Stellaria and France’s Compact Nuclear Heat Race for Heavy Industry

A small team of physicists on the edge of Paris is quietly wagering that compact nuclear heat could soon power heavy industry.

Rather than focusing on vast reactors and national electricity networks, a French start-up is championing a different nuclear model: small modular units designed primarily to replace fossil-fuel boilers at factories. France’s nuclear regulator has received its second licence application for a mini reactor of this kind, suggesting that this specialist concept is becoming a genuine contest.

A different nuclear race takes shape in France

France has traditionally been identified with large reactors supplying a centralised electricity network. Yet that approach is now facing a domestic challenge. Two young businesses, Jimmy and Stellaria, have submitted official applications to construct small modular reactors (SMRs) intended not for households, but for industrial sites.

These projects have moved beyond the laboratory. In France, an application for authorisation to create (DAC) places a project in the same legal framework as the country’s major nuclear operators. That step itself reflects growing belief in the technology.

France’s nuclear regulator now has two mini-reactor projects on its desk, both targeting fossil-fuel boilers in industry rather than the power grid.

The driving force behind this development is simple: industrial heat is still among the most difficult emissions sources to decarbonise. Steelworks, cement works, glass manufacturers and chemical plants burn coal and gas at high temperatures, frequently without interruption. Grid-scale renewable power finds it difficult to provide that same operating profile. Smaller, modular nuclear heat may be able to do so.

Stellaria, the Paris-Saclay nuclear start-up

Stellaria is based in the Paris-Saclay research hub, which includes the French Alternative Energies and Atomic Energy Commission (CEA). The business was spun out of the CEA in 2022 by a purposely lean group of nuclear engineers, physicists and fuel-cycle specialists.

That background gives the start-up an unusual advantage: it can draw on decades of advanced-reactor research and dedicated experimental facilities. Concepts previously confined to technical papers are now being developed into equipment for factories and industrial estates.

Rather than pursuing another EPR-scale generating station, Stellaria is seeking to create something closer in form and function to a highly efficient industrial boiler, with nuclear physics replacing gas combustion.

Stellarium: a molten-salt mini reactor for industrial heat

At the centre of Stellaria’s plan is Stellarium, its leading design. The reactor belongs to the Generation IV family, employing molten salts and fast neutrons. This distinguishes it from France’s existing reactor fleet, which uses pressurised water reactors.

Stellarium dissolves its fuel in heated molten salt. The salt has a dual purpose: it contains the nuclear fuel and serves as the coolant moving around the installation. In other words, the reactor’s core is liquid.

This is more than an unconventional engineering choice. For industrial customers, it offers three direct benefits:

• Heat is spread more evenly through the core, reducing hot spots and thermal strain.
• The system does not operate at extreme pressure, removing the need for thick high-pressure vessels and eliminating certain associated failure modes.
• A conventional “meltdown” scenario takes on a different character because the fuel already exists as a liquid within a salt bath.

Stellarium is designed to deliver around 40 megawatts of thermal power. Although that is small beside gigawatt-scale nuclear power stations, it suits the capacity of substantial fossil-fuel boilers typically used at refineries, chemical facilities and materials plants.

A unit of this scale could be located within a factory boundary, operating continuously and supplying steam or hot gas straight to established industrial processes.

Safety based on physics rather than software alone

Stellaria emphasises a safety principle rooted in fundamental physics instead of complex electronics. Put simply, the nuclear reaction naturally slows if the reactor becomes excessively hot.

As temperatures increase, changes in the fuel-salt mixture and the core geometry lower the reaction rate. The design therefore tends to stabilise without active action from pumps or electrically powered control systems.

Instead of counting on complex back-up systems, the design banks on materials and geometry that cause the reactor to calm down as it heats up.

The chosen salts are non-flammable and chemically stable as well. They do not generate high-pressure steam and greatly reduce the chance of explosions caused by water coming into contact with extremely hot fuel. Such features matter to public authorities still shaped by past nuclear accidents.

Why 40 MW is significant for factories

A thermal output of 40 MW may appear limited at first glance, but it occupies an attractive range for industrial planners. Many major facilities already use boilers of comparable capacity to generate process heat.

Replacing a gas-fired boiler of that size with a nuclear module could allow one site to avoid hundreds of thousands of tonnes of CO₂ during its operational lifetime, while benefiting from much steadier fuel costs. Its footprint is comparatively compact, allowing installation on brownfield land or within industrial estates.

Modular construction would also enable components to be made in factories before being transported and assembled at the location. This differs from the mega-project model of conventional nuclear plants, which involves years of extensive civil engineering and one-off construction.

A 2030 demonstrator and demanding regulation

Stellaria has identified a specific target: a working demonstrator around 2030. Beyond producing heat, this first-of-a-kind installation would need to demonstrate to regulators that the design performs as intended and give industrial customers a practical view of what they would be purchasing.

