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Comment by nine_k | Hacker News Reader

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?

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(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.

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.

> 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 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.

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