BRUSSELS — For the first time in many months, European officials expressed increasing optimism on Tuesday that a revival of the 2015 Iran nuclear deal may actually be agreed upon by Iran and the United States.
Just before midnight Monday in Tehran, Iran sent its promised response to what the European Union has called a final text of an agreement to restore the nuclear deal, known as the Joint Comprehensive Plan of Action, or J.C.P.O.A., which former President Donald J. Trump abandoned in 2018.
The response, after tough internal talks inside Iran, was encouraging, officials said. It was not a complete yes, but it made no significant new objections. Instead, Iran is seeking further clarifications on assurances that the United States would lift punishing economic sanctions and that such assurances would be valid once President Biden leaves office.
Earlier on Monday, Iran’s foreign minister had called on the United States to show flexibility to resolve three remaining issues. Iran’s state-run IRNA news agency offered no details Tuesday, but said: “The differences are on three issues, in which the United States has expressed its verbal flexibility in two cases, but it should be included in the text. The third issue is related to guaranteeing the continuation of the J.C.P.O.A., which depends on the realism of the United States.”
Nabila Massrali, speaking for the European Union, said of Iran’s response: “We are studying it and are consulting with the other J.C.P.O.A. participants and the U.S. on the way ahead.”
Understand the Iran Nuclear Deal
The prospect of a return to power of Mr. Trump, who called the 2015 deal the worst in history before breaking it apart, has hung over these 16 months of talks. Mr. Biden’s hopes for a better deal, “longer and stronger,” as his officials put it early on, have been dashed. But Iran has moved so far ahead on stockpiling highly enriched uranium that it could now build a nuclear weapon, which it has always denied wanting to do.
The 2015 deal reduced Iran’s stockpiles of enriched uranium sufficiently to give the United States and its allies at least one year’s “breakout time” — the amount of time Iran would need to build a bomb if it chose to. That “breakout time” can now be measured in days or weeks, which is why Mr. Biden wants to restore the old deal, even if it is weaker than before, because Iran would have to give up nearly all the enriched uranium it has stockpiled since 2018.
Still, Iran has in the interim developed technical knowledge about enrichment that cannot be deleted and learned how to build advanced centrifuges, banned under the original deal, which critics say makes even a revival of the deal less valuable.
As of the last public count, Iran has a stockpile of some 3,800 kilograms of enriched uranium, well beyond the limits of the 2015 deal. Under that deal, Tehran could enrich uranium only up to 3.67 percent purity, while maintaining a stockpile of uranium of 300 kilograms under constant scrutiny of surveillance cameras and international inspectors.
But now, Iran enriches uranium up to 60 percent purity — useless for civilian applications and a short step away from bomb grade.
At the same time, in a separate dispute with the International Atomic Energy Agency, the U.N. nuclear watchdog that is supposed to monitor any deal, Iran has turned off key surveillance cameras and not allowed the agency to replace memory cards.
Notably, the Iranian response did not include its earlier stated demand that the I.A.E.A. drop its three-year investigation into unexplained deposits of highly enriched uranium, which Iran has refused to account for. But officials said that the dispute with the I.A.E.A. was an important but separate issue from the nuclear deal, which would not block agreement or implementation of the deal.
On Monday, in Washington, the State Department spokesman, Ned Price, said the United States would share its own response with the European Union, which is chairing the talks, and urged Iran to drop “extraneous demands” that go beyond the nuclear deal.
“The only way to achieve a mutual return to compliance with the J.C.P.O.A. is for Iran to drop further unacceptable demands that go beyond the scope of the J.C.P.O.A.,” Mr. Price said.
Farnaz Fassihi contributed reporting from New York.