Relations with North Korea had been going from bad to worse until this past week when the North suddenly agreed to send a delegation to next months' Winter Olympics in South Korea. But one thing - the most important thing - hasn't changed. North Korea insists it will never give up its nuclear weapons and is keeping them aimed at the United States. For years, the North has been boasting about its growing nuclear arsenal in an effort to convince the U.S. it cannot be pushed around. Much of that is propaganda, but all good propaganda has an element of truth. To make sure the U.S. got the message, the North Koreans handed - literally handed - an American scientist one of their most valuable nuclear secrets. And not just any scientist - but a man who used to be in charge of designing American nuclear weapons.

Sig Hecker, the American scientist who toured North Korea's nuclear facilities CBS News

Sig Hecker: I was immensely surprised by how much they showed me and with the openness with which they showed and explained that to me.

For eleven years, Sig Hecker had been director of Los Alamos National Laboratory, the birthplace of the American atomic bomb. So he was more than a little surprised when in 2004 he was invited on a tour of North Korea's nuclear complex.

Sig Hecker: There's no way in the world they're going to let me in. By the way, I also thought the U.S. government wouldn't let me go but it turns out I was wrong on both accounts.

"Nobody would believe them otherwise, right? People would say, 'Oh, they're just posturing. Oh, it's propaganda.' So how are you going to convince the Americans? You get an expert who knows plutonium when he sees it and you, you hand it to him."

The North Koreans took him to a place called Yongbyon where they had been operating a small nuclear reactor.

Sig Hecker: I would call it primitive but functional. And in fact, all of the instrumentation sort of reminded me when I first got to Los Alamos in 1965 you know, no modern electronics or anything of that nature.

Sig Hecker: This is a reactor that was not very good for producing electricity, but it was very good for making plutonium.

After showing him the reactor, the North Koreans took him to a building where they claimed to be reprocessing spent fuel from the reactor into weapons-grade plutonium.

Sig Hecker: They just showed me the facility and basically said, "Look, you have to believe us, we extracted the plutonium."

David Martin: Did you believe them?

Sig Hecker: The answer was yes, but I didn't let them think that I believed them.

So Ri Hong Sop, hecker's guide and director of the nuclear complex, offered to show him the plutonium.

Sig Hecker: They bring in, and it's a red metal box about yea big, about this thick. They open the metal box. They take out a white wooden box. White wooden box has a slide off top. So they slide off the top. I look in there. The director says, "Over here, this glass jar. That's our product. That's the plutonium.

David Martin: You know plutonium when you see it.

Sig Hecker: Plutonium by itself is sort of a silvery color if it's not oxidized. If it rusts, oxidizes a little bit, it sort of turns gray and black and this stuff was gray and black.

This is what plutonium looks like -- the radioactive element which produced the first nuclear explosion in July 1945.

Sig Hecker: So I said... I'd like to hold the jar with the metal in it. And they allowed me to hold it. So what do I learn from holding? Well, first of all plutonium is dense... It ought to be heavy. It was. The other thing plutonium is radioactive. So it… Glass jar ought to be warm and it was warm.

Robert Carlin: They wanted to show Sig that they really did have plutonium.

Robert Carlin has spent his entire career studying North Korea, first as an intelligence analyst at the CIA and State Department, now as a consultant to CBS News.

David Martin: Here's a piece of plutonium, which in any government in the world would be one of the most tightly guarded secrets, and they hand it to an American?

Robert Carlin: Nobody would believe them otherwise, right? People would say, "Oh, they're just posturing. Oh, it's propaganda." So how are you going to convince the Americans? You get an expert who knows plutonium when he sees it and you, you hand it to him. You say, "Here it is. What do you think?"

Robert Carlin, who has spent his entire career studying North Korea CBS News

David Martin: What impact did the information you came back with have on U.S. intelligence assessments of the North Korean nuclear program?

Sig Hecker: It changed from one of "we don't know exactly what they have, if they have enough to make anything" to the fact that they actually could have four to six bombs.

David Martin: Well, that's a fairly major change.

Sig Hecker: That's a big change.

U.S. intelligence relied on satellite photos of the Yongbyon Nuclear Complex to monitor how much plutonium was being produced by the reactor.

David Albright: This area is where the small plutonium production reactor is.

David Albright is director of the Institute for Science and International Security and a leading expert on North Korea's nuclear weapons program.

David Martin: How do you know when it's operating and when it's just-- in idle?

David Albright: Basically evidence of heat. And-- and what you see in this picture is there's steam rising here.

But satellite photos could not solve the mystery of whether North Korea was also building a second type of bomb made of uranium -- using gas centrifuges to enrich the uranium to bomb-grade levels.

David Albright: I had many meetings with North Korean officials where they vehemently denied they had a gas centrifuge program. Denying to the point where they're pounding -- you know, almost pounding their fists on the table, getting very angry.

In fact, the centrifuge plant was hiding in plain sight -- but no one knew it until 2010 when Sig Hecker was invited back and taken inside that blue-roofed building.

The blue-roofed building where North Korea was enriching Uranium CNES 2015, Distribution Airbus DS

Sig Hecker: On the way in I had a chief engineer and he actually, he stopped outside and he said, "Dr. Hecker, we didn't want to show you this facility but our superiors made us do it."

Sig Hecker: And so we got up to the second floor, looked down at a hall...

David Martin: And you saw?

Sig Hecker: I was just flabbergasted. I could not believe what I was seeing. Essentially 2,000 centrifuges lined up, looked beautiful, modern.

David Martin: So, why so flabbergasted though? Everybody suspected they were secretly enriching uranium. Here they are, secretly enriching uranium.

