As diplomats from Iran and the world’s big powers assemble in Istanbul on the inauspicious day of Friday 13, the key to managing the Iranian nuclear crisis lies deep in a mountain near the Shiite theological center of Qom.

There at a facility called Fordow, Iran has been enriching uranium to 20% U-235, the isotope that in 90% concentration can be used to make nuclear weapons.

Iranian officials have been offering to curb enrichment at the 20% level since last fall, when President Mahmoud Ahmadinejad raised the subject with reporters on the sidelines of the UN General Assembly. However, Iran dithered over accepting new talks with the P5+1 (the five permanent members of the UN Security Council plus Germany) until February 2012, when the pressure of new economic sanctions began to mount.

From the point of view of the P5+1 — and a nervous Israel — Iran has to do something dramatic to inhibit its ability to make a nuclear weapon quickly if it so chose. The best way to insure that it cannot “break out” and dash to a bomb is by freezing its stockpile of 20% enriched uranium, which can be converted in a matter of few months to weapons grade.