One of the critical arguments that Iran apologists bring when discussing the Iran nuclear problem is that Iran has, under the tenets of the Nonproliferation Treaty, the "right" to the development of peaceful nuclear energy.
And indeed Iran has a history of nuclear research that goes back to the the Atoms For Peace program, where the US provided a small (5MW) research reactor.
But these research programs all but disappeared when Khomeini took over.
The problem is that the distinctions between peaceful and military nuclear research, even for experts, are sometimes not very simple.
But this shows why there is a problem.
The key is maraging steel. Maraging what?
Maraging steel is a low-carbon martensite that is highly resistant to cracking under stress. It has only a few uses: thinner rocket and missiles skins; firing pins and breech blocks; high performance engine components; dies and high-wear assembly line components; fencing foils for competition (foil and epee); golf club shafts; centrifuges for uranium enrichment.
Why are the Iranians buying maraging steel?
The kind of centrifuges that you use for normal uranium enrichment, the kind that is allowed, don't use maraging steel. The kind that Iran uses are called the P1 centrifuges. The other kind, called P2, use maraging steel, and they enrich uranium far in excess of the requirements for reactor use.
When the Iranians broke the seals at Natanz, they broke the seals on a significant supply of maraging steel that it needs to continue work on the P2 centrifuges.
But it goes further: where did Iran get the blueprints for these centrifuges? They didn't design them on their own, but rather acquired them from the Pakistani Dr. Khan, part of an 18-year old plan of deception that Iran used to hide its nuclear technology development from the IAEA.
This documents how Iran has hidden its actual research. So does this.
Iran knows at this point how to build centrifuges and also knows the processing chain from yellowcake to weapons-grade uranium. Knowing what needs to be done puts them way ahead of the learning curve: the Iranians are not stupid and their scientists are competent. Buying the blueprints from Khan meant skipping years of tedious design work: they're not interested in learning how to build the processing on their own: they want to be able to process uranium up to and including weapons-grade quality.
That is why the Iranian argument that they have the right to process uranium for peacful purposes is a straw-man argument. A straw-man argument uses the rhetorical trick of refuting not the argument that the opponent is bringing, but rather an alternative that appears to be related. Iran is accused of trying to enrich uranium for weapons; they reply that they have a right to enrich for peaceful purposes. This isn't an answer: it's a misrepresentation that is set up to be easily defeated.
But Iran still hasn't answered the questions asked: what are they doing with a development program that leads to weapons-grade uranium, a program that they have been actively hiding and misrepresenting to the IAEA, a program that only has the goal of making weapons-grade uranium?
Smoking gun. The answer the Iranians are giving has nothing to do with the question asked.