Sig Hecker: We had no idea they had this many centrifuges and that modern. And the most amazing thing… They put a blue roof on this facility that is so visible from overhead satellite imagery and nobody knew.

David Martin: So, they'd built this modern uranium enrichment plant under the noses of U.S. spy satellites?

Sig Hecker: Of all the spy satellites. So, a lot of people say, well, that shows how bad, you know, our intel agencies are. It doesn't. It shows how easy it is to build those centrifuge facilities and hide them.

Hecker visited North Korea seven times. Each time the North Koreans were inviting into their midst a nuclear scientist who would report everything he saw to U.S. intelligence.

Sig Hecker: In all my visits, I think that's a calculus that they always made. How much do they show me in order to convince me of something that they'd like me to take away but not show me so much that it actually makes them more vulnerable because they've given me information that they'd rather not give me. So they took that chance with every visit.

Since Hecker's last visit that blue roofed building which held 2000 centrifuges has doubled in size and it is almost certainly not the only uranium enrichment plant in North Korea.

David Albright, director of the Institute for Science and International Security and a leading expert on North Korea's nuclear weapons program CBS News

David Albright: They went out and decided that "now we're going to buy enough materials, equipment to build, 8,000 centrifuges, 10,000 centrifuges."

David Martin: When they go out on the market to buy that much material does that become evident?

David Albright: Yes, in a sense it was a smoking gun that, that North Korea was trying to scale up its gas centrifuge program.

U.S. intelligence estimates that by now North Korea could have enough bomb-grade material for as many as 60 weapons, an estimate considerably higher than Albright's, who believes those centrifuges break down a lot.

David Martin: What's your estimate of the number of nuclear weapons that North Korea has?

David Albright: 13 to 30 nuclear weapons as of the end of 2016.

David Martin: Thirteen to 30?

David Albright: Yeah.

To Hecker, the precise number is not what's most important.

Sig Hecker: What's even more important than the 30 or 60 is how small can they make them?

Which is what made 2016's show and tell by Kim Jong Un such a watershed event. It was, Kim said, a nuclear warhead small enough to be mounted on a missile.

Sig Hecker and correspondent David Martin observe Kim Jong Un with the 'disco ball' bomb CBS News

David Martin: This is what everybody called the disco ball.

Sig Hecker: Right.

David Martin: What does it look like to you, though?

Sig Hecker: This to me it is one that I would call a spherical fission bomb, in other words, the atomic bomb.

"We think if we apply enough pressure, we'll cause them to buckle. And my impression over the years is this is not a country that's going to buckle."

Remember, Hecker was once the director of a lab which designs American nuclear weapons, so he knows exactly what to look for in a bomb.

Sig Hecker: It looks like a simple bomb. However, what I found most important about this is the size of it. It looks to be about 60 centimeters.

60 centimeters across means it would fit on some of North Korea's missiles.

David Martin: How do you determine the dimensions of something in a photo like that?

Sig Hecker: You try to determine Kim Jong Un's midsection and then you compare.

David Martin: So, you measure the bomb against his girth.

Sig Hecker: Right. So the other way of course, you have some general idea as to how tall these guys are.

Hecker has a very good idea how tall the man on the far left is. That's Ri Hong Sop, the scientist who showed Hecker the piece of plutonium in 2004.

Sig Hecker: Director Ri Hong Sop is very highly respected by Kim Jung Un.

This past September when North Korea unveiled a sleeker warhead design, one it said was a much more powerful thermonuclear weapon, there was Ri Hong Sop again, briefing Kim Jung Un. It looked very much like nuclear weapons Hecker had seen when he was director of Los Alamos National Laboratory.

Kim Jong Un unveiling a 'sleeker' warhead design in September CBS News

Sig Hecker: That shape is consistent with what we would call the two-stage thermonuclear weapon. What that essentially means is sort of a modern hydrogen bomb.

David Martin: Could they put that peanut shaped device on a missile?

Sig Hecker: They definitely want us to think so. In the background they actually show the warhead positioned in the nose cone of the missile which we interpreted to be an ICBM.

Just hours after these photos were released, North Korea conducted its sixth and largest nuclear test in a remote underground site. U.S. intelligence estimated the device was many times more powerful than the bomb which destroyed Hiroshima.

Sig Hecker: Their confidence level, you know, is amazing. I mean, it's amazing. They go and show this thing and two hours later they detonate this weapon.

But there is no way of knowing whether the device they detonated is the same as the one they showed off.

David Martin: Do you believe that North Korea has that kind of weapon today?

David Albright: I don't think they have it today… There's just a lot of engineering challenges and when you look at other countries like North Korea it's taken them quite a long time to master these things.

David Martin: This is their moon shot.

David Albright: Yeah, very much so.

If you believe North Korean propaganda, Kim Jung Un is ecstatic with the progress his nuclear scientists have made. Here he is holding hands with Hecker's old tour guide, Ri Hong Sop -- in full military uniform -- now promoted to director of the nuclear weapons program.

Robert Carlin: There's nothing that appears that they don't want us to see. Very calculated.

Robert Carlin's four decades of studying North Korea has taught him not to believe everything he sees, but it has also taught him that Americans who look down on North Korea as a backward nation are making a big mistake.

Robert Carlin: I think they understand us better than we understand them. We're still weighed down with a lot of stereotypes and they're going to trip us up.

David Martin: Trip us up how?

Robert Carlin: We think if we apply enough pressure, we'll cause them to buckle. And my impression over the years is this is not a country that's going to buckle.

Sig Hecker: They're going to get there, you know, that's, that's one thing you can count on. We've tried to sanction them into submission. They've not submitted. They just keep testing and keep evolving.

Produced by Mary Walsh. Associate producer, Tadd J. Lascari.