When you think of efforts to pare down the world's nuclear weapons stockpiles, maybe you imagine heads of state and uniformed generals sternly staring down their military rivals across a huge table.

Reality, though, looks very different.

Picture instead a white-haired, US weapons scientist sidestepping the summit meetings and heading directly to research labs in Russia, China, Pakistan and even North Korea to chat about physics and build the direct ties that may be more effective at establishing trust than edicts from the top brass.

That man is Siegfried Hecker, former director of Los Alamos National Laboratory and now a professor emeritus at Stanford University. He's one of the few people in the world who can appreciate exactly what it meant when a member of North Korea's nuclear weapons program handed him a glass jar warmed by the radioactive decay of the plutonium inside. Or the dramatic unveiling in 2010 of thousands of centrifuges at North Korea's Yongbyon nuclear site to make weapons-grade uranium.

Hecker -- Sig for short -- has been working on nuclear weapons diplomacy for decades. In the 1990s, after the Soviet Union dissolved and the Russian economy faltered, he was central to a program that led to the dismantlement of thousands of nuclear warheads while ensuring jobs for his Russian counterparts. That work led to similar cooperation with other countries: Hecker has traveled to Russia 56 times, China 38 times, North Korea seven times, India six times and Pakistan once.

"Everywhere I go in the nuclear world, Los Alamos is considered the mecca of all places nuclear. The doors open up," Hecker says. "I feel this special responsibility -- when they open the doors, I need to walk through."

That peer-to-peer contact arguably is more important than ever. The US withdrew from the 1987 Intermediate Nuclear Forces Treaty this month after alleged Russian noncompliance. Russia is talking about deploying hypersonic nuclear missiles that some say the US military can't stop. And the 2011 New START treaty reining in US and Russian nuclear stockpiles likely will expire in 2021.

But even with the frosty US-Russia relationship these days, Hecker will make his 57th trip there in November.

Hecker is comfortable enough donning a suit and tie when giving congressional testimony or speaking to Energy Department officials, but when I meet him in his office at Stanford's Freeman Spogli Institute for International Studies, his red polo shirt, tan slacks and brown leather shoes reflect the more casual look typical of Los Alamos or Silicon Valley. He's soft-spoken but intense, his passion showing when he's excited about scientific breakthroughs or frustrated by what he sees as political regression.

"I don't think the Trump administration is going to be interested in renewing," Hecker says of New START. "We've already cut back so much contact between nuclear scientists and the nuclear military. If you now go ahead and chop off the treaties, that's very dangerous."

An optimist despite it all

Today's geopolitical climate is grim in Hecker's view. He speaks regretfully of the demise of the US-Russian collaboration. He enthusiastically endorsed the US-Iran nuclear accord that President Donald Trump withdrew from last year and that Iran now is taking steps to defy. He worries about the dangers of Trump's 2017 threat of unleashing "fire and fury" on North Korea.

Yet he calls himself an optimist. "Fundamentally, I'm a believer in international cooperation."

That might sound surprising given the strident nationalism on display across the world stage, but Hecker has been pushing his agenda for decades with both Republican and Democratic administrations.

And he's made a difference.

"We can only guess how many catastrophes have been avoided because of Sig's work on nuclear safety and security," says former Georgia Sen. Sam Nunn, now co-chair of the Nuclear Threat Initiative, an organization dedicated to reducing the dangers of weapons of mass destruction. "We do know that global risks have been significantly reduced because of his cooperative efforts with scientists from the former Soviet Union in the 1990s."

Siegfried Hecker examines evidence in 2008 that the North Koreans had disabled some of their nuclear weapons manufacturing equipment. Courtesy of Siegfried Hecker

A global view

Even before mastering plutonium chemistry and leading a premier nuclear weapons lab, he knew there was a world beyond the US border. Hecker was born in Poland in 1943, but his father disappeared fighting for the Germans on the Eastern Front of World War II. After the war, he lived in converted Army barracks in Austria, having a good time skiing and playing soccer despite the lack of running water or central heating.

In 1956, at age 13, he moved to the United States. Four years later, he was named valedictorian of Cleveland's East High School and won a scholarship to Case Western Reserve University.

He measures his milestones from his arrival in the US: five years to obtain US citizenship, nine years to get a security clearance for a summer job at Los Alamos, 30 years to become director of LANL. Although he spent the early 1970s as a metallurgist at General Motors, he returned to Los Alamos in 1973 and rose through the ranks of materials science.

