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.
Q: What is a coupon in this context?
It is also a term for strips of metal that you mount in a corrosive environment as a way to monitor corrosion of other susceptible components. So if you were worried about your bronze bushings, you would buy some matching bronze coupons, put them in an area of similar flow, and measure their thickness periodically.
https://www.alspi.com/coupons.htm https://en.wikipedia.org/wiki/Fatigue_testing#Coupon_tests
>strips of metal
I mean in essence you just have to look at the word. Coupon is from French couper, which means to cut. So it's a cut-out. It has the same meaning when you look at extreme couponing and metal testing.
A weld coupon, for example, is a piece of test material of the same thickness and weld preparation as the main part being welded but is not integral to the part itself. That coupon is then sawed off and submitted for destructive tests like impact testing (to determine minimum operating temperature), tensile strength testing (specimen is pulled apart to find how strong and stretchy it is), and crystallographic etching (looking at the crystal structure of the metal to see if it has defects).
It is usually an artifact of materials like metal which are hard to cut, and can’t efficiently just be cut as needed during a task, but are prepared and pre-cut into small pieces you can grab easily. Each of those pieces is called a coupon.
I used to store mine in a Chinese takeout container box.
They would provide the school with bins of scrap from their manufacturing free of charge. Those were our practice coupons in class. We had to be careful to return the welded scraps back to the proper bin so the manufacturer could recycle them.
Our class was just a very small part of their recycling flow. It was a great relationship, we needed practice materials, and they hired welders.
Graphite is the current favorite non-metal option. It is already widely used in traditional reactors, so it is approved, and its interaction with radiation is well understood. On the other hand, its interaction with radiation and molten salt is not as well understood, but hopefully this test reactor and others like it will solve that.
One of my favorite solutions seals the reactor in with a crane and 8 graphite containment vessels. The best estimate of the lifespan of structural graphite in this environment is ~7 years, so the plan would be to monitor the system and move the reactor to a new vessel as needed. The goal is 50 year life-span for these reactors.
Anyway, there is research into alternate materials, but to really test them you need to expose them to both molten salts and radiation. And if they pass, you need to get government approval. The last material to get govt approval was Hastelloy N, and I heard the process was a slog.
So unless materials is solved, you need scalability and replacement. At least the OTHER problem with solid fuel, lots of waste, is generally not a problem with breeder reactors and liquids. You remove the fission products from the fluid and feed it back in. I'm no chemist so I don't know all the separation nastiness involved, but it's better than carting ten thousand year waste across the country to Yucca.
So the question is, what about the other reactor designs, don't they need replacement with respect to the vessel? And as I understand it, fusion reactors also have issues with high speed neutrons so their vessels would need periodic replacement, even if they get to sustained ignition and positive energy.
Your replacement containment layers seems like the "constant replacement" strategy. What if you could simply inject a new layer that hardens and pushes out the older layer?
Also, why not have solid uranium or some similar material as the inner container? Could simple saturation of the existing uranium in the salt prevent excessive wear?
I wonder how much of this approval is because the Chinese brought one online.
Terrestrial Energy: The whole reactor, including heat exchanges and so on is defined for 7 years of life. After that a second reactor is running. After a few years the original reactor is put into a storage silo and then a new reactor is placed their ready to be switch to.
From memory they don't seem to pump the fuel from 1 to the other.
Flibe Energy: They are doing the famous lifter. Simular concept, 2 reactor cores with graphite moderation, after 7 years the fuel is pumped to the second reactor, the first is getting its graphite core replaced. This will have longer lead time to deployment.
Moltex Energy: They have totally different approach. Instead of 7 year lift-time they are building a traditional reactor with much longer life. They are basically building a sodium cooled fast reactor, but replace sodium with salt solution. And then in the fuel assemblies are also like those in sodium reactor, but contain liquid salt with uranium in them. They want to produce the fuel salt from spent Canadian CANDU fuel.
Kairos energy: This is a molten salt cooled reactor that uses pellet fuel but little balls instead of the traditional pellets.
You don't care if the thorium or uranium captures neutrons I would think. Thorium neutron capture is a good thing.
So depending on how long the solid "breeder" inner shell hold up, when you "recondition" the inner shell, I assume you can just dissolve it into salt, feed it into the normal fission products processing that the salt fuel use, and put in a new solid uranium/thorium salt shell.
Or maybe thorium could be alloyed for more endurance properties as the containment. Of course I have no idea about the various cracking / strength / fatigue properties of thorium as a metal.
I wonder if pellets/spheres could use thorium as a surrounding material.
So how nuts is all of that?
Maybe you could do a weekly monthly re-coat of the inner layer with more thorium or uranium to replace that which gets dissolved/degraded.
Edit: ORNL on thorium properties in a nuclear environment
> weekly monthly re-coat
Any operation inside the reactor is a bad idea.
