French start-up Stellaria has achieved a regulatory milestone for its proposed Alvin prototype reactor intended to demonstrate the molten chloride fast reactor (MCFR) technology of its planned Stellarium Generation IV fast-neutron molten-salt reactor. By submitting a Creation Authorisation Decree (DAC – Demande d’Autorisation de Création) with the Minister of Ecological Transition, responsible for nuclear safety, to establish a Basic Nuclear Installation (INB – Istallation Nucléaire de Base), Stellaria moves beyond the conceptual design stage to regulatory authorisation. The application is currently being technically examined by the Nuclear Safety Authority (ASN – Autorité de Sûreté Nucléaire).
In July 2025, Stellaria, launched by the Alternative Energies & Atomic Energy Commission (CEA – Commissariat à l’énergie Atomique et aux énergies Alternatives) and Schneider Electric in March 2023, raised €23m ($26.7m) to support its reactor development. The new funding is in addition to €10m in non-dilutive financing obtained by Stellaria through its success in the France 2030 Innovative Reactors call for projects. Stellaria has recently secured a power purchase arrangement with Equinix to support AI data centres starting in 2035.
Stellaria’s prototype reactor, Alvin, is a 100 kW experimental reactor designed as a “proof of concept” for the full-scale Stellarium. It will validate the core physics, recycling 100% of its fuel in-core. It is planned for construction at the CEA’s Cadarache nuclear site. Construction and testing are scheduled to target startup in 2030. The reactor is named after Alvin Weinberg, the nuclear physicist who pioneered molten salt reactor technology in the 1960s.
Stellarium is designed to act as a “breed and burn” system using liquid chloride salt fuel to operate in a closed fuel cycle. It is intended to provide high-temperature heat and electricity for heavy industry and data centres.
While a single unit has an output of 250 MWt/110 MWe, deployment is planned in pairs. It delivers high-grade heat at 570°C to 650°C for industrial steam or high-efficiency electricity generation using a nitrogen-based Brayton cycle. The compact core occupies only four cubic metres.
Stellarium is designed to use uranium, plutonium, mixed-oxide (mox) or thorium fuel and is an isogenerator, regenerating its fuel in-core and capable of incinerating long-lived nuclear waste from existing plants. The liquid core allows for rapid load following, scaling production from 0% to 100% in around three minutes (at a rate of 30% per minute) to balance intermittent renewable energy on the grid.
Once Alvin has validated Stellaria’s concept, the company plans to build a 10 MWe prototype reactor, MegAlvin. It is intended to serve as the bridge between the small-scale Alvin experiment and the full-scale commercial Stellarium. Its primary purpose is to prove the technical and economic viability of the 100% fuel regeneration (isogeneration) process at a more significant power level. It is planned for construction after the Alvin prototype completes its initial validation phases, which are currently aimed at a 2030 startup.
“The year 2025 marked a turning point for Stellaria.” said Stellaria President Nicolas Breyton. “After a fundraiser structuring, then the signing of a first agreement pre-ordering the power of our reactors with Equinix, the global data centre giant, the submission of the DAC helps us take a new step. This step is decisive for Stellaria because it gives credibility to the work carried out so far by its teams and partners, and fully commits the company to its responsibilities as a nuclear operator. By filing this request, Stellaria leaves the concept stage to enter a structuring regulatory phase, where its fundamental choices are now made.”
The DAC comprises 15 documents totalling more than a thousand pages. These cover: the safety demonstration, including a detailed description of the installation; the safety principles adopted; the analysis of incidents and accidents, including serious accidents; the management of external attacks (earthquakes, floods, aircraft falls); further radiation protection of workers, the public and the environment; assessment of the environmental impact of the installation; conditions provided for the dismantling of the installation at the end of its life; a presentation of the technical and financial capabilities of the operator; and all the technical documents describing the installation in depth.
Stellaria is the first of the companies to have submitted its application to the authorities on the French fast neutron nuclear market, and the second among the eleven French start-ups to work on the development of SMR or AMR afterwards Jimmy Energy.
Stellaria said it is the first company to have filed such an application with the authorities in the French fast-neutron nuclear market, and the second among the 11 French start-ups working on the development of advanced and small modular reactors, after Jimmy Energy.”
