Reply #1: They can go from 20% to 90% quickly and easily

[bananas]

http://lewis.armscontrolwonk.com/archive/2620/iran-to-enrich-20-percent-leu

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David Albright explains why Iran could go from 20 percent HEU to a bomb in relatively short order:

Meanwhile, enriching uranium under the guise of medical needs will get Tehran much closer to possessing weapons-grade material. Iran insists it has no interest in nuclear weapons. But Albright said 70 percent of the work toward reaching weapons-grade uranium took place when Iran enriched uranium gas to 3.5 percent. Enriching it further to the 19.75 percent needed for the reactor is an additional “15 to 20 percent of the way there.”

Once the uranium is enriched above 20 percent, it is considered highly enriched uranium. The uranium would need to be enriched further, to 60 percent and then to 90 percent, before it could be used for a weapon. “The last two steps are not that big a deal,” Albright said. They could be accomplished, he said, at a relatively small facility within months.

It must seem odd for casual readers to see 20 percent and 90 percent U235 lumped together as highly enriched uranium or to be be told that Iran will find it much easier to go from 20 to 90, than from 5 to 20. That’s not how everyday math works, where 5 and 20 are closer to “ten” and 90 rounds to “one hundred.”

For many readers (especially of this blog) the answer is obvious. But for those to whom it is not obvious, Francesco Calogero found a nice way to illustrate the same point to students at a previous ISODARCO meeting. The essential concept is understand enrichment as a process of removing undesirable isotopes (or more specifically, isolating the desirable ones).

So, imagine 1000 atoms of uranium. Seven of them will be the fissile isotope Uranium 235. The rest are useless Uranium 238. (If you are the sort of person who just said, “Hey! What about Uranium 234?” or other nitpicks this post is not aimed at you.)

To make typical reactor fuel, Iran or any other country would removes 860 of the non-U235 isotopes, leaving a U235:U238 ratio of 7:140 (~5 percent).

To make fuel for the TRR, Iran removes another 105 non-U235 atoms from the 140, leaving a ratio of 7:35 (20 percent).

To make a bomb, Iran needs only to remove 27 of the remaining 35 atoms, leading a ratio of 7:8 (~90 percent).

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