Just an aside, Thorium fuel in itself doesn't actually offer safety advantages. The cooling configuration largely determines the safety characteristics of any reactor, and decay heat removal after shutdown has been the biggest challenge (Fukushima, TMI). Coolants that can move heat with passive natural circulation give the "walk-away" safety that Gen-IV reactors flout. This was demonstrated in the EBR-II sodium metal cooled reactor in 1986, (mere weeks before Chernobyl), and it is game changing. Any reactor with low-pressure coolant and some gas-cooled reactors can achieve this passive decay heat removal without backup power. Options include sodium metal coolant, molten salt coolant (fluoride or chloride, note: very different from sodium metal), lead metal, lead-bismuth eutectic, etc.

People often think of Thorium reactors in the molten salt configuration (i.e. the Thorium-MSR), which should be very safe. The Thorium gets conflated with the molten salt, but there are uranium MSRs too which are just as safe.

There are some other advantages to Thorium fuel besides the inherent safety of the plant design.

It's harder to turn Thorium into nuclear weapons, so a nuclear state could provide fuel to a non-nuclear state. Thorium is more abundant and easier to mine. There's much less nuclear waste from a Thorium plant and it decays to safe levels of radiation in a few hundred years making safe storage easier.

Partially true on all accounts. See [1].

[1] https://whatisnuclear.com/thorium-myths.html

I think that article is mostly agreeing with my points:

"Not only can they technically (but with much difficulty!) be used to make bombs.... It may be difficult to do this several times without going subcritical, but it certainly could be done"

So yeah, it's technically possible to make a bomb with byproducts from a Thorium reactor, but it's not trivial.

"Considering that the oceans contain 1.4x1021 kg of water, that amounts to 56,000 tonnes of Th and 4.62 billion tonnes of Uranium. Moreover, mining the entire crust is difficult, whereas the ocean delivers to you. While seawater extraction of uranium is not yet competitive with traditional mining (it’s hovering around 4x more expensive), it is possible and may become economical in the near future"

Pointing to a source that's not feasible and use some handwavy "Well it might be done in the future" doesn't really disprove the point that Thorium is more abundant in the places we can actually extract it from.

"In fact, the long-term decay heat from Thorium-MSRs can be orders of magnitude lower than that from traditional reactors"

On this point the article agrees.

> So yeah, it's technically possible to make a bomb with byproducts from a Thorium reactor, but it's not trivial.

It's not trivial to make a nuclear bomb period.

Yes, but sodium/potassium have their own share of problems like ignition/explosion in contact with air or water. Gas coolant can easily leak and doesn't have much heat capacity, lead requires high temperature to melt but it's otherwise non-corrosive, lead-bismuth eutectic needs lower temperature but corrosive. Molten salts are also corrosive, so reactors using them would most likely required to be built from special steel alloys. Using low pressure coolant with natural circulation is indeed a very attractive option.

Thorium is attractive because it's widespread and lower cost and supposedly doesn't have as many proliferation issues as LEU, since its fuel cycle also creates U232 along with fissile U233. U232 is highly gamma radioactive and thus quite difficult to separate from U233. OTOTH U232 would keep being radioactive for at least 700 years (10 half lives), so we woldn't want any U232 contamination occuring.

And lead-bismuth becomes contaminated with polonium.

SMRs have a big problem though: the number of people needed to operate a reactor does not scale linearly with the power of the reactor. This is why commercial reactors got so big in the first place: to reduce operating cost per unit of produced energy.

SMRs are getting attention now only because big reactors are essentially dead commercially. So I propose they be renamed HMRs: Hail Mary Reactors. They are the last shot nuclear has, and not a very good one.