The vast scope of North Korea’s atomic program means ending it would be the most challenging case of nuclear disarmament in history. Here’s what has to be done to achieve — and verify — the removal of the nuclear arms, the dismantlement of the atomic complex and the elimination of the country’s other weapons of mass destruction.

President Donald Trump said he agreed to meet Kim Jong Un because the North Korean leader has signaled a willingness to “denuclearize.” But that word means very different things in Pyongyang and Washington, and in recent weeks Trump has appeared to back away from his earlier insistence on a rapid dismantlement of all things nuclear — weapons and production facilities — before the North receives any sanctions relief.

However, the task of “complete, verifiable, irreversible denuclearization” — the phrase that Secretary of State Mike Pompeo keeps repeating — will be enormous. Since 1992, the country has repeatedly vowed never to test, manufacture, produce, store or deploy nuclear arms. It has broken all those promises.

North Korea has 141 sites devoted to the production and use of weapons of mass destruction, said a 2014 Rand Corp. report. Just one of them — Yongbyon, the main atomic complex — has 663 buildings, said the Institute for Science and International Security, a private group in Washington.

Some nuclear experts argue that unwinding more than 50 years of North Korean open and covert developments is too large for outsiders. The best approach, he contends, is for inspectors to monitor disarmament. The time estimates range from a few years to a decade and a half. Here’s what is involved in each of the major disarmament steps:

Dismantle and remove

John Bolton, Trump’s hawkish national security adviser, has argued that before any sanctions are lifted, the North should deliver all its nuclear arms to the U.S., shipping them to the Oak Ridge National Laboratory in Tennessee, where inspectors sent Libya’s uranium gear. It’s almost unimaginable that the North would simply ship out its weapons.

Siegfried S. Hecker, a Stanford professor who formerly headed the Los Alamos weapons laboratory in New Mexico, argues that the only safe way to dismantle the North’s nuclear arsenal is to put the job, under inspection, in the hands of the same engineers who built the weapons. Otherwise, he said, outsiders might accidentally detonate the nuclear arms.

Halt uranium enrichment

Factories holding hundreds of centrifuges spin gaseous uranium until it is enriched in a rare form of the element that can fuel reactors — or, with more enrichment, nuclear arms.

It’s easy to shut down such plants and dismantle them. The problem is that they’re relatively simple to hide. Because uranium can also be used to fuel reactors that make electricity, North Korea is likely to argue it needs to keep some enrichment plants open for peaceful purposes.

After arguing that the Obama administration made a “terrible deal” by allowing modest enrichment to continue in Iran, it is hard to imagine how Trump could insist on less than a total shutdown in North Korea.

Disable reactors

Inside a reactor, some of the uranium in the fuel rods is turned into plutonium, which makes a very attractive bomb fuel. Pound-for-pound, plutonium produces far more powerful nuclear blasts than does uranium. In 1986, at Yongbyon, North Korea began operating a 5-megawatt reactor, which analysts say produced the plutonium fuel for its first atomic bombs. The North has commissioned a second, larger reactor. Reactors are hard to hide: they generate vast amounts of heat.

Close nuclear test sites

Since 2006, the North has detonated nuclear devices at least six times in tunnels dug deep inside Mount Mantap.

Last month, the North blew up test-tunnel portals as a conciliatory gesture before the planned denuclearization talks. Experts said it leaves open the question of whether the damage is irreversible.

End H-bomb fuel production

At the heart of a missile warhead, an exploding atomic bomb can act as a superhot match that ignites thermonuclear fuel, also known as hydrogen fuel. The resulting blast can be 1,000 times more powerful than the Hiroshima bomb. North Korea is suspected of having at least two sites for different aspects of H-bomb fuel production. The Trump administration stance is unclear on such sites.

Inspect anywhere, forever

Under past agreements, inspectors from the International Atomic Energy Agency have lived in North Korea, but their movements were limited. For inspections to be effective, they must cover the whole country . (One of Trump’s complaints about the Iran deal was that inspectors were inhibited from going anywhere.)

Destroy germ weapons

Biological weapons can be more destructive than nuclear arms. A single gallon of concentrated anthrax is said to have enough spores to kill every person on Earth. The challenge is how to deliver the living weapons.

North Korea is suspected of having a large complex for making germ weapons. The problem is learning its true dimensions, and verifying its dismantlement. Experts argue that the gear for producing germ weapons is often identical or similar to that of medicine and agriculture, making it extremely hard if not impossible for outsiders to verify that germ-weapon work has ended. The Trump administration wants the North to end all work on biological weapons.

Destroy chemical weapons

Last year, the deadly nerve agent VX was used to assassinate Kim Jong Nam, the half brother of the North’s leader. The killing cast light on the North’s chemical weapons. Although the North denies having any, experts rank the nation as among the world’s top possessors, saying it harbors thousands of tons of the banned armaments. The Trump administration’s negotiating list with the North includes chemical disarmament.

Curb missile program

In November, the North tested an improved intercontinental ballistic missile that flew farther than any other — far enough to threaten all of the United States. Curbing the missile program is high on the Trump administration’s negotiation list. A key question is whether negotiators will also try to redirect the North’s large corps of rocket designers and engineers into peaceful activities, such as making and lofting civilian satellites.