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Comment by marcosdumay | Hacker News Reader

A 1MW research reactor.

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.

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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

https://www.moltexenergy.com/

is well ahead of the others.

While at it, I never understood the worry about the plutonium surfacing in the MSR cycle. Of course it can be diverted to make nuclear warheads. But countries like the US, or France, or Russia, or China, or India already are able to produce nuclear warheads, even from plutonium extracted from more conventional reactors. I don't understand where is the risk of proliferation.

Is this about international treaties and ease of inspection? About currently non-nuclear countries obtaining nuclear weapons more easily?

(1) There is fear that any advance in nuclear power technology will lead to corresponding advances in nuclear weapons technology. For instance, if somebody built a perfect system for separating out protactinium from a thorium MSR, that protactinium could be allowed to decay outside the reactor and produce pure U233 that could be used to make weapons. That perfect system is probably not practical, but in general there is fear that any new approach to fuel processing could have unintended consequences. Would it be possible, for instance, to make something like the EBR-II that breeds weapon grade plutonium in a blanket and uses some form of pyroprocessing to produce pure metallic plutonium? Such a system might be able to make enough material to build several weapons a year.

(2) The published information about nuclear proliferation is incomplete and the mental models behind it are broken. For instance, the "little boy" bomb was made with uranium produced with a

https://en.wikipedia.org/wiki/Calutron

but for all the fear that countries like Iran would develop centrifuges, there has been little fear expressed about Calutrons... Except that when Iraq tried to develop a bomb it used the exact same approach used by the US! A scientist at CERN had been contacted by an Iraqi scientist who was interested in a magnet which could have been used for a Calutron and the proliferation authorities just blew him off.

See https://nuclearweaponarchive.org/Iraq/andre/ISRI-95-03.pdf

A country like Japan has large amounts of plutonium which is contaminated with Pu240 and Pu241 and not weapons usable but it's plausible that a modified Calutron could be used to purify non-weapons grade plutonium and make it weapons grade.

Although the conventional model is that a threat would make plutonium by irradiating uranium with neutrons from a fission reactor, it's also possible that a fusion reactor or particle accelerator could be used as a neutron source to do the same. The later would actually have less heat output per unit of Pu and might be an easier device to hide. Current particle accelerators aren't reliable or economical enough for this purpose, but this is just one of many paths to proliferation which are ignored.

> There is fear that any advance in nuclear power technology will lead to corresponding advances in nuclear weapons technology.

It seems like it would be a good idea to get the United Nations’ Treaty on the Prohibition of Nuclear Weapons (https://www.un.org/disarmament/wmd/nuclear/tpnw/) signed by more nations then, so that this technology could be safely developed without a risk of nations weaponizing it.

Ukraine let go its Soviet-era nuclear weapons in 1991. In 2014 Russia occupied a large part of it, and in 2022 is trying to occupy it entirely. Aster this, I suppose no other nation will ever voluntarily relinquish their existing nuclear weapons, and many nations that don't have nukes will try hard to obtain them.

This ship has sailed, to my mind.

I can't find a source, but my recollection is that the majority of the U-235 for Little Boy was generated by gaseous diffusion? I know the original process was gaseous-diffusion followed by Calutron and that once gaseous diffusion was improved they shut down all of the Calutrons, and the timing is such that all but the first little boy bomb must have been made by gaseous diffusion, but it's unclear how the fissile material for the bomb dropped on Hiroshima was generated.

I think one fear is about the proliferation of reprocessing technology. So currently, with a traditional LWR, you buy X fuel rods, then later IAEA inspectors come by and check that those X rods are in the spent fuel pool. Reprocessing, to the very little extent it's done at all, is done by countries that already have nuclear weapons (except maybe Japan?), so the horse has already left the barn.

But a MSR, at least for some of the more advanced concepts, would have online reprocessing on site (continually siphoning off fission products and adding small amounts of fresh fuel). Additionally, since the fuel wouldn't be in discrete bundles, I think there's a fear that small amounts of fuel could be diverted to a clandestine weapons program.

Are there any MSR designs that require NO reprocessing (whether online, batch or other)?

I'd love to find actual data but, anecdotally, it seems like a large amount (vast majority?) of the nuclear legacy costs for countries like the UK, at least, come from the back-end - ie reprocessing. If you just store used rods (in perpetuity) near the reactor they were used in then the overall legacy footprint is pretty modest. Indeed, I think that's what the UK does now - our reprocessing facilities are now in decommissioning mode and the US did the same a long time ago.

Yes - part of the decision to stop reprocessing was proliferation risk. But I think it's also because reprocessing is so insanely messy and so easy to get wrong.

