ARI SHAPIRO, HOST:
When President Trump and North Korean leader Kim Jong Un discuss denuclearization in Hanoi, they are likely to focus on one North Korean nuclear facility in particular. NPR's Geoff Brumfiel introduces us to it.
GEOFF BRUMFIEL, BYLINE: It's called the Yongbyon Nuclear Research Center.
JEFFREY LEWIS: This is really the oldest, best-known nuclear facility in all of North Korea.
BRUMFIEL: Jeffrey Lewis is a scholar at the Middlebury Institute of International Studies at Monterey.
LEWIS: You know, it's basically a small city. There's quite a lot of housing for scientists and workers. There are many facilities.
BRUMFIEL: Including a nuclear reactor that can produce plutonium, a giant facility designed to extract and purify that plutonium and a separate, modern facility filled with centrifuges for enriching uranium. Plutonium and uranium are both used in nuclear bombs.
Sig (ph) Hecker is a former nuclear weapons scientist, now at Stanford University. He's one of the few Westerners who's visited Yongbyon, and he says it's pretty spartan.
SIEGFRIED HECKER: Yeah, most important part - it was cold. This was January in Yongbyon. And particularly, what was striking - it was colder inside the buildings than outside the buildings.
BRUMFIEL: On other trips, Hecker saw scientists growing their own vegetables and using primitive safety equipment while working in potentially dangerous areas. But he is convinced that Yongbyon is effective at producing material for nuclear weapons.
HECKER: These facilities may be old, but they're functional. And the people were professional and very competent.
BRUMFIEL: If the North does give up Yongbyon, as it suggested it might in statements...
HECKER: They would significantly curtail their abilities to continue the nuclear weapons program. Of course, it doesn't mean that they're giving up their bombs because I'm quite certain the bombs are not in Yongbyon.
BRUMFIEL: Hecker estimates North Korea's current nuclear arsenal at several dozen weapons that are stored somewhere else. North Korea is also believed to have covert facilities where it can make uranium for nuclear weapons.
On top of all that, it's not clear what giving up Yongbyon would mean. Last year, North Korea closed its underground nuclear test site by dynamiting the entrances in front of foreign journalists...
(SOUNDBITE OF EXPLOSIONS)
BRUMFIEL: ...But foreign inspectors were never allowed to examine the site first. Jung Pak, with the Brookings Institution, says that North Korea is paranoid about foreigners.
JUNG PAK: North Korea does not want people running around their country looking at their nuclear facilities or their missile facilities.
BRUMFIEL: But Yongbyon is a big, technically complex site. It can't just be blown up. Pak believes that any deal should include inspections.
PAK: It would be a big change and a good signpost of North Korean sincerity on denuclearization if they did allow inspectors into their facilities.
BRUMFIEL: Those inspectors would probably be let in in exchange for some kind of sanctions relief and, perhaps, a peace declaration between North Korea and the U.S., which have remained formally at war since an armistice in 1953. For now, Jeffrey Lewis says the satellite photos indicate the reactor is running, making more plutonium for nuclear bombs.
LEWIS: You know, it's really business as usual at Yongbyon at the moment. People all show up for work, and material comes in. And it looks pretty much like it's looked for the last, you know, 10 or 15 years.
BRUMFIEL: It remains to be seen whether a second summit can change that. Geoff Brumfiel, NPR News, Washington. Transcript provided by NPR, Copyright NPR.