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Investigating Terrestrial Energy's Molten Salt Reactor Design

Matt Wald
Today, we welcome Matt Wald to NEI Nuclear Notes. Matt, who is senior director of policy analysis and strategic planning at NEI, joined us in April after 38 years at the New York Times.

Around the world, most nuclear power reactors work by splitting uranium to make heat, and using water to carry the heat away so it can be used to make electricity. The uranium is a solid, sometimes in metal form and sometimes ceramic with a metal support system. The design works well, but it dates from the 1950s, and some engineers are re-thinking the whole package.

Enter Terrestrial Energy, of Mississauga, Ontario. Its engineers say that water works fine, but they point out that at reactor temperatures, the water has to be kept under very high pressure to keep it from boiling away. That means heavy, expensive pipes and vessels, and a lot of safety systems designed to kick in if a pipe breaks. A reactor builder could avoid most of that by replacing the water with salt, melted into a liquid, to move the heat. Salt can carry far more heat per unit volume than water, at atmospheric pressure.

And why use solid fuel, which could melt if it gets overheated? Terrestrial Energy starts with uranium in liquid form, mixed into the molten salt. That makes it easy to add a bit more fuel from time to time, and thus to control the system partly by managing the amount of material in the reactor that can be split to sustain the nuclear chain reaction.

Terrestrial calls the result an Integral Molten Salt Reactor. Building on pioneering work done by the Oak Ridge National Laboratory in Tennessee, Terrestrial Energy’s design is for a plant that shuts for refueling just once in seven years. Like all uranium-based reactors, the IMSR produces plutonium as it runs. But the fuel is in the reactor for so long that much of the plutonium is consumed as the reactor runs, aiding energy production and making the design unattractive for anyone who wanted fuel for a bomb. Burning off more of the plutonium also makes the wastes easier to handle.

And while water is essential to most reactors, the water molecules have a tendency to trap neutrons, the sub-atomic particles that sustain the chain reaction. Build a reactor with fewer materials that absorb neutrons, and it takes far less uranium fuel to produce a given amount of energy.

Most current reactors are very large, because if the machine is full of parts and structures designed to handle high pressures, there are economies of scale to building them big. But that is far less true for equipment that runs at atmospheric pressures, so Terrestrial’s reactor is intended to be built in a factory, which is good for cost and quality control, and shipped by truck. Small has a variety of advantages, including ease of financing. “There’s less sticker shock,’’ said David LeBlanc, the company’s chief technical officer.

So far the work is preliminary, and the company is still months from having a design it can submit to Canadian regulators for approval. But Terrestrial, which recently joined the Nuclear Energy Institute, is another demonstration that as with so many other technologies, from aviation to medicine to computing, innovative thinking is going strong and for nuclear energy, more good things lay ahead.


Anonymous said…
Sounds good on paper. Congratulations on your new position
Paul Blanch
Levis said…
Terrestrial Energy's reactors are more suited to act as a source of industrial heat than conventional reactors because of the more limited role of economies of scale and the higher temperature at which they are designed to operate.
But the key comparative advantage of the Terrestrial Energy reactor is the lower capital cost because of the lesser need for defense in depth against accident. A Terrestrial Energy reactor can be designed to be cheaper and safer than any conventional reactor. said…
Can some the homogenous fuel be removed or added from time to time and does this have proliferation potential?
Chris Bergan said…
Pardon my nitpicking, but two details seem off to me. "Terrestrial Energy starts with uranium in liquid form,...." That doesn't sound right.

Google tells me that Uranium melts at 1132°C and boils at 3818°C.
Thorium melts at 1755°C and boils at 5061°C.
Whereas Plutonium melts at only 639°C & boils at 3235°C.
I believe the IMSR will operate below 700°C, right?

The other point is " is for a plant that shuts for refueling just once in seven years." From the link imbedded within the article I read; "At the end of its 7-year design life, the IMSR Core-unit is shut down and left to cool. At the same time, power is switched to a new IMSR Core-unit, installed a short time before in an adjacent silo within the facility." The IMSR plant is a dual-core - so that as one is powered down the second is powered up, allowing the plant to provide continuous energy for several decades. Conceivably the plant never shuts down. Old powered down core-units can be left in place to cool down until a new replacement core needs that slot 7 years later. Or have I misread TE's website?
Bill said…
@Chris Bergan -- The uranium will be dissolved in a flouride salt eutectic, with a melting point about 460 °C.
@Chris Bergan

If pure, you are correct. When mixed in salt form, the melting point is considerably dropped. I can't be certain but it could be explained with an Eutectoid Point (

As for the power plant life cycle, I believe the objective is to bring back old units for recycling and decontamination.
Anonymous said…
The uranium isn't molten - it's dissolved in molten salt.
Nathan Wilson said…
@ Chris, the liquid uranium they use is not in pure metallic form, but is reacted with fluoride to form a salt (UF), which is then dissolved in another molten salt such as FLiBe (containing fluoride, lithium, and beryllium), which melts at around 459C (see

@ Dansolitz, the proliferation resistance of these reactors is excellent, since the uranium they use is low-enriched and after a short time in operation becomes much more radioactive than natural uranium, which complicates handling.

The plutonium produced in power reactors rapidly degrades to non-weapons-usable isotopic quality if not quickly removed from the reactor. That requires that specialized equipment be part of the plant design, and is easily detected by inspectors years before the plant begins operation.
Nick said…
@C Bergan: the U in the reactor is part of a fluoride salt, which melts at a much lower T than U metal.
KitemanSA said…
@Chris Bergan;
He actually means that they start with UF4 which is dissolved into a eutectic of other molten fluoride salts.
KitemanSA said…
As it turns out, Terrestrial Energy is specifically keeping their U235 content in the low enriched uranium regime specifically to avoid that.
Anonymous said…
"The plutonium produced in power reactors rapidly degrades to non-weapons-usable isotopic quality if not quickly removed from the reactor."

It degrades to non-weapons-GRADE plutonium, which is defined as Pu with less than 90% Pu-239. But that's not the same as non-weapons-USABLE.

The US Department of Energy (the people who make our nuclear weapons) and the National Academy of Sciences have both acknowledged that effective nuclear explosives can be made using so-called reactor grade plutonium.

I'm not saying power reactor spent fuel is the biggest proliferation threat we face -- clearly it's not -- but you don't help your case by misstating the facts.
Anonymous said…
I seem to notice a tendency of molten-salt reactor fans to downplay the problem of long-term storage of nuclear waste. Sure, it's great if this produces "less" waste than other designs, but that's not very concrete. So: What kinds of waste come out of this reactor, how much, how long would it have to be stored under what conditions...?

I would be very interested in "clean" nuclear power, but articles like this always give me the impression that the authors are holding something back.

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