And since in the US its essentially impossible to get a license for anything but a PWR, that isn't rally possible.
Technology independent regulatory framework is one of the main reasons Canada has so many reactor startup, even those that started in other countries.
https://nuclearsafety.gc.ca/eng/reactors/power-plants/pre-li...
Canada's vast tracts of land wouldn't hurt either. If a meltdown or containment breach happens, and no population centers are within 400km, that's a much better bad scenario.
And thus the reason for all this regulation.
In fact, nuclear reactor to be built have a higher barrier of 'prove of safety' then almost anything else.
In fact, nobody in the US ever died because of civilian nuclear reactor research or at least not in the last 40+ years.
Additionally, the public is relatively unaware of how nuclear plants fail. If people at nuclear reactor research labs suddenly stop showing up to work - what happens? People kind of assume it will explode, or become a radiation hot-zone rendering the local area unusable for a hundred+ years.
And considering the experimental reactors being discussed need parts replacement due to corrosion, what happens if those replacements dont happen?
The dangers of human inaction seem much higher for nuclear than other things. With standard fuel sources, if people stop showing up to work then power simply stops being made - that's pretty much it. And while they are at work, the process is pretty simple and robust (compared to nuclear) with a lot of room for error.
I know I've been surprised (in a good way) to learn about some of the safety mechanisms existing within nuclear reactors, but I still only sort of understand what anything means that I read - and the safety mechanisms that gave me some sense of relief is based on a lot of assumptions I had to make about the way nuclear works as a lay person.
how about nuclear scientist stop saying "trust me bro" and more aggressively educate people on nuclear fail-safes? People have trouble voting for things they do not understand. And educating the public on something they are either uninterested in or incapable of understanding is an unfair burden to put on nuclear scientists, but they are the only ones qualified to do so.
the ball will move a lot faster once lay people can exchange stories about nuclear safety that go beyond "we barely use nuclear, and no one has died yet, it's actually really safe because ... reasons? scientists said so?"
I would think a major one would be their failure mode. Metals flex and expand before they eventually fail. Glass/ceramic is fine until suddenly it isn't and has a total failure.
Think of a window being hit. If it were metal it would probably deform but if it is glass it shatters.
Next would be joining them on-site. If needed, metal piping can be bent and welded in-place. what do you do with a glass pipe that needs a join? what do you do if there is a small variation in the plans and the pipe needs an adjustment?
I think there are a host of reasons why glass is not used for pipes.
Metals also have weird properties. Like tempering and hardening based on temperature. In an industrial setting you need expert welders with deep knowledge of the materials or a weld is going to fail and ruin your day.
So it doesn't seem like a huge leap to me actually, assuming ceramics or glass actually have desirable properties.
I would think that putting liquid salt in a a glass pipe is somewhat asking for trouble. One of the many issues with glass is "thermal shock".
Let's say there is a fire and water based sprinklers are activated.. what will the 1,400F glass do once water touches it?
Or there is an accident and someone bangs into the glass. Metal can deform and not fail, glass cant.
A molten salt reactor is a material science problem. Conventional “metal” doesn’t work because of the corrosion. If glass has some desirable property then we can overcome the “bumping in to it” problem. Maybe with a hand rail. Or staying away from the operational nuclear reactor.
I’m not suggesting glass actually be used. I’m saying if it was I wouldn’t be surprised.
What point are you trying to make here?
Salt melts at 1474 F.
801C
(or
1978K
1074K)
Not sure if it's suited to the chemistry and temperatures (and radiation) of a molten salt reactor, but it seems like an interesting technology.
Their design doesn't require the 7 year swap cycle most MSR do.
One additional advantage of separating the fuel salt from the coolant salt is that the coolant salt volume can be increased as desired, making the thermal inertia of the reactor as large as one likes independent of reactor power or size of the fuel load.
The problem with that is that you would likely require a different design and go threw full licensing. So in practice this will likely not be used.
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.
Starting with graphite makes sense imho for a university.
Since the fluid is "thinned", neutron economy plummets and chain reactions stop.
In addition, I though the fluid would expand a bit under high heat in the reactor, which would drop neutron economy as well, so a fluid can somewhat self-moderate.
So unless the fluid is in a ball or tank where the neutron economy is maintained in three dimensions, if you drain it out it no longer has the neutron economy.
So what they probably are saying is that since the fuel is fluid, and if you need to replace the moderator, you drain the fuel (and as stated above, that stops the reaction as well) and replace the moderator.
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.
So no crazy misinformed graphics all over the news.
So even if you drop a hand-grande into that reactor, it just gone be like throwing a rock into a bucket full of toothpaste. Its gone be a mess but its not gone result in a lot of airborne materials.
I'm sure there's all sorts of practical reasons why that wouldn't work, but it's an interesting thing to think about.