He's still an avid skier -- he served as president of the Los Alamos Ski Club that once ran the local Pajarito Mountain Ski Area -- and he wears a Fitbit activity tracker on his wrist today. But even after so long in the US, he remembers his experience as an immigrant.

"I have a soft spot for refugees and immigrants. I will never forget how this country welcomed me with open arms," he says. The US granted similar opportunities to the refugees and immigrants who escaped Hitler's Germany before the war and helped to build the first atomic bomb at Los Alamos during the Manhattan Project, he adds.

Hecker stepped down as LANL director in 1997 -- he spent his last day on the job at Tomsk-7 (or Seversk), a Russian nuclear weapons production site in Siberia. But he still returns often to an office in Los Alamos where he keeps mementos like the diploma marking him a member of the Russian Academy of Sciences.

At LANL, he became an expert in the weird ways of plutonium, the radioactive metal made in nuclear reactors to fuel modern nuclear weapons. "Plutonium is without question the most complex and interesting of all metals," Hecker and two colleagues wrote in a 1983 publication. For one thing, as a solid, plutonium can take six different forms called allotropes, each with different properties (a seventh allotrope occurs when plutonium is under pressure). For comparison, iron has only four allotropes. Another tricky factor is that solid plutonium expands dramatically when it gets warmer -- except sometimes it contracts.

Its properties are crucial for the challenge of maintaining the US nuclear weapons stockpile as it ages decades beyond its original expected lifespan. But if you read Hecker's 46-page assessment of plutonium, written after stepping down as LANL director, you'll see he appreciates plutonium's weird physics, not just its military and political importance.

Life after the lab

Over a dozen years, Hecker has taught about 3,000 students at Stanford about the intersection of technology and national security. Accolades and mementos adorn his office; there's a Chinese print of flowers, a pair of gold-rimmed engraved plates from Russia's nuclear weapons lab in Sarov. A copy of his book about the US-Russian nuclear collaboration, Doomed to Cooperate, is jammed into a bookshelf covering two walls of his office. A window overlooks Stanford's green lawns, towering oaks and sandstone arches.

"He's got a ton of credibility," says Miles Pomper, a senior fellow at the James Martin Center for Nonproliferation Studies (CNS). "The hardliners are not going to just be able to dismiss someone like Hecker. He can physically eyeball things in a way very few people can."

North Korea says it's developed a hydrogen bomb -- a powerful thermonuclear weapon -- and indicated it's small enough to fit on a missile. In this Korean Central News Agency photo from September 2017, leader Kim Jong-Un inspects the design. AFP photo/KCNA via KNS/Getty

His worldview is grounded in the idea of looking at the world from others' perspectives -- something he says he learned in Austria but still applies when he visits scientists across the world. "In the United States, we tend to be so incredibly America-centric. We're only 300 million out of 7 billion people, for heaven's sake."

Seeing other weapons researchers on their own turf has been crucial, he adds: "You can only understand them by being there."

It's why he believes that the North Korean regime isn't suicidal -- so belligerent it would provoke the US into a nuclear war. And he's learned from visiting many Russians in their homes. "They're so much like us -- it's a civil society, one that's into the traditions of music and art and family culture."

Grounded in physics

Hecker knows exactly how nuclear weapons work, which is handy when it comes to verifying treaties or figuring out North Korea really has a hydrogen bomb.

Take that moment in 2004 when he scrutinized that jar of North Korean plutonium. The funnel-shaped sample looked like oxidized plutonium, but Hecker asked to hold it, too. That let him check if it was heavy enough to match plutonium's high density and warm enough to indicate radioactive decay.

"It was both -- heavy and warm," Hecker says. (Glass stops the relatively slow, heavy alpha particles of plutonium's radioactive decay, though Hecker wore gloves in case the jar's outside was contaminated.)

"It was Sig's firsthand experience in some of North Korea's nuclear facilities that really first affirmed that North Korea's nuclear capabilities were the real deal," says Grace Liu, a CNS analyst.

Siegfried Hecker (center) visits the plutonium facility at North Korea's Yongbyon nuclear research center in 2007. Courtesy of Siegfried Hecker

Hecker also got a good look at North Korea's single working nuclear reactor on that trip, scrutinizing its control room, tracking the spent fuel rods crucial to making plutonium, and getting a measure of its plutonium production ability. And he confirmed the country's reprocessing plant, for extracting uranium and plutonium from spent fuel rods, operated at industrial scale.

To assess a country's nuclear weapons capability, Hecker uses a three-point evaluation: its knowledge of how to build a nuclear weapon; its supply of weapons-grade materials like plutonium and enriched uranium; and its missile technology to deliver a bomb.

