TOKYO -- With a week remaining before U.S. President Donald Trump and North Korean leader Kim Jong Un hold their historic summit, a raft of details remain unsettled. Chief among them: How might North Korea go about giving up its nuclear arsenal?

North Korea has pledged to "denuclearize" a number of times already this year. But the hows and whens remain unclear.

The summit, despite already being hailed as the "deal of the century," remains at risk of deteriorating. In fact, the talks between the superpower and its 1950s foe that have regained so much momentum in the past week or so could still break down.

Even if the U.S. and North Korea can agree on some details, history shows that following through will not be easy.

During earlier preparations for the meeting, the U.S. appeared to be operating under the assumption that North Korea would unilaterally abandon the nuclear weapons it has spent decades obtaining -- and that it would do so in a complete, verifiable and irreversible manner. In practical terms, this would entail the swift removal of nuclear warheads and inspections by the International Atomic Energy Agency, a United Nations watchdog.

North Korea has had something else entirely in mind: a phased approach. It would decrease its nuclear capabilities step by step in return for the lifting of sanctions and other economic benefits. Kim has already lined up allies for such a process, receiving approval from Chinese President Xi Jinping and Russian Foreign Minister Sergei Lavrov.

The U.S. appears to have relented.

When Trump on Friday announced he would go through with the summit as originally set on June 12 in Singapore, he said he told North Korean envoy Kim Yong Chol to "take your time" with denuclearization. Perhaps "relented" is an understatement. The author of "Trump: The Art of the Deal" granted a major concession as his negotiators took a pragmatic turn.

But "take your time" is rather vague. Does Trump have months' worth of patience or years' worth? And does Kim intend to stick to any timeline?

The fog hanging over this issue may never lift.

But there is a more immediate issue: What does 'denuclearization' mean?

North Korean envoy Kim Yong Chol talks with U.S. President Donald Trump at the White House on June 1.   © Reuters

North Korea is demanding "denuclearization of the Korean Peninsula." What this entails is anybody's guess. South Korea has no nuclear weapons. The U.S., which had kept nuclear arms in the country during the Cold War, removed them in 1991. To Kim, "denuclearization" could mean drawing a no-fly arc somewhere around the peninsula that long-range B-52 bombers stationed on the South Pacific island of Guam would have to obey. It could also mean a similar no-go zone for U.S. submarines. In other words, North Korea would want the U.S. to pull its nuclear umbrella away from South Korea. Perhaps North Korea would insist that the U.S. do so in a complete, verifiable and irreversible manner.

Such a demand would lead to quite a debate. But there is also speculation that a nuclear-free zone like the one New Zealand declared in 1984 would suffice.

"There are some ways to potentially craft a nuclear-free zone where you may still have a nuclear umbrella," said Jonathan Berkshire Miller, a senior visiting fellow at the Japan Institute of International Affairs in Tokyo.

This brings another nuclear-free country into the debate, Japan, which also falls under the U.S.'s nuclear umbrella.

"The nightmare scenario," Miller said, "is that ... the U.S. operationally feels it has gotten rid of its umbrella, but North Korea was basically pretty much lying the entire time."

Another issue that could arise during the summit and following talks is the peaceful use of nuclear energy. Does North Korea have the right to use nuclear materials and operate reactors?

South Korea is awash in nuclear power, and its neighbor is likely to demand that it also be entitled to this source of electricity were it to return to the non-proliferation treaty and take other steps.

"North Koreans would say that the denuclearization of the peninsula should be on equal terms," said Frank Jannuzi, the president and CEO of the Mansfield Foundation, a Washington-based non-governmental body promoting U.S.-Asia relations.

This issue has its own set of hows and whens.

Experts say North Korea would have to accede to a number of conditions. At a minimum, it would have to allow IAEA personnel to carry out inspections of the known nuclear facility in Yongbyon, north of Pyongyang.

Yongbyon has a 5-megawatt reactor, equipment to extract plutonium from spent nuclear fuel and a uranium enrichment plant.

Nuclear weapons make use of plutonium or highly enriched uranium.

Besides returning to the non-proliferation treaty, North Korea could also be pushed toward signing the Comprehensive Test Ban Treaty. This would bring a degree of awkwardness to the difficult negotiations: The U.S. is not a signatory to the CTBT.

The U.S. negotiators will also have to deal with some invisible hurdles.

"The real problem is not with the facilities that the IAEA and the rest of the world know about," said Frank Pabian, an analyst monitoring non-proliferation at 38 North, a Washington-based website tracking nuclear weapons activity in the reclusive communist state. "It is the facilities that are clandestine."

Some scholars estimate that North Korea has a plutonium stockpile of 20-40kg, or enough for eight nuclear warheads.

But the exact amount is not known.

"A full production operational history has to have been recorded, and all accounting would have to be turned over and then verified by an organization like the IAEA," Pabian said.

The odds of North Korea disclosing all this are long, Pabian said, noting Pyongyang's refusal to allow experts to take a look before it conducted explosions that it said destroyed an underground nuclear test site in Punggye-ri. Since no experts were on hand, there is no way of knowing if the explosions were anything more than a show.

North Korea has a history of clandestine activity. In 2002, it was found to be enriching uranium despite having signed an agreement with the U.S. in 1994 to freeze its nuclear program.

"Given the historical precedents," Pabian said, "there is little reason to be optimistic."

To ensure that North Korea denuclearizes, the U.S. is expected to demand that all weapons-related materials, equipment and facilities be exported out of the country. This would be anathema to North Korea. It would allow international eyes to take a look at exactly what North Korea has -- and perhaps glean how and from where the country obtained the technology.

"It is hard to imagine such a secretive and isolated regime actually giving up its hard-won nuclear weapons arsenal anytime soon," Pabian said.

If the North is cooperative, Washington is prepared to make a range of concessions, Jannuzi of the Mansfield Foundation said. These include holding a conference to discuss a peace treaty to formally end the Korean War, opening liaison offices as a step toward eventual diplomatic normalization and removing North Korea from the list of state sponsors of terrorism.

"The U.S. is not likely to provide any significant sanctions relief until the North takes some serious concrete steps toward denuclearization," Jannuzi, a former State Department official, said.

Trump on Friday indicated he would not seek a specific agreement at the historic summit. "We're not going to go in and sign something on June 12th, and we never were."