So, in a conventional reactor, you use nuclear fission to heat/pressurize water and then use your hot, slightly radioactive steam turn a turbine. This mostly works because moving even very hot, very high pressure water around is kind of a solved problem in industry.
In a molten salt reactor, you use nuclear fission to melt various corrosive salts into a fluid, and this is good because molten salts store a lot more energy per unit (volume, presumably?) at low pressures, so you can transfer heat indirectly to nearby turbine-turning water without irradiating the water or relying on high-pressure water to cool your reactor. Cool.
But I was under the impression that the main stumbling block for molten salt reactors was that high-energy corrosion resistant materials for containing / moving molten salt simply don't exist (yet). I suppose this is less of a problem for a research reactor, but it doesn't sound like there's been a materials breakthrough here that's allowing them to get started. Are they just plowing forward and they'll need to replace the containment infrastructure every few years?
You are correct about the corrosion issue. I've done some work developing molten salt resistant claddings. The main culprit is chromium leaching, which de-alloys most of the metals approved for reactor design. The leeching happens at the grain boundaries, so you will hear 'intergranular attack' as a research focus.
A close second problem is the radiation itself. Elements in both the containment vessel and salt transmute. One study I read estimated that pure tungsten (a viable salt resistant material) would transmute to rhenium at a rate of 1% a year. The radiation also causes void-swelling in both the metals and pure graphite.
The standard way to test material's resistance to molten salt is to put a coupon in a crucible full of salt for a few hundred hours. A paper from 2015 showed that the material the testing crucible is made of greatly effects the rate of chromium leeching. They found that both graphite and nickel act as chromium sinks. Many designs call for graphite or nickel parts to be used alongside chromium containing steels. This reactor appears to be stainless steel with graphite moderators.
Another paper strongly suggested that radiation induced void-swelling can squeeze together the grain boundaries, greatly slowing down intergranular attack. Very little corrosion testing has been done under exposure to radiation as it is logistically difficult.
Basically, the next best step is test reactors. You can only get so far testing things in isolation.
but it was believed that some small change in the formula such as adding Niobium could clear the problem up. What's needed to move forward is not a big conceptual breakthrough but rather testing of materials under realistic conditions... A new test reactor.
What is more problematic with the MSRE design is that it incorporates graphite as a moderator and the graphite swells and goes bad over time. Possibly you can take the graphite core out every few years and replace it with a new one, but people have also found designs that don't require a moderator outside the fuel salt.
When I went to the first conference on Thorium Energy years ago David Leblanc had done some very simple calculations that showed you didn't need the graphite -- it works just fine with a faster spectrum. He's refined that idea and is running with it. Others are pursuing chloride salts and plutonium fuel with a very fast spectrum.
Annoyingly, "molten salt reactor" is used to describe two different technologies. What you describe is a traditional reactor that uses molten salt to move heat. This typically leads to higher efficiencies, but does have corrosion issues. Other power generation systems can also benefit from molten salt loops - namely solar energy collectors.
In the research field, "molten salt reactors" (MSRs) usually means the other tech - a reactor where the fissile material is dissolved in a salt. This not only brings efficiency increases, but many safety improvements. Many designs also use a 2nd molten salt loop as a temperature step-down before steam power generation.
Molten salt loops are not as difficult with current technology as they were when they were first introduced.
There are some very interesting startups in this field working on delivering these reactors on an industrial scale rather than the "artisanal" reactors that dominate today:
Copenhagen Atomics [1] is one. They offer a molten salt loop for rapid prototyping [2] if you want to try it yourself.
Seaborg Technologies is also building a compact molten salt reactor. [3] They have a subsidiary, Hyme, to use the same molten-salt technology to provide grid-scale energy storage to balance electricity grids with variable generation from e.g. wind and solar power. [4]
> So, in a conventional reactor, you use nuclear fission to heat/pressurize water and then use your hot, slightly radioactive steam turn a turbine.
I was under the impression that there was a heat exchanger in the path - that is the reactor turns water into slightly radioacive steam, which is sent through a heat exchanger to turn different water into non (or way less anyway)- radioactive steam for the turbines. So both are indirect.
