• The plan would permit North Korea to work on its nuclear energy and spaceflight programs, but end its efforts to build nuclear-tipped missiles.

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Donald Trump and Kim Jong Un's summit in Singapore has ended, but the four-hour meeting may be just the first step in a long, drawn-out series of nuclear weapons talks.

Some experts called the initial agreement between Kim and Trump "depressing" due to its lack of concrete steps to close or dismantle nuclear weapons facilities in North Korea. However, many others see it as a useful starting point.

The Trump administration says it eventually wants a deal that meets something called CVID: complete, verifiable, and irreversible denuclearization (or dismantlement, depending on whom you ask).

But under that kind of all-or-nothing agreement, North Korea would have to stop pursuing civilian nuclear energy and spaceflight programs in addition to its weapons development. For that reason, nuclear weapons experts at Stanford University's Center for International Security and Cooperation think Kim and his envoys would likely reject CVID.

So the experts at Stanford crafted a rough plan that they think the Trump administration should pitch instead. The three-phase proposal comes close to CVID, but would face less risk of rejection from North Korea.

An alternative to total and immediate denuclearization

After the summit, David Wright, the co-director of the Union of Concerned Scientists' global security program, told Business Insider in an email that "it appears to have set the two countries on a path to trying to solve the biggest issues between them, which led to fears of war only months ago."

After all, Trump was only recently calling Kim "little rocket man" and threatening him with the size of his "nuclear button."

Optimists say that if the US and North Korea are lucky, both nations could get a mutually beneficial agreement that lowers the world's risk of nuclear war. North Korea might get stifling economic sanctions relaxed or eliminated and see the end of "provocative" military exercises by the US and its allies, while Kim's threatening arsenal would be reduced or eliminated.

But there's a lot of work left to do, and probably years of it, which necessitates a long-term plan.

In anticipation of a Trump-Kim summit, three researchers at Stanford - Siegfried Hecker, Robert Carlin, and Elliot Serbin - published a report in May titled "A technically-informed, risk management roadmap to 'denuclearization.'" It includes a color-coded chart that outlines which risks to take care of and when.

"The approach suggested here is based on our belief that North Korea will not give up its weapons and its weapons program until its security can be assured," they authors wrote. That security would involve guarantees that the US won't invade North Korea, depose Kim, or squeeze the country's economic independence.

"Such assurance cannot be achieved simply by an American promise or an agreement on paper, it will require a substantial period of coexistence and interdependence," they wrote.

The report says a workable agreement would likely "be forged during long and complicated negotiations." It recommends a long-term approach in which nuclear weapons capabilities are gradually curtailed, inspected, taken offline, banned, or destroyed over six to 10 years.

They advise tackling this work in three phases: Halt, roll back, and eliminate.

"The phased approach will also provide an effective way to build trust and interdependence, which are required for a viable long-term solution - complete demilitarization of North Korea's nuclear program," the authors said. "We believe it has a chance of being supported by Pyongyang based on our experience in dealing with North Korea's diplomatic and technical communities."

Below are some of the most pressing risks the three phases aim to control.

Phase 1: Halt - Year 1

Since 1992, North Korea has been working to develop nuclear weapons despite the risk of sanctions. The effort is a cornerstone of the country's quest to assert its sovereignty, and possibly intended to be a bargaining chip to attain international acceptance and status.

The report proposes an agreement in which the Trump administration placates Kim by stopping nearby US military exercises, loosening economic sanctions, and meeting other demands. In exchange, North Korea would hit the pause button on most aspects of its nuclear program within one year. More specifically, that would involve:

This crucial first step would help slow or even stop production of nuclear weapons and development. To get there, the authors say the Trump administration should be prepared let North Korea keep any non-military programs that overlap with its development of bombs and missiles - including nuclear power plants and satellite launches.

"Our experience in dealing with the North has also taught us that retaining a civilian nuclear program and a peaceful space program are of great importance to the North," the authors wrote.

Phase 2: Roll back - Years 2 to 5

The next phase seeks to open up North Korea to international inspectors who would help verify future progress.

The country would have to:

Phase 3: Eliminate - Years 6 to 10

The next phase seeks to demilitarize North Korea's nuclear and missile programs while also putting the country under the watchful eyes of the international community. That would require North Korea to:

Though the plan suggests six to 10 years to complete these three phases, its authors say the timeline can certainly change.

"Political development will, of course, determine whether or not that time frame can be shortened or lengthened," they wrote.

Whatever the Trump administration chooses to do, the authors say to avoid immediate CVID, which would "eliminate everything up front and virtually all at once" and "is tantamount to a North Korean surrender scenario" - something they say "is unimaginable that Kim will agree to."