[PolLambert]

The reverse is actually true. Nobody uses Thorium since you can't make bombs out of them. Thorium is very unsuited to weaponize since one of the byproducts of the fuel process (U-232) decays in to a hard gamma-emitter. This not only makes the material dangerous to handle, it will also wreak havoc with bomb electronics, will require heavy shielding and be very easy to detect.

Due to its unsuitability for bombs, the militaries of the world (in the '50s) didn't invest into researching Thorium. Industrial inertia (and sunk costs) and Uranium being good enough lead us to where we are today.

It is 'technically' possible to make a U-233 bomb but the effort required is so far above U-235 bombs that for any nation able to do the former, it's much easier and cheaper to do the latter.

[jandrese]

Emitting hard gammas seems like it would be troublesome for a power plant as well.

[PolLambert]

A nuclear power plant has thousands of tons of shielding. There are also no timing critical electronics sitting right next to the fuel.

[cthalupa]

It's a much easier problem to solve in a plant setting than it is in something that would be suitably sized for use as a bomb