(This is just a nit comment, I think your main points about efficiency still hold, and your materials questions are good!)
There are a number of obstacles. Neutron damage to the reactor structure is more of a problem, since the fuel is dissolved in salt in direct contact with that structure (unlike a reactor with solid fuel rods, which are separated from the reactor vessel by a thickness of moderator, in the case of LWRs is water.)
See here for a (somewhat old) list of some technical issues:
Instead of pumping the salt around, they plan to leave it sitting in stainless steel tubes, and use simple convection to extract the heat. Oak Ridge rejected this idea in the 1950s because they were trying to power an aircraft, but convection makes more sense when the reactor isn't moving.
For anyone worried about this, the longest lived unstable isotope of oxygen (that is heavier than stable oxygen) has a half life of 26 seconds. Hydrogen can become deuterium which is stable, and finally tritium which is not.
Tritium has a long half life of 12 years, but is low energy and very easily shielded (just don't eat it).
There is very very little tritium - first you'd have to make deuterium (there isn't much), and then a deuterium would have to become a tritium, i.e. a rare event on top of a rare event.
I think one key aspect is that they are less susceptible / immune to loss of coolant incidents. In a PWR if there is a loss of pressure, or coolant in any other way, and emergency cooling doesn't work, the core overheats and might melt down.
An uncooled pool of molten salt will keep on generating heat even after the reaction is stopped, so will continue heating up, but it is possible to design the reactor so that the whole thing remains stable. Since the pressure is low, there is no risk of explosion, or release of the radioactive materials.
So the energy density is i think a secondary benefit, if at all.
> But I was under the impression that the main stumbling block for molten salt reactors was that high-energy corrosion resistant materials for containing / moving molten salt simply don't exist (yet).
Is this also an issue for molten salt / liquid metal batteries[1][2] that have been proposed as a grid scale energy storage solution for renewables?
The way I understand it, molten salt is used as the membrane separating the electrode and electrolyte layers. But I was under the impression that there are actual molten salt batteries prototypes with industrial scale facilities currently under construction.
Are the requirements to contain the molten salt in a battery different from a nuclear reactors? Or do they have the same challenges and are simply able to overcome economic feasibility whereas nuclear reactors aren’t?
> So, in a conventional reactor, you use nuclear fission to heat/pressurize water and then use your hot, slightly radioactive steam turn a turbine.
Only in the more primitive reactor designs (BWR, Boiling Water Reactor). Most are of the PWR, Pressurized Water Reactor, design. In these, the water in the reactor is still liquid due to being held at pressure. This pressurized water is run through a steam generator [1] that boils non-radioactive water that never comes into contact with the reactor.
I hope we manage to improve the design over the 1960s version MSRE which cost $130m to clean up due to unforeseen problems including a near-criticality incident. Certainly there is a lot of research to be done.
Way, way overdue! China has taken all of our research from the 50's and has been charging ahead. Very sad that politics and ignorance has severely kneecapped our nuclear industry :(
It's also sad that industry completely failed to convince the population that they can provide a safe solution. For all the credit we give BigCo for creating propaganda, it's curious that they were unable to be successful here, given the stakes.
Particle physics dominated US research grants for several decades with some results to show for it. Now that the focus has shifted elsewhere (e.g. material science) and another country is leading in nuclear research, we will be able to observe their results and act accordingly (just like China did in the 50s). Why is it so important which government is facilitating which research?
Fluoride salts are good for fissile uranium + fertile thorium. If you want to work with a plutonium/uranium 238 cycle then chloride salts are a better choice. Plutonium doesn't dissolve very well in fluorides.
Molten chloride reactors can have performance characteristics right out of science fiction, it seems possible for such a reactor to not only breed more fuel but to destroy the long-lived (500 year) fission products such as cesium and strontium.
It never gets upvoted on HN when I link it but I've been following MSRs for a while and even spoke at the first thorium energy conference and I've been watching people's thoughts about designs evolve and this one
"Molten Salt Reactor (MSR) was designed to operate at high temperature in range 700 - 800°C and its fuel is dissolved in a circulating molten fluoride salt mixture. Molten fluoride salts are stable at high temperature, have good heat transfer properties and can dissolve high concentration of actinides and fission product."
