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Comment by macrael | Hacker News Reader

Are there serious arguments against Thorium? Or is it just early stage waiting to be a bit more experimented with before going mainstream?

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yes. a recent nature article pointed out how thorium can be used to make uranium-233 which can be used to make nuclear weapons.[1] (see the conclusions section at the end of the article)

here's a more detailed explanation: the "old" idea before this nature article was that the uranium-233 produced in thorium reactors would be inevitably mixed with uranium-232 (which isn't useful for making bombs). but the nature article pointed out that the real decay path is thorium-233 --> protactinium-233 --> (uranium-232 AND uranium-233), and that by separating the protactinium-233 from the thorium reactor's neutron flux after 1 month, you can ensure that the protactinium-233 converts to uranium-233 instead of uranium-232.[2] ... step 3 you can make a weapon.

[1] http://www.nature.com/nature/journal/v492/n7427/full/492031a... (sorry for the paywall :( ) [2] http://en.wikipedia.org/wiki/Thorium_fuel_cycle

But this isn't practical in reality. First of all, you need a core of U-233 to start the process but once you start you're adding thorium at different points in time so the fuel slurry is going to be always a mix of thorium, protactinium, u-232, u-233 and other various products which makes extracting anything a royal pain. There's much easier ways of isolating weapons grade uranium instead of trying to extract it from a ongoing fuel process.

The question isn't whether there are easier ways of producing, but whether the thorium reactor doesn't provide better concealment for producing what would bring unwanted attention when done in the 'easier' way.

If you're trying to conceal what you're doing, it would be best by far to avoid U-232 being involved in the process.

1) If you take away Uranium from a thorium reactor you will rather quickly lose criticality, so you won't be making any new protactinium. Thorium breeders run very close to steady state, and only make a very small fraction of fuel more then they need.

2) I'm pretty sure there is a rare reaction that you will get protactinium-232, which has a much shorter half life the protactinium-233 (about a day instead of about a month) but it can't be chemically separated. It will decay to U-232.

The nature article is alarmist, and not practical. It would only make sense if you had a separate neutron source that was not a thorium breeder, and probably will still have a good amount of U-232. Since you have a neutron source (probably a light water reactor) why you wouldn't use the extremely well understood methods to make plutonium from U-238 is beyond me. You quite literally just need to put uranium metal in the neutron flux for 1 month and chemically separate out the plutonium. Much easier, if I was designing a nuclear weapons program I sure as hell wouldn't pick Thorium.

But that means that while thorium doesn't present an obstacle to state actors in acquiring nuclear weapons, it would considerably lower the risk of theft of non-polluted U-233 by non-state actors.

as far i understand, yes. the real nasty thing about uranium-232 is that it emits highly dangerous gamma radiation [1], which is extremely unhealthy==difficult==expensive to handle. if i understand correctly, the gamma radiation also makes it much easier to detect from afar, making stealth difficult.

[1] http://en.wikipedia.org/wiki/Isotopes_of_uranium#Uranium-232

It will also toast the electronics in your nuclear weapon, essentially making a weapon with a considerable enough portion of U-232 impossible.

It's only tangentially relevant that a given reactor type can be used to make nuclear weapons. Yes, it means that there are states on the planet that can't be trusted with them, so be it. But there are plenty of countries that already have nuclear weapons and/or are stable and transparent enough that they can be trusted to submit to inspections - these could start Thorium projects and the world wouldn't be worse off on the proliferation front for.

Also, any fission reactor is a proliferation risk, because it will be a strong neutron source. Simply put hunks of natural Uranium around the core of your reactor and replace it on an appropriate schedule and you'll breed weapons-grade Plutonium which can be easily chemically extracted. It's just may not be as efficient as other methods.

FOLLOW THE NUMBERS!!!

U235 & Pu239 bombs extant ... ~20,000

U233 bombs extant ... ZERO

Seems pretty conclusive. It is MUCH easier to make bombs the way everyone always has than to put your lives on the line fiddling with U233.

The only really sustainable Thorium reactors would be if you could net positive U-233 production in one. This is somewhat challenging.

Also, the reactors that claim operational advantages against existing designs (particularly LFTR) are somewhat novel, which casts some doubt on their feasibility.

The operational advantages are definitely necessary for them to catch on, as this article points out, if you could get your fuel rods for free, you only save 14%.

Indeed. Uranium reactors are pushing the 4th generation, with unprecedented levels of safety and reliability in the reactor designs. Whereas LFTRs have yet to get to the 1st generation. The idea is certainly worthy of research, but the idea that we could be switch our base power to LFTRs in the next few decades is almost certainly a fantasy.

Thorium has significantly less delayed neutrons [1], which means that the reactor is closer to prompt criticality. In addition, browsing the relevant parts of wikipedia [2], there seem to be some problem with the neutron economy. And to start an reactor you need to breed some U 233 from Thorium first ( Which will alter the reaction of the reactor to a given neutron flux). Since the neutron flux in the reactor is the main variable which controls the power output it seems, that a Thorium reactor is significantly harder to control than a Uranium one.

[1] .6 delayed neutrons per 100 fissions for U233 compared to 1.6 for U235. http://www.reak.bme.hu/Wigner_Course/WignerManuals/Budapest/...

[2] Especially https://en.wikipedia.org/wiki/Liquid_fluoride_thorium_reacto...

I've heard this concern raised before, and the real question is whether the thermal expansion of the fuel salt acts as a strong enough negative coefficient of reactivity.

Answer, yes it does. This was proven in the Molten Salt Reactor Experiment.

"Since the neutron flux in the reactor is the main variable which controls the power output it seems, that a Thorium reactor is significantly harder to control than a Uranium one." This is nonsense. LFTRs are self controlling with a significant negative coefficient. It really makes no more sense to think about putting Thorium in a solid fuel reactor than it does about using diesel in a spark ignition engine.

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