The “other” nuclears: the molten salt reactor

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Published on 05/05/2021

Nuclear power is an industry of the future. Developed with the future in view, part of the future, necessary for it to happen. Of all the anti-nuclear arguments against this little-known energy, the most fundamentally wrong, wrong from the start and wrong for a long time to come, is that nuclear energy is a thing of the past. They have confused “nuclear” with what is just “one step” in what others-than-us will call the origins of nuclear.

Not only must we intensify nuclear research, but we must also share as widely as possible the prospects that it offers, make known the nuclear of the future as much as the nuclear of today, and above all, never oppose the two, since they are links in the same chain.

The sheer number – in the hundreds – of existing reactor concepts and their multiple variations (architecture, heat exchange modes, type of core, choice of moderator, shape and composition of the fuel, safety provisions, size, of course, and obviously, field of application) is a way of measuring the potential nuclear energy has in store.

In 2000, the Generation IV International Forum attempted to clarify this potential by establishing a list of six major types of reactors on which to focus research, including the underdog. The one that books describe as unusual, futuristic, and, above all, promising. The Molten Salt Reactor, long considered difficult and on which research, and its few successes, marked the 1970s without meeting the economic test, are now experiencing a second birth. If governments, research institutes, but also entrepreneurs and students are again taking an interest in them, it is because, although the challenges ahead are great, their potential is even greater.

To put the accent, here, on this nuclear technology is to shed light on all the others, all those which will gradually, patiently, and resolutely decarbonize our societies, and make them viable.

for the Voices

The potential of molten salt reactor technology

By Maxim Romain and John Laurie

John Laurie is President of Progrès Nucléaire and a bilingual British eco-modernist. He lives in the Paris region where he earns his living as a member of Renault’s collaborative innovation laboratory. Concerned about the contradiction between human development and climate change, he created the Fission Liquide blog in 2012.

Maxim Romain is Vice President of Progrès Nucléaire and co-founder of Dott, a leading startup in shared micro-mobility. He is a member of Voix du Nucléaire, a business angel and eco-modernist with a passion for technologies and the transition to a carbon-free world.

The Molten Salt Reactor (MSR) is part of the family known as Generation 4 reactors, which aim to improve nuclear safety, minimize waste, and reduce the costs of building, operating and dismantling reactors.

However the MSR is quite an old concept. It dates from the same period as the Pressurised Water Reactor (PWR) and the Boiling Water Reactor (BWR), used today for the vast majority of the world’s nuclear fleet.

In the 1960s Alvin Weinberg, the “father” of civilian nuclear power, developed and operated the MSRE (Molten Salt Reactor Experiment) at the Oak Ridge national laboratory. This 7.4 MW (thermal) MSR demonstrator operated successfully for four years, in particular with a load factor of 85% – an exceptional performance for a prototype reactor.

Despite its success, the program was scrapped under former President Nixon, because the development of alternative technologies for civilian nuclear power was not a political priority.

Alvin Weinberg in 1967, celebrating 6000 hours of full power production for the MSRE.


The molten salt reactor family uses a technology radically different from current nuclear power plants. In this type of reactor the fuel is a liquid – a mixture of molten salts (typically fluorides or chlorides) and fissile material – rather than the solid fuel used exclusively in conventional reactors.

Molten salts are thermally and chemically stable and are excellent coolants, ideal for capturing and dissipating heat from the fission process.

In this mixture of molten salts, a fissile material is dissolved:

  • Uranium 235
  • Plutonium 239
  • Uranium 233
  • or a mixture of minor actinides from current nuclear waste

… such that the fuel and the coolant are one and the same.

Example: IMSR Concept – Terrestrial Energy

Advantages and obstacles

MSRs offer multiple advantages in terms of safety, cost, scalability and waste management.

Firstly, the liquid fuel makes it possible to reduce or eliminate the main hazards of current technologies such as pressure, volatile source term, active reactivity control, active cooling, chemical reactivity, proliferation, excess reactivity, or the transformation of liquids into gases. For example, a core meltdown is not possible in a molten salt reactor – the fuel is already a liquid.

The safety profile of current pressurised water reactors is the main driver of their cost and size. The safety advantages intrinsic to liquid fuels therefore make it possible to propose reactor concepts with a significant reduction in cost. Some MSR concepts are being developed in the form of SMRs (Small Modular Reactors), where most of the components can be manufactured in a factory and transported to site by truck before being assembled relatively easily on site. Molten salt reactors could thus be mass-produced and easily exported all over the world. The decommissioning, relocation or replacement of the reactor also become potentially much easier.

On the waste side, using liquid fuel improves burn-up, thus using less fuel and producing less waste. The liquid state facilitates reprocessing and allows certain concepts to separate the minor actinides from the fission products, thereby closing the nuclear fuel cycle by removing from the system only the “real” nuclear waste, the fission products.

Finally, whereas current reactors, with operating temperatures limited to just over 300°C, are used only to generate electricity, a molten salt reactor operates at a much higher temperature – around 650-700°C, which makes it possible to envisage new markets such as industrial heat, or the production of molecular energy carriers such as hydrogen or synthetic fuels. With excellent load following capabilities, molten salt reactors are the ideal partner to accompany intermittent renewable energies, so as to ensure regular and reliable energy supply.

As well as these advantages, molten salt reactors face a number of obstacles. The technology has never been commercialised, and regulatory approval remains a long and expensive process. Experimentation will be required to qualify some new material concepts and applications, in particular those requiring corrosive materials. In addition, securing funding is difficult due to the long-term commitment required and the high risk of implementing innovative technology in a highly regulated environment.

Projects around the world

Since the re-discovery of MSR technology in the 2000s and sharing of MSRE technical data in the public domain, many projects have been started around the world.

The technology has won over the Chinese who have the ambition to have a new operational industrial sector before 2030. Their TMSR-LF1 prototype is in the final phase of construction in the Gansu province, and should start up this year.

China (Gansu) – 2020 – Offline installation and startup of the TMSR-LF1 reactor body

In North America, multiple start-ups, with a mix of private and public funds, plan to launch products before 2030. The most advanced projects are Terrestrial Energy (IMSR), Terrapower (MCFR), Moltex (SSR), Elysium Industries (MCSFR) and Thorcon Power. It should also be noted that the ONE (Office of Nuclear Energy) in the United States is supporting the development of five Advanced Reactor concepts, with an investment of $600M over the next five years. Among these concepts, two are molten salt reactors.

In Europe, the only commercial project is Seaborg (CMSR) in Denmark which recently raised 20 million euros from private investors. Their concept is interesting because it aims to create a very compact reactor that can be easily delivered by sea anywhere in the world.

The Seaborg team (2019)

In France, the MSFR research project has been led by Daniel Heuer and Elsa Merle at the CNRS (National Center for Scientific Research) in Grenoble for 21 years with a very limited budget. Since 2017 there has also been increased attention within the energy research agency CEA, with the establishment of an MSR concept study group one year ago. However, to date there is no real project to develop and launch an MSR in France.

Nuclear power needs a new start – a new era even. The molten salt reactor, with its potential for superior safety, lower costs and reduced waste, is a disruptive technology that can support human development while fighting climate change.