It’s getting down to the number of centrifuges allowed for Iran:
“We need at least 100,000 IR-1 (first generation) centrifuges to produce enough fuel for each of our (civilian) nuclear (power) plants. We have informed the International Atomic Energy Agency about our plans to build 20 plants,” a senior Iranian official said on condition of anonymity. U.S. officials, however, have made clear for months that the number of centrifuges they are willing to tolerate operating in Iran over the medium term is in the low thousands to ensure that Tehran’s ability to produce a usable amount of bomb-grade uranium, should it go down that road, is severely limited.
Iran, which has demonstrated a readiness to curb higher-level enrichment, says such draconian limitations would be a violation of its right to enrich – an issue Supreme Leader Ayatollah Ali Khamenei has said is a “red line” for Tehran.