Iran’s production of nuclear fuel plunged in recent weeks, the United Nations atomic agency reported on Monday, following alleged sabotage of its main nuclear facility in April that Tehran has blamed on Israel. The drop in production, detailed in the International Atomic Energy Agency’s confidential report circulated Monday, gives the first substantive insight into the impact of the incident, which took out the power supply at Natanz and destroyed potentially hundreds of centrifuges, machines that enrich uranium.

Israel has declined to comment on the incident, one of several at Iran’s nuclear facilities over the last 18 months. Tehran accused Israel at the time of orchestrating the attack to undermine the start of talks in Vienna involving Iran and the U.S. aimed at restoring the 2015 nuclear deal. Former President Donald Trump withdrew the U.S. from the deal in 2018.

Iran’s stockpile of enriched uranium increased by an estimated 273 kilograms in the three months ending May 22, the IAEA said, just over half the 525 kg stockpile increase in the previous quarter. The Natanz incident happened April 11, halfway through the reporting period. The agency also reported that on May 24, there were 20 cascades of centrifuges working at the fuel enrichment plant at Natanz, a sharp decrease from before the April incident.

Iran’s total stockpile is now estimated to be around 3,241 kilograms, about 16 times higher than the permitted stockpile in the 2015 agreement. Experts say that amount is likely enough nuclear material, if purified to weapons-grade level, for up to three nuclear weapons.

Iran responded to the Natanz incident by starting to produce uranium enriched to 60% for the first time, a level close to weapons grade, and by saying it would install more advanced centrifuges at the site.

Iran’s Nuclear Program: What We Know About Tehran’s Key Sites
While Iran says it isn’t trying to build nuclear weapons, a look at its key facilities suggests it could develop the technology to make them. WSJ breaks down Tehran’s capabilities as it hits new milestones in uranium enrichment and limits access to inspectors. Photo illustration: George Downs

In its report, the IAEA said Iran had only produced 2.4 kilograms of 60% enriched uranium. Its stockpile of 20% enriched uranium reached 63 kilograms. The rest is low-enriched material.

At the time of the Natanz incident, Iran played down the damage done. Ali Akbar Salehi, the head of Iran’s atomic agency, said the country’s enriched- uranium production was progressing vigorously. Iran also introduced more advanced centrifuges, which would increase its production speed.

Last week, IAEA Director General Rafael Grossi called Iran’s broadening nuclear program “very concerning,” saying only countries producing nuclear weapons were enriching uranium to 60% levels.

Biden administration officials have said Iran’s advancing nuclear program underscores the need to restore the 2015 deal, which placed strict but temporary restrictions on Iran’s nuclear activities.

Iran lifted its uranium stockpile above the 2015 deal’s permitted levels in July 2019, responding belatedly to the Trump administration’s decision to quit the agreement and impose economic sanctions on Tehran.