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During World War II the United States spent a large amount of money on developing nuclear technology. Much of that investment went into industrial systems designed to separate U-235 from U-238, or to transform U-238 into Pu-239. Here such systems were built in Oak Ridge, Tennessee, My childhood hometown. The Uranium separation technologies were gaseous diffusion, thermal diffusion and electromagnetic separation. In addition, Plutonium was produced in the Graphite Reactor. Of the 4 technologies, thermal diffusion was the least successful.

 
The almost unknown Oak Ridge thermal diffusion plant, S-50, was capable of tripling natural uranium's enrichment, but that level of enrichment was not very useful. During WW II thermal diffusion processed uranium was used to speed up the output of K-25, the gaseous diffusion plant. As soon as World War II was over, the S-50 plant was shut down, The electromagnetic plant was hugely expensive to build and operate, and not nearly as effective as gaseous diffusion separation. Electro magnetic was accomplished by means of a device called the calutron. Oak Ridge Y-12 calutrons were expensive to build, because of the amount of silver that went into them. They were also expensive to operate, because their operation required a large amount of electricity. By 1946 all but one of the Y-12 Calutrons were shutdown. However, the calutron was considered sufficiently successful to under development as a proliferation tool by Iraq prior to the First Gulf War.

Gaseous diffusion was a successful but expensive to develop and operate technology Gaseous diffusion production was developed by the United States, the Soviet Union, China, the United Kingdom and France. The major drawback to gaseous diffusion is the expense of operation. The technology requires a lot of electricity. For this reason gaseous diffusion plants are being shut down and replaced by centrifuges.

The Oak Ridge X-10 Graphite reactor was never intended to produce plutonium for weapons, instead the reactor was intended to support research, that would contribute to the development of the Handford weapons complex.However, given enough time the X-10 Graphite reactor could have produced enough weapons grade Pu-239 to build one or more nuclear weapons. In fact the United Kingdom's first two military grade Pu-239 facilitated designs that were closely related to that of the graphite reactors. The design of later Pu-239 production reactors used graphite cores. Finally the design of the reactor which North Korea uses to produce its nuclear devices is closely related to the UK's Second Generation weapons productions Reactors. The Handford Reactors were water cool graphite piles, a design that was also adopted by the Soviet Union.

I should mention two more Uranium separation technologies. The first is is the centrifuge. Ye centrifuge uses far less electricity than the Gaseous diffusion method, but is relatively simple to build. Even Nations that are nor highly advanced industrially, are capable of producing Centrifuges, and centrifuge arrays, capable of producing weapons.grade U-235. Pakistan's nuclear weapons program is based on centrifuge technology, even though Pakistan is not advanced industrial state.

I should mention one more successful Uranium separation, the Aerodynamic separation process. This system is in expensive to build, and has been successfully used by South Africa to produce the U235 needed to manufacture 6 nuclear weapons. The major drawback of Aerodynamic separation is that it requires a lot of electricity. Yet if a small country wishes to produce a few nuclear weapons at a relatively low cost, Aerodynamic separation is the way to go. On the other hand, Aerodynamics separation is not as low cost as centrifuge separation for a bigger5 nuclear program.

Finally, we come to heavy water reactors, a class of reactors that uses water with one atom of deuterium rather than simple hydrogen. Heavy water is a superior moderator, better even than graphite. and thus heavy water reactors are excellent producers of Pu-239. Three nations have used Heavy water reactors, to produce weapons grade plutonium for nuclear weapons. they are the United States, India and Israel. The American reactors were large and complex reactors deployed as part of an industrial system designed to produce Hydrogen Bombs. Both the Indian and the Israeli reactors were the result the heavy water reactor project carried out by British, French and Canadian scientists during World War II. After the war, France assisted Israels program to develop nuclear weapons. The modified copy of the WW II jointly developed reactor was passed by the French to the Israelis/ The Indian reactor was quite similar and was sold to India by Canada.

Thus any nation would have a wide variety of tested options available to it, If it wished to produce locally made weapons grade materials. The cost of producing a limited amount of such material is not great, and there are tested production methods.
 
Rational decisions and decision makers and nuclear proliferation
 
Decision makers who are on the whole rational are much more likely make decisions that lead to successful nuclear weapons programs. On the whole, small programs are more likely to be successful than large programs. Lower cost nuclear materials are more desirable than higher cost materials, that have no more military usefulness. Programs using proven technology are more likely to sauced than programs that use unproven technology. Programs using simple technology are more likely to succeed than programs using complex technology.
 
The first explosion of a nuclear weapon was a test of a Pu-239 weapon. The first test of a U-235 weapon was occurred when it was used against the Japanese. the triggering mechanism of the U-235 bomb was very simple, while the triggering of the Plutonium bombs triggering was far more complex. Of course if you have got help. you may not need a test. Israel has, as far as we know, never tested a nuclear weapon, yet its weapons appears to use Pu-239. leads to easier for bomb design to vaporize than U-235. Thus U-235 is the weapons material of choice for nations with limited weapons technology skills.
 
There are well defined and tested paths to nuclear proliferation, most of which use World War II and Cold War era technology. In contrast, new nuclear technology, for example the LFTR, do not provide a tested path to weapons production. Given potential costs, and other uncertainties which new nuclear technology poses to nations seeking to acquire nuclear weapons, more traditional routes would be far more appealing.
 
Thus the LFTR and other generation IV reactors do not make nuclear proliferation more likely, and they are not particularly useful proliferation tools. It should be noted that while the United States government regards laser isotope separation to pose a serious proliferation threat, it does not hold the Molten Salt Reactor technology in the same regard.
 
Not all paths to nuclear weapon development technologies are created equal. Some are simpler, lower cost, and better tested than others. Would be proliferators are most likely to choose the simplest, best tested and lowest cost path available to them. The development of new, more expensive, and more challenging paths to the spread of nuclear weapons is very unlikely to increase the spread of nuclear weapons. These warnings about the proliferation dangers of new nuclear technologies may represent bogus expressions of an anti-nuclear stance. It is not enough to claim that a new nuclear technology poses a proliferation danger. The proliferation argument must show how the development of new nuclear technology increases the likelihood that the new technology will lead to nuclear proliferation.