Right now, North Korea has all three -- though with limitations, Hecker believes. Iran has the know-how and the missiles, but is a bit short on ingredients.

Types of nuclear weapons

There are two basic types of nuclear weapons: fission and fusion bombs. Fission bombs use the release of energy that accompanies the splitting of heavy uranium or plutonium atoms. Fusion bombs use the energy released by the merging of lighter atoms such as hydrogen and lithium.

The most basic fission bomb is the gun type, which the US set off over Hiroshima in 1945. In it, a detonation of conventional explosives slams two pieces of highly-enriched uranium together. The uranium reaches critical mass -- atoms split and release energy and neutrons that trigger more splitting -- and explodes. You can't make a gun type bomb with plutonium -- it releases more neutrons than uranium, causing a premature, feeble detonation, Hecker says.

Because gun-type weapons are easy to design, the most effective way to limit the spread of nuclear weapons is to control the nuclear materials needed to build them, says Ferenc Dalnoki-Veress, a high-energy physicist at CNS.

"With modern weapons-grade uranium, terrorists would have a good chance of setting off a high-yield explosion simply by dropping one half of the material onto the other half," he says.

A more sophisticated fission bomb uses implosion. A carefully constructed shell of conventional high explosives detonates on the outside of a sphere of plutonium or highly enriched uranium, compressing the core and causing the explosion. The US used plutonium-based fission bombs in both its Trinity test in New Mexico in July 1945 and the Nagasaki attack a month later. Adding tritium -- a variety of hydrogen with two neutrons instead of the more common zero -- can boost the power of implosion weapons.

"These designs are more sophisticated, and you really need to test it to get it to work," Dalnoki-Veress says.

But it's fusion bombs -- the thermonuclear or hydrogen weapons that make up all modern nuclear arsenals -- that are most explosive. They begin with a smaller fission bomb "primary" that releases enough energy to trigger the fusion "secondary." This two-stage reaction is more complicated, but it delivers more explosive power, which makes it more efficient for weight-constrained missiles.

Thermonuclear bombs are the world's most powerful weapons. Warheads like the United States' B83 have an explosive yield the same as 1.2 million tons of TNT, about 80 times the explosive power of the Hiroshima and Nagasaki bombs and 600,000 times more powerful than the 1995 fertilizer bomb that destroyed the Alfred P. Murrah Federal Building in Oklahoma City. The Soviet Union holds the record in explosive power with the 50-megaton Tsar Bomba, a thermonuclear bomb that swept ground zero as smooth as a skating rink in 1961 with a detonation more powerful than all bombs dropped in World War II.

Newer nuclear weapons emphasize accuracy over explosive power, but you wouldn't want to be near an explosion. A 1.2 megaton bomb can flatten homes more than 4 miles away and cause third-degree burns 8 miles away.

No matter what design is used, weapons designers want plutonium when launching missiles. "Plutonium is so much more potent than uranium," Hecker says. Although you can make a uranium-triggered hydrogen bomb, "plutonium is much preferred for nuclear warheads for ICBMs."

Visiting North Korea

Fifteen years ago, when North Korea had a much younger nuclear weapons program, the US was fixated on the early days of the war on terror. In Hecker's view, North Korean leaders believed at the time their country wasn't getting the attention and respect it deserved.

Hecker's Stanford colleague John Lewis, an expert in Asian political science, made several trips to North Korea, and in 2004, the country invited him to Yongbyon. Lewis persuaded Hecker to come along to offer technical expertise, Hecker recounts. North Korea, eager for recognition, was amenable.

In fact, it was harder to sell the US government on the idea of sending one of its senior weapons experts, Hecker said. As he put it, then Vice President Dick Cheney's attitude was, "We don't talk to evil. We destroy it." But Hecker's allies in Washington, D.C., prevailed.

Skeptics didn't believe North Korea's claims that it could make nuclear weapons, but Hecker became convinced they could during his visit to the Yongbyon Nuclear Scientific Research Center, a site about 60 miles north of the capital, Pyongyang. It's also home to a nuclear reactor along with nuclear weapons research and manufacturing facilities.

"When you spend time with the scientists, discussing the density of plutonium in the delta phase [a metallic, workable form], you get insights you can't possibly have from the outside or get around a negotiating table," he says.

His 2007 and 2008 visits confirmed some disablement of North Korea's weapons program. But then came a difficult period after President Barack Obama took office. He'd told dictators, "we will extend a hand if you are willing to unclench your fist." But then North Korea announced a satellite launch that Western powers saw as a threatening display of nuclear missile expertise.

