If you're going to count DU shells as nuclear proliferation, you should also count Po-210 poisoning (Litvinenko, and perhaps others) and that guy who poisoned his gf with I-131.

Though, when talking of nuclear proliferation, we almost always mean just the proliferation of nuclear and thermonuclear explosive devices.

North Korea used a power reactor to produce all of the Plutonium for its nuclear explosive devices, some of which were supposedly of a weapon form-factor have been used. If by "used" you mean "used in anger", then of course, there are only the two instances. (It also should be noted that in the context of nuclear reactors and proliferation, Little Boy is only tangentially related. Nuclear reactors weren't used in the production of Little Boy.)

That didn't happen by accident, but was the result of caution (and probably lowish number of attempts).

I would be curious to hear how/if thorium presents a qualitatively different proliferation problem.