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Iran’s announcements that it is going to produce 20 percent Highly Enriched Uranium is bad news — not least because the Iranians said they would do so without respect to the presumably doomed TRR fuel swap. (Though this thing has risen from the dead so many times, maybe I should start calling it the Zombie Fuel Swap.)

I made some comments to Le Figaro, but they are in French. (Well, they’ve been translated. I can barely say Châteauneuf-du-Pape.)

Glenn Kessler has a news analysis on the implications of the deal, in which 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).

This is simplified illustration, of course, since some of the U235 ends up in the depleted stream as “tails” — but you get the idea.

You can see why separative work is measured as mass — the interesting question is the amount of material separated — and why the lower levels of enrichment actually require more work.

As a result, as Kessler notes, Iran’s announcement “marks a new and potentially dangerous turn” in the situation. If Iran enriches a significant amount of U235 to 20 percent — and that’s a stated goal right now, not yet an actual achievement — then Iran would be able to “top off” the enrichment a small, clandestine facility like the one revealed near Qom. I did a few calculations, but I don’t need to wade into the middle the FAS–ISIS steel cage death match that Josh has so ably chronicled. I will just encourage people to do their own back-of-the-envelop calculations. I think the answer is pretty obvious.