Federation of American Scientists:

The only proven and practical source for the large quantities of neutrons needed to make plutonium at a reasonable speed is a nuclear reactor in which a controlled but self-sustaining 235 U fission chain reaction takes place.

Discussion of the particle accelerator route which would enable the bootstrapping of a non U-235 route eventually (producing a critical mass of 10+ kg of plutonium using superconductors and huge amounts of energy and accelerator time), but only with much increased difficulty:

Wilson died in 2000 but a paper he wrote on this topic in 1976 has now found its way onto the arXiv and it highlights some thought-provoking ideas.

At the time, Wilson was director of Fermilab where he was building an accelerator called the Energy Doubler/Saver, which employed superconducting magnets to steer a beam of high energy protons in a giant circle. These protons were to have energies of up to 1000 GeV.

The Energy Doubler was special because it was the first time superconductivity had been used on a large scale, something that had significant implications for the amount of juice required to make the thing work. “One consequence of the application of superconductivity to accelerator construction is that the power consumption of accelerators will become much smaller,” said Wilson. And that raised an interesting prospect.

Imagine the protons in this accelerator are sent into a block of uranium. Each proton might then be expected to generate a shower of some 60,000 neutrons in the material and most of these would go on to be absorbed by the nuclei to form 60,000 plutonium atoms. When burned in a nuclear reactor, each plutonium atom produces 0.2 GeV of fission energy. So 60,000 of them would produce 12,000 GeV.

Using this back-of-an-envelope calculation, Wilson worked out that a single 1000 GeV proton could lead to the release of 12,000 GeV of fission energy. Of course, this neglects all the messy fine details in which large amounts of energy can be lost. For example, it takes some 20MW of power to produce an 0.2MW beam in the Energy Doubler.

But even with those kinds of losses, it certainly seems worthwhile to study the process in more detail to see if overall energy production is possible.

In a natural uranium fuelled reactor, the actual fuel (at startup) is U235, present in the natural uranium at a concentration of about 0.7%. No U235 in nature = no easy plutonium.

Those two "distinct" pathways both rely on properties of 1 highly unusual nucleus, U235, which is both easy to fission, and stable enough to still be around after ~5 billion years. How unusually stable is it? Well, from the date of, say, 40 million years since explosion of the supernova that formed Solar system, it was for all intents and purposes the only one such isotope left. (Every other fissile isotope was gone not because of fizzling or spontaneous fission but because of alpha decay and such)

I explained it in greater detail here .