Without knowing the specifics as apply to nukes, anion reactivity is inversely proportional to atomic reactivity, fluoride is far less reactive than chloride or bromide or iodide, respectively in order of increasing reactivity. Ergo fluoride and fluoride containing anions are quite common in molten salts/ionic liquids.
In English: the flouride, having so voraciously devoured that 8th electron it was missing, really doesn't want to give it up, whereas chlorine et al have lots of spares.)
Anyone know of any software work being done around Nuclear? I don't care what it is, just looking to play a bit more with the space, Open Source, data sourcing. I really mean I don't care what it is, just wanna learn a bit more about it and be a bit more involved.
This is so fantastic. That a 'Christian' university would do this is kind of insane to me, but I take it.
So many problems with nuclear in the last 50 years are that there simply are not enough experiments and research reactors.
If you don't have them, then how can you ever test new materials.
The original molten salt experiment reactor was only not that much bigger then this university reactor and didn't cost a huge amount, but it lead to such amazing research results including new materials that were designed when making it.
New materials can't easily be qualified without having the necessary research infrastructure.
This is really exciting. Fission tech is largely stuck in with incremental improvements from the 1950s. The last time the NRC got an application for a research reactor was 30 years ago.
The reactor is a graphite-moderated, fluoride salt flowing fluid design. If this works well, fission will have a very bright future as it should be more efficient and much safer to operate than current reactor designs.
[Layman, but I've read up quite a bit on thorium and molten salt reactor designs]
The 1959 Sodium Reactor experiment was to demonstrate the feasibility of a sodium-cooled reactor as the heat source for a commercial power reactor to produce electricity. It used fuel rods similar to rods used in reactors in operation today.
Conversely the The Molten Salt Reactor experiment[1] used uranium fuel dissolved into a salt as both fuel and coolant for the reactor - this design enables significant advantages over traditional reactors powered by solid fuel.
I'm surprised the reactor needs a moderator, I would assume that a liquid fuel reactor could be controlled far better by controlling flow into and out of the critical mass. It's all at normal pressures, just hot (temperature and radiation wise), so just bog standard plumbing practices for handling fluid levels should work.
Being able to scramble the reactor by just dumping the contents through a set of diverters into separate flat bottom chambers to cool and eventually freeze is an awesome safety feature.
A moderator is not used to control the reaction, but to make it possible. The neutrons produced by fission, have high kinetic energy, and fuel has a small cross section for neutrons at such high energy. A neutron moderator slows down the neutrons, making them more likely to interact with the uranium and split them. typically water is a good moderator, but here they use graphite for obvious reason (high temperature salts doesn't play well with water)
One of the great practical challenges with molten salt is to design a reliable valve. At temperatures this high and with the high corrosive properties of the medium, this is not trivial.
I thought these were outlawed, per a LFTR video I saw once. To emphasize, I'm asking if that is the case, or was the LFTR video was engaging in misinformation, or if something changed in regulations.
I believe that the video was misinformed or your recollection is a bit fuzzy. The closest true approximation I can muster is "there is no commercial molten salt reactor design yet approved by the NRC, and the NRC approval process may need updates to consider reactor designs that significantly deviate from the common water-moderated types."
[+] [-] dangerlibrary|3 years ago|reply
In a molten salt reactor, you use nuclear fission to melt various corrosive salts into a fluid, and this is good because molten salts store a lot more energy per unit (volume, presumably?) at low pressures, so you can transfer heat indirectly to nearby turbine-turning water without irradiating the water or relying on high-pressure water to cool your reactor. Cool.
But I was under the impression that the main stumbling block for molten salt reactors was that high-energy corrosion resistant materials for containing / moving molten salt simply don't exist (yet). I suppose this is less of a problem for a research reactor, but it doesn't sound like there's been a materials breakthrough here that's allowing them to get started. Are they just plowing forward and they'll need to replace the containment infrastructure every few years?
[+] [-] ortusdux|3 years ago|reply
A close second problem is the radiation itself. Elements in both the containment vessel and salt transmute. One study I read estimated that pure tungsten (a viable salt resistant material) would transmute to rhenium at a rate of 1% a year. The radiation also causes void-swelling in both the metals and pure graphite.
