First, let me remind you I am a nuclear physicist by education. Secondly, I worked nearly a decade in the defense industry.
For nearly 40 years, America and Russia worked hard on nuclear weapons. Long after Hiroshima, Russia and America continued to improve on the original designs. They became smaller , more efficient. The very nature of being an engineer is the challenge of achieving theoretical potentials. In layman terms, radioactive fallout is the result of lack of perfect execution of converting all the mass to energy according to Einstein’s’ equations. Shielding transportation, etc. all become engineering problems to be solved. Upgrading from delivering 1 to 16 warheads on a warhead is a significant engineering challenge. Shielding radiation for handling by workers is no simple task either.
However, let me say this. If you have abundant amounts of enriched uranium or plutonium, YOU DO NOT NEED A WEAPONS PROGRAM. If you don’t care about efficiency, and only about an ugly result, it is remarkably easy to construct disastrous weapons. So running 300 or more gas centrifuges 24 hours a day is a weapons program as much as it is a fuel program. There is no difference, only intent. So a fuel program for one politician is the same weapons program for the next one.
Greater than critical mass amounts of fuel can be put in situations of water or pressure that create very dirty thermal explosions. They are easily shipped piecemeal and assembled on site. While you may not physically destroy everything for 50 miles, the radiation will have a practical result that is quite equivalent.
So why would Iran reject an offer of free fuel? I cannot know their intent. What I do know is that the world absolutely does not need more producers of radioactive fuel. First Iran then who? This is not an issue of national sovereignty, but rather an issue of universal security. If there was ever something constructive the United Nations might do, it would be to solve this problem.