U.S. President Donald Trump says many things that aren’t true, but his repeated assertion during a press conference that “it does take a long time” to “pull off complete denuclearization” stacks up pretty well. It will probably take a decade at the very least.

It’s not hard for a nation to dismantle or destroy its nuclear weapons and related technological infrastructure. North Korea may have as many as 50 weapons, and could probably dismantle them in a few weeks. Destroying test facilities, including underground test sites, as well as centrifuges and other technology, would take a little longer. But the really hard part of “de-nuking” is managing to do it in such a way that other parties can verify your actions are legitimate.

And that presents a host of difficult challenges. To begin with, no one really knows how many nuclear weapons North Korea currently has. If it dismantled 25 or 30 in front of international observers, no one would know how many it might have stashed away in its estimated 10,000 underground shelters. Nuclear experts from the International Atomic Energy Agency could inspect all known nuclear facilities, examine their records, and test these for consistency against the known history and technical capabilities of the facilities. But records can be altered, or faked.

There are many other problems too. Other nations would need some way to know if North Korea has any other uranium-separation facilities, in addition to those they disclose. But centrifuge facilities for enriching uranium can easily be hidden in an underground tunnel. They use little energy, and don’t betray themselves with detectable thermal or chemical emissions, and would be next to impossible to find. Another issue fraught with uncertainty is estimating how much weapons-grade nuclear material North Korea has created. North Korea might make a claim, but how can anyone trust it?

Finally, there’s also uncertainty over how much nuclear material was actually used up in the various nuclear tests North Korea has carried out, another important piece of information in working out how much material North Korea might have left. Only a very small fraction of nuclear fuel gets consumed in an explosion, and the rest is just scattered. Scientists can use seismic data to estimate the explosive yield of the various tests North Korea has conducted, but this doesn’t indicate how much uranium or plutonium fuel was really used up. North Korea might claim that more was consumed than actually was, and maintain quite a lot in reserve.

So “de-nuking” isn’t likely to be happening quickly, if at all. It’s not something that can easily be achieved with a few brash political gestures, but necessarily involves the build-up of trust. And trust, by its very nature, comes slowly over a period of repeated interaction and successful cooperation.

The task for the U.S. and North Korea is that much harder given the deep suspicion each nation harbors toward the other. Thus it will probably take at least a decade to achieve high confidence in the accuracy and completeness of North Korean declarations.

Only one nation has truly de-nuked, that being South Africa; and it managed the feat under quite unusual circumstances. The National Party of the apartheid-era, loathe to give control over nuclear weapons to the incoming African National Congress, gave inspectors access to its nuclear sites.

It would be a profound achievement if Trump and Kim were able to manage a similar success, but the inherent challenges, and the two leaders’ unpredictable temperaments, suggest the road will be long and punctuated with doubt and recrimination. Full denuclearization might not even be a realistic goal. The best outcome to be hoped for might be a more limited inspection program that aims to control North Korea’s nuclear and ballistic testing.

Mark Buchanan is a physicist and science writer..

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