The standard way to test material's resistance to molten salt is to put a coupon in a crucible full of salt for a few hundred hours. A paper from 2015 showed that the material the testing crucible is made of greatly effects the rate of chromium leeching. They found that both graphite and nickel act as chromium sinks. Many designs call for graphite or nickel parts to be used alongside chromium containing steels. This reactor appears to be stainless steel with graphite moderators.
Another paper strongly suggested that radiation induced void-swelling can squeeze together the grain boundaries, greatly slowing down intergranular attack. Very little corrosion testing has been done under exposure to radiation as it is logistically difficult.
Basically, the next best step is test reactors. You can only get so far testing things in isolation.
[+] [-] PaulHoule|3 years ago|reply
https://haynesintl.com/docs/default-source/pdfs/new-alloy-br...
(which is practically stainless steel without the steel) was good for this use but when it was tried in this system it did not hold up very well
https://en.wikipedia.org/wiki/Molten-Salt_Reactor_Experiment...
but it was believed that some small change in the formula such as adding Niobium could clear the problem up. What's needed to move forward is not a big conceptual breakthrough but rather testing of materials under realistic conditions... A new test reactor.
What is more problematic with the MSRE design is that it incorporates graphite as a moderator and the graphite swells and goes bad over time. Possibly you can take the graphite core out every few years and replace it with a new one, but people have also found designs that don't require a moderator outside the fuel salt.
When I went to the first conference on Thorium Energy years ago David Leblanc had done some very simple calculations that showed you didn't need the graphite -- it works just fine with a faster spectrum. He's refined that idea and is running with it. Others are pursuing chloride salts and plutonium fuel with a very fast spectrum.
[+] [-] ortusdux|3 years ago|reply
In the research field, "molten salt reactors" (MSRs) usually means the other tech - a reactor where the fissile material is dissolved in a salt. This not only brings efficiency increases, but many safety improvements. Many designs also use a 2nd molten salt loop as a temperature step-down before steam power generation.
[+] [-] mjul|3 years ago|reply
There are some very interesting startups in this field working on delivering these reactors on an industrial scale rather than the "artisanal" reactors that dominate today:
Copenhagen Atomics [1] is one. They offer a molten salt loop for rapid prototyping [2] if you want to try it yourself.
Seaborg Technologies is also building a compact molten salt reactor. [3] They have a subsidiary, Hyme, to use the same molten-salt technology to provide grid-scale energy storage to balance electricity grids with variable generation from e.g. wind and solar power. [4]
[1] https://www.copenhagenatomics.com/ [2] https://www.copenhagenatomics.com/products/molten-salt-loop/ [3] https://www.seaborg.com/ [4] https://www.seaborg.com/press-release-hyme
[+] [-] sophacles|3 years ago|reply
I was under the impression that there was a heat exchanger in the path - that is the reactor turns water into slightly radioacive steam, which is sent through a heat exchanger to turn different water into non (or way less anyway)- radioactive steam for the turbines. So both are indirect.
(This is just a nit comment, I think your main points about efficiency still hold, and your materials questions are good!)
[+] [-] pfdietz|3 years ago|reply
See here for a (somewhat old) list of some technical issues:
https://gain.inl.gov/SiteAssets/MoltenSaltReactor/Module2-Ov...
"Nickel-based alloys embrittle under high neutron fluxes at high temperature"
"Over 40% of [fission products] leave core [in offgas]"
"Large fraction of cesium, strontium, and iodine end up in offgas"
"MSRE was approaching end of allowable service life when shut down" (after four years at 40% capacity factor)
[+] [-] p1mrx|3 years ago|reply
I'm interested to see what Moltex can do to simplify matters:
https://www.youtube.com/watch?v=7qJpVClxzVM&t=758s
Instead of pumping the salt around, they plan to leave it sitting in stainless steel tubes, and use simple convection to extract the heat. Oak Ridge rejected this idea in the 1950s because they were trying to power an aircraft, but convection makes more sense when the reactor isn't moving.
