IMSR (Integral Molten Salt Reactor)

What is an IMSR?

Liquid Core Molten Salt Reactors (“LCMSRs”) are nuclear reactors that use a fluid fuel in the form of a molten fluoride or chloride salt.  This is a fundamentally different approach compared to conventional nuclear systems (LWR, Light Water Reactors) that use solid fuel. A liquid fuel offers unique advantages not enjoyed by reactors that use solid fuel.  As an MSR fuel salt is a liquid, it functions as both the fuel (producing the heat) and the coolant (transporting the heat away and ultimately to the power plant).  This represents a revolutionary paradigm in nuclear reactor safety: a reactor that cannot lose coolant and cannot melt down, a reactor with a completely fresh narrative on civilian nuclear safety.


Since the inception of civilian nuclear power over 70 years ago, nuclear technology has evolved slowly and incrementally, without substantial departure from the original reactor design concept. The rudiments of the technology have largely remained the same – burning solid fuel in a water-cooled core. Conventional reactors must be built large for the economies of scale necessary to create a competitive commercial proposition. For any reactor system, there is a direct link between the reactor safety profile, engineering complexity and capital cost. Safety concerns with Conventional Nuclear reactors have driven cost inflation, as extensive engineering complexity is required for safety assurance. For the Light Water Reactor, the singular standard of Conventional Nuclear has led to immense capital costs and resulted in a commercial imperative to build very large reactor units.

Other LCMSR development programs, including the extensive original U.S. program from the 1950s to 1970s, are generally focused on two key objectives: i) to use thorium-based fuels, and; ii) to “breed” fuel in an MSR-Breeder reactor.

Thorium is not currently licensed as a fuel.  Liquid thorium fuels are the nuclear fuel equivalent of wet wood (they are fertile and not fissile).  Wet wood cannot be lit with a match; it requires a torch.  That torch must come in the form of, for example, a very small amount of highly enriched uranium (HEU) to start the reaction.  The thorium fuel cycle would require its own involved regulatory process to become licensed for use on a wide commercial basis. The liquid uranium fuel of an IMSR can be lit easily with already licensed fuel, it is dry tinder.

It is the design of the IMSR itself that achieves exceptional fuel efficiency and a much reduced waste profile, and not the fuel.   The IMSR will use Low Enriched Uranium, or Slightly Enriched Uranium, each of which are broadly available, and have a long regulatory history and long-established supply chain.  The only difference is that the fuel will be in a liquid form and not solid, meaning that far less fabrication will be required.  Uranium fuel is licensed, it is in common use, its fuel cycle is widely understood, and it can be supplied as fuel through an existing industrial chain.

The breeding of fuel creates substantial regulatory hurdles, as well as substantial technology hurdles, leading to substantial additional research and development costs.  All of this is unnecessary if a reactor is a simple burner.  The only reason to breed nuclear fuel is if there is a concern that fuel supplies are scarce and declining.  Uranium is geologically abundant and available in quantities sufficient to supply the world’s power needs for centuries.  There is no commercial case for breeding, and to attempt to do so creates unnecessary regulation, costs and delays.




Advantages of an IMSR

For the IMSR, the safety profile-engineering complexity-capital cost relationship is completely different. The IMSR has completely different economies of scale, owing to the fact that it is an entirely different reactor system. This is commercially very significant. The IMSR can be built small and modular, yet remain highly cost competitive. The IMSR first-of-a-kind reactors will be constructed in a variety of power outputs, from the very small, 30 MegaWatts-Electrical (MWe) or smaller, up to 300 MWe and larger still. IMSRs can also be arrayed in a multi-unit facility for greater output if necessary. IMSRs are modular and designed to have very small land footprints. The IMSR can be manufactured with material readily available in today’s industrial supply chains, with methods common in modern factory production and in high unit volume. The IMSR can be shipped to a power plant located at point-of-demand via flatbed truck or rail car. The IMSR is fuelled with uranium, a terrestrial source of energy with a density many times greater than the expensive combustion energy of fossil fuels. The IMSR unit is scalable, its production is scalable, its industrial use is scalable — scalable enough to displace coal as an electricity source.

When nuclear fuels “burn”, they release thermal energy and fission products are created. When fossil fuels burn, they release thermal energy, and CO2 and water are created.  The fission products are the real waste from fission energy, as CO2 waste is to fossil fuel combustion.  Fission products must be removed or the fission reaction will stop, just as CO2 must be vented from a car engine or the engine will stop.  When fossil fuels are burned inefficiently, they create toxic by-products, which are, partially burned fuel. The same is true of a nuclear reactor system.  When fission nuclear fuels such as uranium are burned inefficiently they create plutonium.  However, if a reactor system can remove the fission products, it can burn its fuel far move efficiently and potentially leave no plutonium.  This scenario is not possible for the solid fuelled reactor systems of Conventional Nuclear, as the fission products are trapped in the fuel by design.  For Conventional Nuclear reactor systems fuel and waste efficiency can only be achieved by the centralized reprocessing of the waste fuel.  Reprocessing of solid fuel waste to create new solid fuel is very difficult, both technically and commercially.   For these reasons, waste reprocessing has limited use today despite the issues of plutonium waste.  Furthermore, in practice, solid fuel waste can only be partially recycled; only a partial improvement in efficiency for all that complexity, effort and cost.

This is not the case with the IMSR using a liquid fuel.  Firstly the IMSR allows many fission products to be removed continuously and in-situ.  They simply vent from the salt and are captured by the IMSR.  Hence, even without any reprocessing of waste IMSR liquid fuel, the IMSR is far more efficient, like a well-tuned engine.  In fact, the IMSR is six times as efficient as a Conventional Nuclear reactor – an IMSR power station is expected to leave less 1/3rd less fission product waste and far less plutonium waste.

While the basic IMSR cannot remove all fission products and leave zero plutonium, there are processes under development that can be added to the basic IMSR to allow the achievement of near zero waste.  IMSR liquid fuel recycling can be done centrally or in-situ i.e. “on-line”, and both methods can achieve a near zero plutonium waste profile for the IMSR.  In this key respect, the IMSR represents a completely new waste paradigm for civilian nuclear power – nuclear power without plutonium waste.  The commercial vision is only possible because the IMSR uses a liquid fuel, and chemical reprocessing of waste IMSR liquid fuel into new IMSR liquid fuel is far easier than chemical processing of waste solid fuel into new solid fuel.  Terrestrial Energy believes that both online and centralized reprocessing of IMSR waste fuel are highly viable and will become standard for the next generation of IMSR power plants, possibly by the end of the next decade.  This future does not exist for Conventional Nuclear using solid fuel.


Companies working to bring IMSR to life.

Terrestrial Energy’s IMSR is a small modular design, with its three first-of-a-kind models ranging from 29 MWe to 290 MWe – ideally suited for remote communities and industrial operations, and to permit on- and off- grid power and heat provision. These models together capture the broadest set of civilian applications. Canada’s nuclear jurisdiction is principles-based, flexible and technology neutral, ideal for the Company’s IMSR development and licensing program. Terrestrial Energy’s management and board consist of executives from the nuclear, oil-sands, mining and finance sectors. – See more at:

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