Their goal is not to build another giant power plant, but a compact nuclear boiler that could slip inside an industrial site and replace gas or coal burners. In doing so, they hope to tackle one of the toughest climate headaches: high‑temperature heat for cement, glass and chemicals, where renewables still struggle.
A new kind of nuclear ambition at Saclay
The project comes from Stellaria, a start‑up spun out of France’s CEA nuclear research agency in 2022 and based at Paris‑Saclay. The team is small on purpose: a tight group of physicists, nuclear engineers and fuel‑cycle specialists, with direct access to the CEA’s experimental platforms and decades of research on advanced reactor concepts.
That heritage matters. For years, many so‑called “Generation IV” reactor ideas stayed buried in technical reports and lab notebooks. Stellaria is trying to turn one of them into a real industrial product, without the mega‑project complexity and public scrutiny that surround massive reactors like France’s EPR design.
France’s regulator has now received two formal creation requests for mini‑reactors, signalling that this is no longer just a PowerPoint market.
Alongside Stellaria, another French start‑up, Jimmy, filed the first such request in early 2024. For a country whose nuclear sector long revolved around national champions and gigawatt‑scale plants, the arrival of nimble, venture‑backed players marks a notable shift.
Stellarium: a molten‑salt mini‑reactor built for heat
Stellaria’s flagship design is called Stellarium. Technically, it is a molten‑salt, fast‑neutron reactor, part of the Generation IV family that international programmes have been studying for years.
The departure from classic reactors is radical. Instead of solid fuel rods cooled by pressurised water, Stellarium uses liquid fuel dissolved in a bath of molten salts, which also act as the coolant. The reactor core is essentially a hot, circulating liquid.
- The temperature inside the core stays more uniform.
- The system runs near atmospheric pressure, so large pressure vessels and steam‑explosion scenarios disappear.
- A traditional “meltdown” no longer applies, since the fuel is already in liquid form.
For Stellaria, this is not a futuristic gadget but a design choice tailored to industry. They are not chasing maximum electrical output. They want reliable, controllable heat that can feed pipes and heat exchangers on a factory site.
Safety that leans on physics, not software
Safety is the core selling point. Stellaria’s engineers favour passive behaviour over complex electronics. The idea: if something goes wrong, the physics should naturally steer the system to a safer state, without any operator or power supply intervention.
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As the temperature rises, the fuel salt expands and the neutron economy changes, which reduces the nuclear reaction rate. The reactor effectively throttles itself down. On top of that, a typical molten‑salt fast design includes a “freeze plug” — a section of salt kept solid by active cooling. If the system overheats, that plug melts and gravity drains the fuel into subcritical storage tanks.
The salts themselves are non‑flammable and chemically stable at high temperature. That removes the risk of hydrogen explosions and steam‑driven pressure spikes that have haunted past nuclear accidents.
Stellaria promotes a design where gravity and thermal expansion are part of the safety system, not enemies to be controlled.
Forty megawatts: small on the grid, big in a factory
Stellarium is sized at around 40 megawatts of thermal power. On a national grid, that is modest; a single large reactor can exceed 3,000 MW thermal. In an industrial context, though, 40 MW is roughly the output of a major gas‑fired boiler tucked behind a chemical plant or refinery.
That scale is deliberate:
- It matches existing industrial heat needs, so factories can switch without redesigning their entire process.
- The footprint is smaller than a conventional power station, which matters on crowded industrial estates.
- Multiple units can be added step by step, following demand rather than betting on a single huge investment.
The design is modular. Key components are intended for factory fabrication and transport by road, then assembled on site. That could shorten construction schedules and reduce the risk of the long, delayed building projects that have damaged nuclear’s reputation.
A 2030 demonstrator and a high regulatory bar
Stellaria has set a firm goal: an operational demonstrator around 2030. This first‑of‑a‑kind unit will not just serve as a test bed; it will be a shop window aimed at regulators, investors and industrial clients.
The company has already taken a decisive step. On 22 January, it filed a creation licence request with France’s nuclear safety authority, the ASN. That single move shifts Stellaria from the research sphere into the tightly regulated club of would‑be nuclear operators.