[+] [-] ars|3 years ago|reply
For anyone worried about this, the longest lived unstable isotope of oxygen (that is heavier than stable oxygen) has a half life of 26 seconds. Hydrogen can become deuterium which is stable, and finally tritium which is not.
Tritium has a long half life of 12 years, but is low energy and very easily shielded (just don't eat it).
There is very very little tritium - first you'd have to make deuterium (there isn't much), and then a deuterium would have to become a tritium, i.e. a rare event on top of a rare event.
[+] [-] rich_sasha|3 years ago|reply
An uncooled pool of molten salt will keep on generating heat even after the reaction is stopped, so will continue heating up, but it is possible to design the reactor so that the whole thing remains stable. Since the pressure is low, there is no risk of explosion, or release of the radioactive materials.
So the energy density is i think a secondary benefit, if at all.
[+] [-] runarberg|3 years ago|reply
Is this also an issue for molten salt / liquid metal batteries[1][2] that have been proposed as a grid scale energy storage solution for renewables?
The way I understand it, molten salt is used as the membrane separating the electrode and electrolyte layers. But I was under the impression that there are actual molten salt batteries prototypes with industrial scale facilities currently under construction.
Are the requirements to contain the molten salt in a battery different from a nuclear reactors? Or do they have the same challenges and are simply able to overcome economic feasibility whereas nuclear reactors aren’t?
1: https://ambri.com/
2: https://www.youtube.com/watch?v=-PL32ea0MqM
[+] [-] Manuel_D|3 years ago|reply
Only in the more primitive reactor designs (BWR, Boiling Water Reactor). Most are of the PWR, Pressurized Water Reactor, design. In these, the water in the reactor is still liquid due to being held at pressure. This pressurized water is run through a steam generator [1] that boils non-radioactive water that never comes into contact with the reactor.
1. https://en.wikipedia.org/wiki/Steam_generator_(nuclear_power...
[+] [-] samstave|3 years ago|reply
https://i.imgur.com/InQHoSI.png
https://i.imgur.com/go5v5QL.jpg
[+] [-] pnw|3 years ago|reply
[+] [-] EricE|3 years ago|reply
[+] [-] powerhour|3 years ago|reply
[+] [-] runarberg|3 years ago|reply
Particle physics dominated US research grants for several decades with some results to show for it. Now that the focus has shifted elsewhere (e.g. material science) and another country is leading in nuclear research, we will be able to observe their results and act accordingly (just like China did in the 50s). Why is it so important which government is facilitating which research?
[+] [-] marcosdumay|3 years ago|reply
Anyway, why is it always a fluoride salt? Is it because of the melting point or because of some nuclear property?
I imagine there is some very relevant reason, because a less reactive anion (even chlorine) would be much easier to work with.
[+] [-] PaulHoule|3 years ago|reply
Molten chloride reactors can have performance characteristics right out of science fiction, it seems possible for such a reactor to not only breed more fuel but to destroy the long-lived (500 year) fission products such as cesium and strontium.
It never gets upvoted on HN when I link it but I've been following MSRs for a while and even spoke at the first thorium energy conference and I've been watching people's thoughts about designs evolve and this one
https://www.moltexenergy.com/
is well ahead of the others.
[+] [-] nickpinkston|3 years ago|reply
"Molten Salt Reactor (MSR) was designed to operate at high temperature in range 700 - 800°C and its fuel is dissolved in a circulating molten fluoride salt mixture. Molten fluoride salts are stable at high temperature, have good heat transfer properties and can dissolve high concentration of actinides and fission product."
https://aip.scitation.org/doi/10.1063/1.4972932
[+] [-] thereisnospork|3 years ago|reply
In English: the flouride, having so voraciously devoured that 8th electron it was missing, really doesn't want to give it up, whereas chlorine et al have lots of spares.)
[+] [-] detaro|3 years ago|reply
[+] [-] pfdietz|3 years ago|reply
Chlorine has a rather high capture cross section for thermal neutrons, so it could only be used in fast reactors.
[+] [-] panick21_|3 years ago|reply
https://www.youtube.com/watch?v=bkZ0ZBAVDu4
[+] [-] LatteLazy|3 years ago|reply
[+] [-] notacop31337|3 years ago|reply
[+] [-] abi|3 years ago|reply
[+] [-] panick21_|3 years ago|reply
So many problems with nuclear in the last 50 years are that there simply are not enough experiments and research reactors.