Surprise -- 2,000 centrifuges

Hecker's last visit to Yongbyon came in 2010, when the North Koreans had a final message to send: They'd built a full-scale uranium enrichment facility.

The facility uses centrifuges to rapidly spin a gaseous form of uranium. Natural uranium is 99.3% Uranium 238 -- a particular variety of the element with 238 protons and neutrons. But weapons require a concentration of at least 90% U-235, a lighter version with three fewer neutrons. Spun fast enough, the lighter U-235 collects toward the center of the centrifuge, where it can be skimmed off and sent to the next centrifuge. This cascading arrangement gradually produces the weapons-grade, highly enriched uranium.

North Korea's Nyongbyon Nuclear Scientific Research Center includes the blue-roofed building for enriching uranium so it's useful in bombs. This satellite photo is from April 2019. Satellite image ©2019 Maxar Technologies

Although spy satellites can monitor plutonium manufacturing in a nuclear reactor, they can't easily track uranium enrichment in centrifuges. So the North Koreans built the facility right under the US's nose, so to speak.

"They showed me these 2,000 centrifuges. Quite frankly, my jaw dropped," Hecker says. "I knew they had centrifuges. I knew they were doing enrichment. But I had no idea they had this many in that modern a facility and in a building I had been in a couple of years before."

Just as in 2004, when showing Hecker they could make plutonium, the North Koreans were using Hecker's expertise to tell the rest of the world they had serious nuclear weapons capability. In effect, North Korea used the centrifuge display to tell Hecker, "Now we have the second path to the bomb," he said.

Hecker is willing to go back, but currently there's no need. He says North Korea now communicates its nuclear capabilities with weapons tests detectable across the globe, missile launches visible from space and government photos of Kim Jong Un inspecting nuclear weapons designs.

North Korea's first five nuclear tests from 2006 to 2016 ranged in power up to the equivalent of about 7 to 14 kilotons of TNT, roughly the same size as the two US atomic bombs exploded over Japan during World War II. Scientists infer the magnitudes from the way the explosions cause shock waves to traverse Earth, in effect ringing it like a bell. But the sixth test, in 2017, now looks to have been about 250 kilotons.

"At 250, this was thermonuclear, and it was a hydrogen bomb," Hecker says.

His deep knowledge of bombs and how North Korea makes them is why he's frustrated by US-North Korea nuclear summits. Even though Trump and Kim are willing to challenge their countries' hardliners, each side was overconfident in the 2019 summit talks at Hanoi, Vietnam, Hecker says. At the last minute, North Korea offered to give up all its Yongbyon operations, he says, but it was too late.

"When Trump walked away from Hanoi, he got applause from both sides of the aisle. But he walked away from what could have been a blockbuster deal," Hecker said.

"That was a deal that would bring Americans back into Yongbyon," where the US can see what's going on. Even if they maintain covert work elsewhere, that's slower and harder, he says. "They make a lot more progress when we're not there."

Back from the brink

It's not Hecker's job anymore to keep the aging US stockpile working, but his expertise is still in demand. One ongoing project is to keep a close eye on 16 aspects of North Korea's nuclear weapons program. What is its ability to make plutonium and tritium? To enrich uranium? How active is Yongbyon? He also tracks related factors like US financial aid and the tone of North Korean diplomatic communications for a broader view.

He refers to the research as he discusses his work with North Korea, calling the charts up on a MacBook perched in front of a standing desk with tape covering its webcam. Color codes offer a roadmap toward denuclearization that the US and North Korea both can accept.

It's all part of Hecker's approach toward improving relations. You don't get everything you want at once. The US normalizes some relations while North Korea takes some early steps toward denuclearization. Next comes some sanction relief, maybe a nonaggression pact, and eventually a peace treaty. "We're talking at least about a 10 year process," Hecker says.

Small steps worked with Russia. The US-Russian nuclear collaboration grew out of his contacts with Russian nuclear scientists who came to the Nevada Test Site for a 1988 treaty enforcement activity called the Joint Verification Experiment. At its peak, more than 1,000 Russians and Americans were involved in the Nunn-Lugar Cooperative Threat Reduction Program. "You get a better sense of where the other side is coming from," he says.

And ultimately, personal connections lay a foundation for trust -- the kind of relationship that can be deeper than a treaty.

"Trust takes a long time to develop, but can be destroyed quickly. The world is on a terrible trajectory right now," he says. But during his tenure as LANL director and scientific shuttle diplomat, he's seen seven presidents come and go.

Give it another decade. Maybe his optimism will be rewarded.