the cost estimates I’ve seen have been all over the place.
But the high concept is pretty clear. The thorium reactors don’t need all the protection that the light water reactors require.
Further, there’s lots of waste thorium being stored in various places which can be burned. As well some designs like Transatomic will burn waste uranium.
Finally, the designs are modular. They can be created on a factory floor and shipped off. This naturally makes their cost curves bend down as they scale up production.
I agree they have lots of expected potential and likely cheaper than traditional nukes. But that is a different comparison...
Breeder reaction:
Thorium + neutron -—> Protactinium isotope + decay = Uranium 233 !