If you don't have them, then how can you ever test new materials.
The original molten salt experiment reactor was only not that much bigger then this university reactor and didn't cost a huge amount, but it lead to such amazing research results including new materials that were designed when making it.
New materials can't easily be qualified without having the necessary research infrastructure.
[+] [-] indymike|3 years ago|reply
The reactor is a graphite-moderated, fluoride salt flowing fluid design. If this works well, fission will have a very bright future as it should be more efficient and much safer to operate than current reactor designs.
[+] [-] thoughtpeddler|3 years ago|reply
[1] https://www.etec.energy.gov/Operations/Major_Operations/SRE....
[+] [-] dragonshed|3 years ago|reply
The 1959 Sodium Reactor experiment was to demonstrate the feasibility of a sodium-cooled reactor as the heat source for a commercial power reactor to produce electricity. It used fuel rods similar to rods used in reactors in operation today.
Conversely the The Molten Salt Reactor experiment[1] used uranium fuel dissolved into a salt as both fuel and coolant for the reactor - this design enables significant advantages over traditional reactors powered by solid fuel.
[1] https://en.wikipedia.org/wiki/Molten-Salt_Reactor_Experiment
[+] [-] mikewarot|3 years ago|reply
Being able to scramble the reactor by just dumping the contents through a set of diverters into separate flat bottom chambers to cool and eventually freeze is an awesome safety feature.
[+] [-] YakBizzarro|3 years ago|reply
[+] [-] Rokid|3 years ago|reply
[+] [-] anshumankmr|3 years ago|reply
[+] [-] rob_c|3 years ago|reply
[+] [-] dang|3 years ago|reply
DOE digs up molten salt nuclear reactor tech, Los Alamos to lead the way back - https://news.ycombinator.com/item?id=32423177 - Aug 2022 (1 comment)
Chinese molten-salt reactor cleared for start up - https://news.ycombinator.com/item?id=32406435 - Aug 2022 (9 comments)
Stable Salt Reactor - https://news.ycombinator.com/item?id=32233485 - July 2022 (2 comments)
What is a molten salt reactor? - https://news.ycombinator.com/item?id=31187423 - April 2022 (101 comments)
1MW Molten-Salt Test Reactor by Copenhagen Atomic for €88k [video] - https://news.ycombinator.com/item?id=25388343 - Dec 2020 (198 comments)
New Design Molten Salt Reactor Is Cheaper to Run, Consumes Nuclear Waste - https://news.ycombinator.com/item?id=24771306 - Oct 2020 (2 comments)
ThorCon 2019 500MW molten salt modular tow-able nuclear reactor [pdf] - https://news.ycombinator.com/item?id=21307378 - Oct 2019 (2 comments)
Molten Salt Reactors - https://news.ycombinator.com/item?id=20424841 - July 2019 (120 comments)
What Is Called Nuclear Waste Is Mostly Fuel for Molten Salt and Fast Reactors - https://news.ycombinator.com/item?id=20191064 - June 2019 (49 comments)
Open source Molten salt nuclear reactor design - https://news.ycombinator.com/item?id=18892919 - Jan 2019 (88 comments)
Remote Maintenance of Molten Salt Reactors [video] - https://news.ycombinator.com/item?id=15543428 - Oct 2017 (6 comments)
A Thorium-Salt Reactor Has Fired Up for the First Time in Four Decades - https://news.ycombinator.com/item?id=15084215 - Aug 2017 (161 comments)
Molten Salt Reactor Claims Melt Down Under Scrutiny - https://news.ycombinator.com/item?id=13863626 - March 2017 (137 comments)
The 500MW molten salt nuclear reactor - https://news.ycombinator.com/item?id=5371659 - March 2013 (22 comments)
[+] [-] bayesian_horse|3 years ago|reply
[+] [-] martin1975|3 years ago|reply
[+] [-] acidburnNSA|3 years ago|reply
https://www.world-nuclear-news.org/Articles/Chinese-molten-s...
[+] [-] formerkrogemp|3 years ago|reply
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