The dossier must show in painful detail how the reactor will contain radioactivity, manage waste, handle accident scenarios and operate reliably for decades. For a start‑up, that means heavy investment in safety engineering, modelling and documentation, alongside the shiny innovation story.
For the first time in France, nuclear creation licences are no longer the exclusive territory of state giants.
A domestic race for mini‑reactors
Jimmy, the earlier applicant, targets industrial heat as well, though with a different technical route. Together, these projects signal a new French niche: small, site‑level reactors designed not for the national grid, but for decarbonising process heat.
Industrial heat accounts for a hefty chunk of France’s emissions and a far larger share globally. High‑temperature steam, kilns and furnaces rarely run on wind or solar alone, because factories usually need constant heat, day and night. Nuclear heat, if accepted, could slot straight into those demands.
Yet challenges abound. Local communities will need to live next to nuclear units embedded in industrial zones, not isolated power stations behind wide exclusion zones. Financing models must convince both banks and plant operators that a long‑lived nuclear asset can coexist with shorter business cycles in industries like chemicals or steel.
Global competition in SMRs and advanced reactors
France is not alone in this push. Mini‑reactors, often labelled SMRs (small modular reactors) or AMRs (advanced modular reactors) when they use next‑generation technologies, are drawing attention from Canada to China.
Some competitors focus on electricity for remote sites; others, like Stellaria, build a narrative around industrial heat and hydrogen production. A few examples illustrate this range:
| Project | Country | Technology type | Typical output | Main use |
|---|---|---|---|---|
| Stellarium (Stellaria) | France | Fast molten‑salt, liquid fuel | ≈ 40 MW thermal | Industrial heat |
| IMSR (Terrestrial Energy) | Canada / US | Molten‑salt, liquid fuel | ≈ 400 MW thermal | Power and heat |
| KP‑FHR (Kairos Power) | US | Fluoride‑salt cooled, solid fuel | ≈ 320 MW thermal | Electricity, hydrogen |
| Xe‑100 (X‑energy) | US | High‑temperature gas | ≈ 200 MW thermal | Electricity, high‑grade heat |
This competitive pressure cuts both ways for France. On one hand, foreign designs might reach commercial deployment first and lock in markets. On the other, if French regulators gain early experience with advanced reactors such as Stellarium, that expertise could become an export asset in its own right, particularly around safety rules and licensing methods.
What this could mean on the ground
Imagine a cement plant on the outskirts of a French city. Today, its towering kiln runs on imported gas or petcoke, and the site emits hundreds of thousands of tonnes of CO₂ each year. A 40 MW nuclear module tucked into a corner of the site could supply steady high‑temperature heat to that kiln, dramatically shrinking emissions without moving the factory.
Or picture a chemical complex with multiple steam loops at different temperatures. Several modular units could sit in a cluster, each feeding a different pipeline. If one goes down for maintenance, the others keep running, and gas‑fired boilers take only the residual load. For operators, that redundancy might feel more familiar than pinning everything on a single giant unit.
Risks remain, and not just technical ones. Public acceptance near industrial zones will hinge on clear communication, robust emergency plans, and transparency about waste handling. A small reactor still produces long‑lived radioactive materials, and communities may ask why waste should be stored near a factory that might shut down decades before the reactor’s legacy is resolved.
On the benefit side, coupling nuclear heat with processes like hydrogen production or synthetic fuels could reshape industrial clusters. High‑temperature reactors can feed electrolysers or thermochemical cycles more efficiently than conventional power plants. That kind of integration might make some low‑carbon products commercially viable without constant subsidy.
A few terms that shape the debate
- Generation IV: a family of advanced reactor concepts aiming for better fuel use, lower waste and enhanced safety compared with current reactors.
- Fast neutrons: neutrons that have not been slowed down by a moderator, enabling the reactor to use a wider range of fuels and even “burn” some long‑lived waste.
- Molten salt: a mix of salts that stays liquid at high temperature and low pressure, acting as both coolant and, in some designs, fuel carrier.
- Industrial heat: heat above about 150°C used in manufacturing processes, often much harder to decarbonise than power generation.
If projects like Stellarium pass regulatory scrutiny and prove their economics, France could end up exporting not only electrons and engineering, but packaged nuclear heat as a service for heavy industry. For now, everything hinges on turning that first licence application into steel, salt and concrete before this decade is out.