Stellaria and France’s Mini Nuclear Reactor Race

As the climate debate increases pressure on governments and industry, a new generation of compact reactors is beginning to emerge from French laboratories.

In the Paris-Saclay area, close to leading engineering schools, a low-profile start-up is seeking to show that atomic technology can fit into a unit the size of an industrial boiler. Its initiative joins another application already under review, opening an unprecedented contest within France itself over the future of mini nuclear reactors.

From laboratory to market: Stellaria

Stellaria is the company behind the latest authorisation application. Founded in 2022 from research carried out by France’s Atomic Energy Commission (CEA), it was established with a clear objective: to develop a compact, modular nuclear reactor intended primarily to provide heat for industry.

Its small team consists mainly of physicists, nuclear engineers and fuel-cycle specialists. Its distinguishing feature is its close connection to the CEA’s experimental platforms, which bring together decades of research into advanced reactors, many of which have never progressed beyond the drawing board.

Rather than pursuing a large EPR-scale reactor designed to supply millions of people, Stellaria is targeting a far more defined market: boilers currently fuelled by gas or coal in factories, refineries and heavy industry.

The ambition is to turn cutting-edge technology into a standardised nuclear “product”, ready to be installed like a high-performance piece of industrial equipment.

France’s second mini-reactor application signals a clear shift

Another start-up had already entered the race before Stellaria. In January 2024, Jimmy became the first company to apply for authorisation to create a small nuclear reactor in France.

With Stellaria’s new application filed on 22 January, the country now has two private ventures competing in the same space: mini-reactors aimed at decarbonising industry rather than solely generating power for the grid.

The development points to the emergence of a French SMR (Small Modular Reactor) ecosystem, in which different technical concepts are likely to seek out a range of market niches.

Why the second application is attracting so much attention

Stellaria’s project, named Stellarium, is more than simply a “smaller reactor”. It is a Generation IV reactor using molten-salt and fast-neutron technology, an approach that remains unusual even among leading nuclear nations.

By submitting a DAC (application for authorisation to create) for such an advanced concept, the company is seeking an early place in France’s regulatory process and, consequently, an opportunity to shape future European SMR standards.

With two applications in less than two years, France is showing that it does not merely want to retain its nuclear tradition, but to reshape it into an industrial model that is more accessible and quicker to build.

How Stellarium works: a mini-reactor unlike the standard model

Stellarium departs from the conventional pressurised-water reactor model used by today’s large power plants. Its nuclear fuel is dissolved in molten salts, which act both as the fission medium and as the coolant. Rather than being solid, the core is liquid.

This design has several tangible implications:

• more even internal temperatures, supporting operational stability;
• removal of the risks typically associated with high-pressure systems;
• the notion of a “core meltdown” no longer applies, because the fuel is naturally in a liquid state.

In practical terms, the reactor operates like a large vessel of radioactive molten salt, surrounded by multiple containment barriers and heat exchangers. These transfer thermal energy to secondary circuits, through which the heat is ultimately supplied to industry.

A safety case based on physics

One of the project’s main arguments is its so-called inherent, or passive, safety. The approach is designed so that the system’s physical properties themselves “apply the brakes” if conditions move outside normal limits.

If temperatures rise too far, nuclear reactions naturally decline because of the fuel’s properties and the geometry of the core. The reactor therefore stabilises without relying on complex electronic controls or active pumps.

Molten salts are non-flammable, do not produce explosive steam and offer high chemical stability. Together, these characteristics help to reduce the number of accident scenarios requiring consideration, a significant factor in a regulator’s assessment.

A reactor designed for the factory boiler

Another central element of Stellaria’s case is its target output: around 40 megawatts thermal. That is modest on the scale of a national electricity system. At factory scale, however, it is precisely the typical range for a large gas- or coal-fired boiler.

Rather than connecting the reactor to a vast electricity turbine, the plan is to use it as a continuous heat source for:

• chemical industries;
• oil refineries;
• cement works and ceramics plants;
• glass and metals industries;
• hydrogen and synthetic-fuel production.

Modularity is part of the commercial model: reactor components would be manufactured in factories in series and then transported to the site of use, cutting construction time and reducing the risk of schedule overruns.

