SEOUL, South Korea (AP) - An unknown number of nuclear warheads. Stockpiles of weapons-grade plutonium and highly enriched uranium. ICBMs. Weapons factories - and the scientists who work at them.

The list of what it would take for the “complete denuclearization” of North Korea[1] is long.

North Korea[2] has said it’s willing to deal away its entire nuclear arsenal if the United States provides it with a reliable security assurance and other benefits.

But there is lingering skepticism ahead of Tuesday’s summit between President Donald Trump and North Korean leader Kim Jong Un that Kim would fully give up the nuclear weapons he has pushed so hard to build.

It wouldn’t be hard to hide at least some of the warheads and radioactive materials in the country’s vast complex of underground facilities.

A look at the many pieces of a secrecy-clouded bomb program that has rattled the region for decades:

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THE WARHEADS

The size of North Korea[3]’s nuclear arsenal is a mystery, with estimates ranging from 10 bombs to as many as 60 to 70....

How sophisticated they are is also unclear. It’s one thing to conduct a nuclear test - North Korea[4] has carried out six underground explosions since 2006, including what it says were two hydrogen bomb tests.It’s another thing to make the warheads small enough to be carried by a long-range missile that can strike the U.S. mainland.Kim said last November that his country had mastered that technology, and many foreign experts and governments believe North Korea[5] is at least getting there.“They are close enough now in their capabilities that from a U.S. policy perspective we ought to behave as if we are on the cusp of them achieving” the ability to strike the United States, then-CIA Director Mike Pompeo said in October.Closer to home, many analysts believe North Korea[6] is able to mount nuclear weapons on shorter-range missiles that could reach South Korea and Japan, where 80,000 American troops are stationed.___THE INGREDIENTSNuclear bombs can be made from plutonium or highly enriched uranium, and North Korea[7] has both.A 2016 South Korean government report says that North Korea[8] is believed to have produced 50 kilograms (110 pounds) of weaponized plutonium, enough for six to 10 bombs.North Korea[9] shut down the plutonium-producing factory at its main nuclear complex in Nyongbyon in 2007 as part a disarmament-for-aid deal, but the accord later fell apart, and satellite imagery indicates the North has resumed extracting plutonium in recent years.Plutonium plants are generally large and generate much heat, making it easier for outsiders to detect. A uranium-enrichment plant is more compact and can be easily hidden from satellite cameras. The centrifuges to enrich uranium can be clandestinely operated underground.Stanford University scholars, including nuclear physicist Siegfried Hecker who visited North Korea[10]’s centrifuge facility

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