This is because if you 'reprocess' fuel, the waste problem just balloons... the rods have to be chopped up, dissolved in nitric acid, taken through a complex chemical process and you end up with vast amounts of liquid waste, various bits of undissolved gunk, a fiendishly difficult-to-decommission reprocessing facility and all the rest. Reprocessing plants are some of the most complicated chemical plants in the world... and when they go wrong (eg the UK's Thorp leak) they're almost impossible to repair owing to the radioactivity.

In the past, the purpose of the early reactors was to generate plutonium for weapons and so reprocessing was, in reality, the key activity, with the reactors just the tedious thing you had to build to provide feedstuff for this extraction process.

But if we don't want any new plutonium then there's no need to reprocess and the waste problem just becomes insanely easier.

To see what I mean, google the history of the UK's Sellafield (specifically the B.205 and Thorp plants) or Russia's Mayak or France's La Hague. So many leaks and accidents, all totally unnecessary if they hadn't been trying to reprocess the fuel. The idea of taking something small and stable (a rod) and turning it into a dangerous liquid and then trying to run it through a fiendishly complex chemical plant just seems nuts on its face.

Hence my question about MSRs... can you build one that doesn't require any of this tricky chemical engineering, whether 'online' or otherwise? If so, great. If not, why isn't this whole avenue just shut down as DOA?

> Are there any MSR designs that require NO reprocessing (whether online, batch or other)?

I think the Terrestrial IMSR (which is probably the one closest to commercializing in the West) is designed for a once-through uranium cycle, similar to current LWR plants. The idea, IIRC, is to replace the entire reactor vessel (including the fuel salt) every 7 years.

Not sure what the plan is for dealing with the spent fuel. If the fuel salt is water soluble (not sure, but salts tend to be, right?) I'd think some form of processing is necessary before geological disposal. But maybe that can be a cleaner and simpler process than a full PUREX.

Perhaps it's just how much more careful you have to be with plutonium products because of proliferation concerns, or possibly diplomacy concerns "legitimizing", or perhaps just paranoia. Or the basic concern of a bunch more plutonium hanging around even if secure.

They are not really ahead.

https://nuclearsafety.gc.ca/eng/reactors/power-plants/pre-li...

Terrestrial Energy Inc. is the furthest along in terms of the regulatory process.

And for Moltex to work they also need to get a CANDU fuel reprocessing facility online.

But I do think its a great and practical design.

They say they have a version that can be a thorium breeder but they have released little to no information.

> We can produce heat for hydrogen, replacing the need for fossil fuels in heating, transport and industry.

What does it mean to produce "heat for hydrogen"? Why is heating hydrogen a useful property (how does hot hydrogen replace fossil fuels in heating, transport, and industry?)? That seems oddly specific, so I assume I'm misreading something?

I think it means we can produce heat that can power the generation of hydrogen, e.g. from water.

Yeah, I didn't realize thermalchemical water splitting was a thing.

Thermochemical water cracking.

It's pretty cool that they designed their plants with a heat reservoir and oversized turbine generators. This allows them to turn the turbines on and off at will, complementing wind and solar. The nuclear plant "charges" the heat reservoir during the day when the turbines are idle.

From your perspective, given that Thorium salt reactors are part of India's 3rd stage Nuclear plan, where is this technology in general? I assume that they're leading the pack, but I don't really know.

The problem with India is that they started with redoing the whole solid fuel thing first and that took a very long time.

Its hard to find information, but based on some talks I saw a couple years ago I don't think they are close to liquid fueled reactors.

They are still working mostly on creating thorium pellets to put into reactors.

Good question - here's a paper and quote on this:

"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

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.)

The concern is not so much about the fluoride losing an electron, but about whatever reacted to it stealing electrons from the reactor.

Which is less likely with fluorine because what fluorine reacts with to form the salt, say Na as in NaF, wants to give up an electron, not receive one. And with fluorine as a partner, Na will have the hardest* time retrieving that electron to give to something else. This can be easily validated by comparing enthalpy of reaction and formation for the various halide salts.

The properties of broad chemical stability are not limited to ionically bonded fluorine either, see PTFE, which also derives its stability from the extreme reactivity of fluorine. Compared to say PTIE (polytetraiodoethylene), which if you could even make would rapidly yellow and degrade in open air/mild sunlight, as iodine compounds are wont to do.

*for a loose definition of hardest.

https://en.wikipedia.org/wiki/Molten_salt_reactor#Fused_salt...

Because fluorine has a low neutron capture cross section.

Chlorine has a rather high capture cross section for thermal neutrons, so it could only be used in fast reactors.

This video has a fair amount of information on exactly that question:

https://www.youtube.com/watch?v=bkZ0ZBAVDu4

It is very stable both chemically and atomically.

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