Stellaria, CEA to consider experimental MSR at Cadarache

The letter of intent concerns the establishment of an 'Alpha' Basic Nuclear Installation (INB) - an installation involved in the handling, processing, or storage of alpha-emitting radioactive materials - at the Alternative Energies & Atomic Energy Commission's (CEA's) Cadarache site. This installation will house a model, a fast-spectrum liquid-fueled (molten salt) demonstration reactor, and a salt production facility to supply the model and demonstration reactor. To this end, an area has been identified to study the feasibility of establishing the Alpha INB at the CEA Cadarache centre.

The Alpha nuclear facility will include not only the Alvin experimental reactor but also MegAlvin, Stellaria's 10 MWe prototype reactor.

The 100 kW Alvin experimental reactor, scheduled to start up in 2030, will carry out a test programme that will definitively validate the company's modelling and calculations of neutron-thermo-hydraulic coupling.

By 2032, after the end of Alvin's experimental programme, Stellaria plans to modify the facility to operate MegAlvin. The prototype reactor will be installed in the building that was used for the critical Alvin experiment, the main modification of which will consist of replacing some systems and the tank with another of a larger size (about 40-100cm).

MegAlvin's objectives are: to conduct endurance and qualification tests on the fuel; testing structural materials and systems specific to molten salt reactors; and to obtain sufficient and available feedback several years before the commissioning of Stellaria's first commercial reactor, the Stellarium, for 2035.

Stellaria - a start-up spun out of the CEA and Schneider Electric - submitted its application for the creation authorisation decree (DAC) for its Alpha INB to the French minister in charge of nuclear safety in December last year.

The CEA - a public research institution - says it plays a key role in nuclear innovation, notably by supporting the French industrial sector, major research programmes, and more recently, by assisting the winning projects of France 2030 in their development and industrialisation. The CEA Cadarache centre, located in Saint-Paul-Lez-Durance in the Bouches-du-Rhône region, is dedicated to research platforms and technological development in low-carbon energy (nuclear fission, nuclear fusion, solar, bioenergy, biotechnology and hydrogen). Contruction of the International Thermonuclear Experimental Reactor (ITER) is under way at Cadarache.

The Stellarium reactor proposed by Stellaria will be very compact (measuring 4 cubic metres) and will be able to use a diversified range of nuclear fuels (uranium, plutonium, mixed-oxide, minor actinides, even thorium). Stellaria says the reactor is "the world's first reactor to operate with a liquid fuel capable of destroying more waste than it produces".

In November 2025, Stellaria signed a pre-order agreement with California-headquartered data centre developer and operator Equinix. Under the agreement, Equinix has secured the first power capacity reservation on the Stellarium, the reactor that Stellaria plans to deploy starting in 2035.

Molten salt reactors (MSRs) use molten fluoride salts as primary coolant, at low pressure. They may operate with epithermal or fast neutron spectrums, and with a variety of fuels. Much of the interest today in reviving the MSR concept relates to using thorium (to breed fissile uranium-233), where an initial source of fissile material such as plutonium-239 needs to be provided. There are a number of different MSR design concepts, and a number of interesting challenges in the commercialisation of many, especially with thorium.