I'm inclined to believe modern civilian reactors are terrible at producing bomb worthy plutonium. Indeed, the main goals quickly became energy and safety.
However, this was probably not the case for the first reactors. If I recall my documentary properly "la face gâchée du nucléaire" (French and German only I'm afraid), the technology was initially chosen because it could produce plutonium. Therefore it got lots of research, and naturally ended up working best.
Submarines were of course responsible for much of the continued funding. Molten salt got cut off in no small part because the very idea of a nuclear plane was not very good.
That said, I'll need to study this subject more closely before I consider myself properly informed.
It is not surprising that a pro-thorium documentary includes the same misstatements about uranium fueled light water reactors that have spread in writing.
The very first reactors were indeed built for plutonium production. Those 1940s reactors built in Hanford, WA, USA used unenriched natural uranium for fissile material, used purified graphite as their neutron moderator, and did not generate any useful energy. In fact they were net energy consumers because they needed an external power source to move water through to keep them cooled.
Shippingport literally started with a surplus naval reactor from a cancelled aircraft carrier project. It was a pressurized light water reactor requiring enriched fuel like most reactors now operating commercially. Everything about it was different from the early plutonium reactors: the fuel, the moderator, the heat exchange... by a funny coincidence, Shippingport was also the only commercial reactor in the US to use the thorium fuel cycle, from 1977 to 1982.
However, this was probably not the case for the first reactors. If I recall my documentary properly "la face gâchée du nucléaire" (French and German only I'm afraid), the technology was initially chosen because it could produce plutonium. Therefore it got lots of research, and naturally ended up working best.
Submarines were of course responsible for much of the continued funding. Molten salt got cut off in no small part because the very idea of a nuclear plane was not very good.
That said, I'll need to study this subject more closely before I consider myself properly informed.