Let’s assume the signals from the White House and Tehran are reliable, and that Iran is serious about an agreement to remove its existing stockpile of 20 percent enriched uranium from the country and stop producing more. What happens then?

This question of “next steps” in the Iran nuclear talks is important, because neither side is likely to commit to the first set of “confidence-building” measures unless it knows where the process is heading. Iran feels it has been tricked in the past by Western peace feelers that didn’t lead anywhere; the U.S. has the same suspicion. Both sides need more clarity.

A compelling framework for future talks has been prepared by analysts from the Carnegie Endowment for International Peace. The authors are George Perkovich, a leading U.S. scholar on proliferation issues, and Ariel Levite, a former deputy director of the Israel Atomic Energy Commission. In preparing the plan, the Carnegie team has had quiet discussions with U.S. and Iranian experts.

The basic idea of the Carnegie proposal is to create a “firewall” between Iran’s civilian nuclear program, which it could pursue, and a military bomb-making program, which it couldn’t.

A big selling point for the Iranians is that this approach is based on Supreme Leader Ali Khamenei’s pledge that Iran won’t build nuclear weapons.

Khamenei’s most explicit statement came on state television in February: “Iran is not after nuclear weapons because the Islamic Republic, logically, religiously and theoretically, considers the possession of nuclear weapons a grave sin and believes the proliferation of such weapons is senseless, destructive and dangerous.”

President Obama sent a back-channel communication to Khamenei in late March that his fatwa banning nuclear weapons would be a good starting point for negotiations. Turkish Prime Minister Recep Tayyip Erdogan delivered the message when he met Khamenei on March 29. Undersecretary of State Wendy Sherman reiterated Obama’s theme during the first round of negotiations with the Iranians in Istanbul on April 14.

So how could Iran fulfill this pledge in a way that reassures Israel and other nations that fear a nuclear-armed Iran? The Carnegie experts propose a red-yellow-green system, like a nuclear traffic light.

In the “green” approved category would be nuclear power plants, medical research reactors and basic academic and scientific research. Forbidden “red” activities would be those directly related to weaponization, such as warhead design and procurement of items used in making and testing bombs.

The “yellow” dual-use activities would be the trickiest problem, and the firewall would have to be carefully constructed. Some enrichment of uranium might be permitted, for example, if it were verifiably limited below 5 percent — so it could be used for only peaceful purposes. So-called “neutron triggers” would be banned, since they could be used to initiate a bomb’s explosion.

Any real reduction of tensions with Iran will also require greater openness about past as well as present activities, so that each side is confident it isn’t being cheated, and that its basic security hasn’t been compromised. That’s part of the Carnegie proposal, too.

“This approach is not a zero-sum game,” argue Perkovich and Levite. “It would require commitments and concessions from both sides.” And by defining the activities that are part of building a nuclear weapon, it would fill a gap in the existing Non-Proliferation Treaty.

The talks that began last month in Istanbul between Iran and the “P5+1” group of nations are at the initial confidence-building stage. They’re aimed at gaining time for a comprehensive agreement like what the Carnegie authors propose.

What’s likely to be on the table at the next meeting in Baghdad on May 23 is a plan for Iran to stop enriching uranium above 5 percent, and ship its existing stockpile of 20-percent-enriched uranium (currently estimated at more than 100 kilograms) out of the country, in return for medical isotopes and fuel rods for a civilian research reactor.

Obama believes this interim agreement would buy time for further negotiations, by delaying Iran’s bomb-making ability. But to have a lasting pact, it will be necessary to translate Ayatollah Khamenei’s words into a clear and verifiable “red line.”

David Ignatius writes about foreign affairs for The Washington Post. He can be contacted at:

davidignatius@washpost.com