The goal of a demonstrator around 2030

Under Stellaria’s plan, an operating demonstrator by the end of the decade is essential. This full-scale prototype would be used to prove performance, validate safety models, train operators and reassure both regulators and prospective industrial customers.

In the nuclear sector, few arguments carry more weight than a real installation operating reliably for years. The company knows that the demonstrator will be the key showcase for unlocking further funding and agreements with regions interested in hosting the project.

SMRs on the global stage: France is not alone

France’s push comes amid intense worldwide activity in the SMR sector. Companies in Canada, the United States, the United Kingdom and China are testing a variety of concepts: high-temperature gas, molten salts, liquid metals, fast reactors and scaled-down versions of conventional reactors.

Several projects mentioned in the French dossier itself illustrate the range of technologies involved:

Project	Country	Technology	Main use
Stellarium (Stellaria)	France	Molten salts, fast neutrons	Industrial heat
IMSR (Terrestrial Energy)	Canada/United States	Molten salts, liquid fuel	Electricity and heat
KP-FHR (Kairos Power)	United States	Molten salts, solid fuel	Electricity and hydrogen
Xe-100 (X-energy)	United States	High-temperature gas	Electricity and industrial processes
CNNC HTGR / Linglong One	China	High-temperature gas and compact PWR	Electricity and heat

This international competition is likely to directly affect the pace at which French projects are approved: the more other countries progress with demonstrators and licences, the greater the pressure on French and European regulators to make swift decisions.

The regulatory leap: from start-up to nuclear operator

By filing the DAC, Stellaria crosses an important threshold: it moves beyond being solely a technology company and seeks nuclear-operator status, with all the associated obligations.

Its dossier must demonstrate structural robustness, containment barriers, emergency plans, strategies for failure scenarios and a clear long-term operating model, including waste management and eventual decommissioning.

For a start-up, entering this regulated world represents a financial and organisational leap in complexity rarely seen outside major state-owned groups.

Risks, questions and promises of mini-reactors

Supporters identify clear benefits: rapid emissions reductions in industrial sectors that are difficult to electrify, continuous supplies of high-temperature heat, less land use than large power plants and the potential for serial manufacturing.

The questions weighing on the other side include:

• the final cost of thermal megawatts compared with natural gas and biomass;
• the financing model for multiple medium-scale projects;
• waste management across a network of reactors distributed throughout the country;
• arranging physical and cyber security for a larger number of sites;
• public acceptance in regions without a nuclear history.

A sensitive issue will be demonstrating that many small reactors can be as safe as, or safer than, a small number of giant reactors, given the need for staffing, surveillance and maintenance across several facilities at once.

Terms worth explaining

SMR (Small Modular Reactor) refers to smaller reactors, generally with an output below 300 MW electrical, designed for serial production, transportable modules and simplified assembly.

Generation IV reactors are advanced concepts under international development that aim for better fuel use, less long-lived waste, greater efficiency and, in many cases, passive safety mechanisms. Molten-salt reactors such as Stellarium belong to this family.

Molten salts are mixtures of salts (fluorides, chlorides and so on) that remain liquid at high temperatures. They can dissolve nuclear fuel while acting both as the reaction medium and the coolant, tolerating higher temperatures than water without boiling.

Future scenarios: from the first factory to a network of mini-reactors

If the two French applications are approved and the first prototypes operate as planned, a plausible scenario for the 2030s is the emergence of “industrial nuclear islands”: complexes with one or more mini-reactors dedicated to a single manufacturing hub.

A cement works could, for example, sign a 20- or 30-year contract to purchase stable heat, replacing gas-fired kilns with nuclear heat. The initial cost would be high, but spread over decades of continuous operation, with low price volatility.

Another potential scenario combines mini-reactors with hydrogen production by electrolysis during periods of lower heat demand. In this case, surplus heat would be converted into low-emission hydrogen for use as a chemical feedstock or fuel elsewhere in the industrial chain.

By accepting two authorisation applications in such a short period, France has placed itself in a position that is both delicate and strategic: it must weigh the risks and benefits of a technology that could redefine not only its energy mix, but also how heavy industry views the everyday use of atomic power in its operations.