuclear thermal propulsion design and a development roadmap.

The Alumni study – a loose abbreviation for “preliminary elements on nuclear thermal propulsion for space application” – started with a review of nuclear thermal propulsion, assessing technology and feasibility while thoroughly considering safety aspects.

With nuclear thermal propulsion system, the nuclear core is not activated before it is far from Earth and in a safe orbit – guaranteeing it will never return to our atmosphere, or land on Earth. Before activation and during launch, the fresh-fuel uranium contained in the nuclear core has very low radioactivity and is non-toxic.

During operation, the engine emits neutron and gamma radiation, and a multi-layer plate radiation-shield would protect the spacecraft and any crew inside during the short, less-than-two-hour boosts. Astronauts travelling to Mars in a nuclear spacecraft would in total less radiation in total than when flying in a traditional spacecraft as travel times could be halved, so less time would be spent receiving harmful natural cosmic radiation. On our planet, life is protected from cosmic radiation by Earth’s magnetic field and atmosphere.

The consortium studied nuclear thermal propulsion programmes and technological options from the 1960s and also investigated a new ceramic-metal core design with an improved thrust-to-weight ratio while keeping the same efficiency as previous designs. The consortium spent over a year analysing the theory in depth, including controlling the reactor and how to restart it – avoiding so-called xenon-poisoning that can prevent the reactor from restarting – as well as heat management of the fuel and propellant.

The Alumni executive summary can be downloaded here. Experts in the space domain and in the nuclear domain concluded that the project is feasible in a long-term, can be safely operated, and offers huge performance gains, while identifying the challenges if Europe wanted to continue development.