I don't know who in the thorium boosters' club started the "light water reactors were meant for weapons plutonium" meme but it's nonsense.
To quote "Technologies Underlying Weapons of Mass Destruction",
Reactor-grade plutonium recovered from civilian reactors differs from weapon-grade plutonium in the relative proportions of various plutonium isotopes. Reactor-grade plutonium has a higher rate of spontaneous fission reactions than weapon-grade, generating neutrons that can initiate the nuclear chain reaction during weapon detonation sooner than would be optimal. As a result, using reactor-grade plutonium in a first generation nuclear weapon can significantly reduce both the predictability and the expected yield of a weapon designed by a proliferant state.
None of the states that have either made nuclear weapons or attempted to do so appear to have selected anything but high-quality plutonium or uranium for their designs. Nevertheless,
from a technical perspective, reactor grade plutonium can be used to make nuclear weapons, and any state possessing significant quantities of separated plutonium should be considered to have the material needed to fabricate nuclear components for nuclear explosive devices in a short period of time.
If you want to seize on the part where it says that one could build bombs from civilian reactor waste even though it's less straightforward and no nuclear weapons state has ever done that, then I'll point out that the thorium fuel cycle likewise could be used for bombs even though no NWS has ever taken that path (Quoting LANL's "Thorium Based Power Systems and Relevant Safeguards Considerations"):
Thorium is not safeguarded until it is converted into fuel materials. It would seem that if the starting point for uranium safeguards is moved up and thorium begins to play a larger role in the global nuclear establishment, then thorium safeguards should move to the equivalent point in the thorium fuel cycle.
Unlike thorium, U-233 is subject to stringent safeguards. An SQ of U-233 is only 8 Kg, the same as plutonium. U-233 is a highly desirable weapons material despite any practical U-232 content. According to the material attractiveness studies done at LANL, PNNL and LLNL, the additional dose from U-232 decay products is insufficient to stop a determined adversary. While it may be somewhat beneficial to physical protection, the dose rate is not high enough to incapacitate workers. Dose rates can be greatly reduced with shielding and military personnel handling a fabricated weapon should be able to have reasonable dose rates using established techniques. For example, the pit can be stored in lead shielding and only placed in the weapon immediately before use.
The actual military lineage of civilian LWRs is that the US Navy developed pressurized water reactors fueled with enriched uranium to power vessels in the 1950s, starting with USS Nautilus. That variety of reactor had the fewest unknowns when the civilian nuclear industry first developed in the US and the path dependency effects continue today.
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.
>I don't know who in the thorium boosters' club started the "light water reactors were meant for weapons plutonium" meme but it's nonsense.
Maybe the misunderstanding started from the fact that the first British power-generation reactors were intended to produce weapons-grade plutonium. Those reactors were quite different from light-water reactors: graphite-moderated, gas-cooled, and using unenriched fuel.
To quote "Technologies Underlying Weapons of Mass Destruction",
Reactor-grade plutonium recovered from civilian reactors differs from weapon-grade plutonium in the relative proportions of various plutonium isotopes. Reactor-grade plutonium has a higher rate of spontaneous fission reactions than weapon-grade, generating neutrons that can initiate the nuclear chain reaction during weapon detonation sooner than would be optimal. As a result, using reactor-grade plutonium in a first generation nuclear weapon can significantly reduce both the predictability and the expected yield of a weapon designed by a proliferant state.
None of the states that have either made nuclear weapons or attempted to do so appear to have selected anything but high-quality plutonium or uranium for their designs. Nevertheless, from a technical perspective, reactor grade plutonium can be used to make nuclear weapons, and any state possessing significant quantities of separated plutonium should be considered to have the material needed to fabricate nuclear components for nuclear explosive devices in a short period of time.
http://www.au.af.mil/au/awc/awcgate/ota/9344.pdf
If you want to seize on the part where it says that one could build bombs from civilian reactor waste even though it's less straightforward and no nuclear weapons state has ever done that, then I'll point out that the thorium fuel cycle likewise could be used for bombs even though no NWS has ever taken that path (Quoting LANL's "Thorium Based Power Systems and Relevant Safeguards Considerations"):
Thorium is not safeguarded until it is converted into fuel materials. It would seem that if the starting point for uranium safeguards is moved up and thorium begins to play a larger role in the global nuclear establishment, then thorium safeguards should move to the equivalent point in the thorium fuel cycle.
Unlike thorium, U-233 is subject to stringent safeguards. An SQ of U-233 is only 8 Kg, the same as plutonium. U-233 is a highly desirable weapons material despite any practical U-232 content. According to the material attractiveness studies done at LANL, PNNL and LLNL, the additional dose from U-232 decay products is insufficient to stop a determined adversary. While it may be somewhat beneficial to physical protection, the dose rate is not high enough to incapacitate workers. Dose rates can be greatly reduced with shielding and military personnel handling a fabricated weapon should be able to have reasonable dose rates using established techniques. For example, the pit can be stored in lead shielding and only placed in the weapon immediately before use.
http://permalink.lanl.gov/object/tr?what=info:lanl-repo/lare...
The actual military lineage of civilian LWRs is that the US Navy developed pressurized water reactors fueled with enriched uranium to power vessels in the 1950s, starting with USS Nautilus. That variety of reactor had the fewest unknowns when the civilian nuclear industry first developed in the US and the path dependency effects continue today.