Maybe it would need more energy for cooling than what it generates? So it would have to be scaled up to a size where the surface-to-content ratio becomes favourable. There might not be enough salt for that :-)
If I recall the video correctly, solid metal oxide fuel produces offgas at a 50:1 ratio, while being insanely dense. This produces pretty insane internal pressures if the goal of the fuel design is to keep everything contained. Like strong enough to eventually cause mechanical failure of the fuel.
I think there's a lot of really difficult constraints intersecting in these fuel assembly designs, and something that seems obviously simple probably isn't.
Ideally you wouldn't need to expend energy to keep the coolant cold enough; rather, you'd use the coolant to boil water to run your steam generator.
> containing molten salt that has been frozen into a plug
Presumably salt can't be both molten and frozen at once, or is there something about this domain that I don't understand?
If the power fails, the cooling fails and the plug melts.
Think pipes in a house in winter, where one little part of the pipe gets frozen while the rest of the pipes have liquid water in them.
Most materials instead contract when freezing.
Yes, the material is the frozen state of the once liquid reactor fluid.
I hope that clears it up, and would welcome a better way of explaining it!
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
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!)
Water enters only to cools the cold side of the external cycle.
I think I was confused by news stories about situations where the reactor has failed in some way, and then there are stories of how radioactive water needs to be stored / disposed of somehow.
Which then becomes irradiated, and pools via gravity in any lower voids, and then eventually needs to be dealt with.
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)
That could, in theory at least, be an advantage if you have a good process for capturing and storing that offgas (reacting it with something to make it solid and then glassifying it, for instance).
In a traditional fission reactor, gaseous fission products cause swelling and cracking of fuel pellets, and builds up pressure in the fuel tubes, which is one of the factors limiting fuel burnup.
Well, a lot of 'common MSR talking points' are overblown, firmly detached from reality. Or at least conveniently ignoring all the significant challenges remaining in industrializing MSR technology. MSR fanboys are the most tedious of the pro-nuclear side of the energy debate, perhaps beaten only by the "this entirely unproven aneutronic fusion concept will imminently solve all our energy woes" crowd. :)
Some other things that you really don't in your fuel, like Xenon and Krypton will bubble out.
So I think overall it is an advantage.
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.
https://www.osti.gov/biblio/4368180-operation-plutonium-fuel...
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.
CANDU (Canadian reactor design) is moderated via deuterium in which case there is a LOT of it circulating in the reactor core.
The heavy water is syphoned off to a tritium separation unit for recovery. With a market value of $30,000 a gram, there is a clear incentive to recover it ;)
Not really. Tritium is Hydrogen. It cannot bioaccumulate. Each atom of Tritium will spread out to become one among the quadrillions of atoms of Hydrogen in our body. Most will get out of the body in a matter of days, long before they've had a chance to decay. Even when they decay, they undergo beta decay, which is not very damaging. But even if it were damaging, the damage would be very localized, it would affect at most one cell, and the immune system is easily able to handle that.
you could not be more wrong about this. Alpha and Beta are fairly safe OUTSIDE the body. In the case of Alpha it is not able to penetrate the layer of dead skin on your hands.
Internal, it is VERY damaging because there is no "dead skin" to protect the internal organs.
https://www.epa.gov/radiation/radiation-basics
"Some beta particles are capable of penetrating the skin and causing damage such as skin burns. However, as with alpha-emitters, beta-emitters are most hazardous when they are inhaled or swallowed."
Tritium is very different. It is chemically just Hydrogen, which is present everywhere in the body. It just can't bioaccumulate.
In any case, if you don't believe me, here's a statement from the FDA regarding the tritiated water released by Fukushima:
Tritium presents an extremely low human and animal health risk if consumed and any health risk would be further minimized with the dilution effects of discharge into the ocean.
To the extend this is true, it implies that it is the square of the radiation dose that determines carcinogenic effects: Half the dose would cause only a fourth of damage.
However some water does come out of the reactor you are correct.
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.
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?
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...
TRIGGERED :).
The BWR was developed after the PWR specifically to be more economical for terrestrial large-scale power generation. The PWR was designed to be compact and to work on a submarine. So the BWR is the more advanced design for power plants, arguably.
https://whatisnuclear.com/reactor_history.html#the-developme...
https://en.wikipedia.org/wiki/Uranium_hexafluoride
which can be stored in tanks. Instead of removing it they let the salt sit, and radioactive decay led to F2 gas being produced by the salt, which caused UF6 to be produced slowly and then migrate.
This mistake won't happen again.
Had that same team received the funding for a next large commercial prototype, the world would be different now.
But sadly Nixon preferred to spend money for nuclear research in California, Tennessee not exactly a priority.
Alvin M. Weinberg also didn't make himself any friends with government higher ups when he criticized PWR programs for civilian nuclear.
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?