The company officially submitted its DAC to France’s nuclear safety authority on 22 January. By filing, Stellaria entered the tightly regulated sphere of nuclear operators, an enormous advance for a start-up.

Its submission must address an extensive range of issues, including core behaviour, containment barriers, accident-scenario management, waste management, resistance to external events and the ability to operate safely for decades.

For decades, only state-backed giants filed such applications in France. The arrival of start-ups at this level signals a deeper shift in nuclear culture.

Regulators are likely to challenge the proposal, request further information and require design amendments. The process may take time. Stellaria is betting that joining the regulatory queue early will allow it to help shape future European standards for mini reactors.

France’s mini-reactor field: Stellaria and Jimmy

Stellaria has competition. In early 2024, Jimmy became the first French start-up to lodge a permit application for a small nuclear reactor dedicated to industrial heat. Taken together, the two schemes are creating an emerging French ecosystem in this area.

Their central premise is the same: instead of pursuing large-scale electricity generation, they aim to provide high-temperature heat directly to factories. This sector accounts for a substantial portion of emissions, but it often receives less focus than domestic heating or vehicles.

Both firms must still demonstrate viable commercial models, including who finances each unit, who runs it, how maintenance is managed and how local communities are won over. Industrial customers must also compare nuclear heat with electrification, hydrogen and advanced biofuels.

Worldwide competition in small modular reactors

France’s new entrants are joining an increasingly crowded market. Across the world, companies and state-supported organisations are developing SMR designs for electricity, heat or a combination of both. Many are still at an early stage, but the direction of travel is evident.

The Stellarium design forms part of a wider group of SMR initiatives:

Actor / project	Country	Technology	Typical power	Main use	Industrial heat	Status
Stellaria – Stellarium	France	Molten salts, fast neutrons	≈ 40 MW thermal	Industrial heat	Core focus	Licence application filed, demonstrator targeted ~2030
Terrestrial Energy – IMSR	Canada / US	Molten salts, liquid fuel	≈ 400 MW thermal	Power + heat	Secondary use	Advanced pre-licensing
Kairos Power – KP-FHR	US	Molten salts, solid fuel	≈ 320 MW thermal	Power, hydrogen	Yes	Demonstrator under construction
X-energy – Xe-100	US	High-temperature gas-cooled	≈ 200 MW thermal	Power	High-temperature heat	Industrial project stage
Moltex Energy – SSR-W	UK / Canada	Molten salts, fast neutrons	≈ 300 MW thermal	Power	Potential	Concept development
Oklo – Aurora	US	Fast neutrons, liquid metal	< 50 MW electric	Off-grid power	Not primary	Licensing in progress
CNNC – HTGR	China	High-temperature gas	> 200 MW thermal	Power + industry	Yes	In demonstration / service
Linglong One	China	Pressurised water SMR	≈ 385 MW thermal	Power + heat	Yes	Under construction

For France, strong overseas rivals create additional urgency. Should domestic projects lose momentum, future industrial customers may import SMRs rather than deploy technology developed in France.

Implications for heavy industry

For managers of chemical plants or steelworks, the offer appears simple on paper: maintain the same heat requirement while replacing a gas boiler with a compact nuclear module on the existing site.

Three prospective gains are particularly notable:

• Major emissions reductions without redesigning essential processes.
• More predictable long-term fuel costs and lower exposure to gas-price shocks.
• High availability because nuclear units can operate continuously.

In practice, matters will be more complicated. Operators will require personnel trained in nuclear safety, emergency arrangements and rigorous supervision. Some facilities may be reluctant to host nuclear installations on private industrial land, particularly close to populated locations.

Environmental organisations and local communities will have their own input. Public consultations, planning enquiries and legal disputes may all delay projects. For mini reactors, public acceptance may prove as decisive as neutron physics.

Key terms and scenarios to watch

Two terms are likely to appear regularly as these projects progress. A “small modular reactor” is a nuclear unit smaller than conventional plants and intended for repeat factory production. “Generation IV” describes advanced technologies, including molten-salt and high-temperature gas designs, that seek improved safety, resource efficiency and waste characteristics compared with current reactor fleets.

One credible outcome is that early demonstrators, including Stellarium’s 2030 target, are initially installed at state-supported or partly public sites: research campuses, major industrial estates or military facilities. After accumulating years of operating experience, private industrial customers may be more willing to enter long-term agreements.

A different route could involve hybrid locations, with an SMR supplying both a factory and a local district-heating network that provides hot water to nearby towns. Combining industrial and urban demand could improve utilisation and economics, but it would also bring nuclear technology physically closer to daily life.

The coming years in France will reveal whether this compact, heat-first nuclear approach can progress from ambitious PowerPoint presentations to quiet modules humming behind the boundaries of operating factories.