But the North has come farther than most experts expected since the infancy of its program in the 1950s, when the Soviet Union began training North Korean scientists in nuclear basics.
It took three decades for the North to assemble the technology it needed to make its own bomb fuel. Finally, from a reactor at Yongbyon, it succeeded in making plutonium: enough for about one atomic bomb a year.
The first North Korean nuclear crisis, in 1994, ended in an agreement with the Clinton administration to freeze the North’s production facilities in return for oil and peaceful reactors. It fell apart early in the George W. Bush administration. In 2006, the first test explosion, while unimpressive, entered North Korea into the club of nuclear powers. Analysts say the first blast was a plutonium bomb, as was a second detonation just months into the Obama administration in 2009.
Dr. Hecker visited Yongbyon in 2010, and the North Koreans showed him a complete uranium enrichment facility, which American intelligence agencies had missed. The message was clear: The North now had two pathways to a bomb, uranium and plutonium. Today, it has an arsenal made up of both, intelligence officials say.
INTERACTIVE FEATURE
North Korea May Be Preparing Its 6th Nuclear Test
Growing evidence suggests that North Korea may soon conduct another underground detonation in its effort to make nuclear arms.
And it is aiming for something much bigger: a hydrogen bomb, with a destructive force up to 1,000 times greater than ordinary nuclear weapons. That is exactly the path the United States took in the 1950s.
Recently, United Nations investigators found evidence that the North’s factories had succeeded in producing lithium 6, a rare ingredient needed to make thermonuclear fuel. Gregory S. Jones, a scientist at the RAND Corporation, said the North might have already used bits of thermonuclear fuel in its 2016 detonations.
A potential clue, analysts say, is that the North’s five blasts over the past decade have grown steadily more destructive.
Shrinking the Bomb
A bomb is useless to North Korea — as an offensive weapon or as a deterrent — unless the country can make a convincing case that it has a reliable delivery system. So when the North flaunts missiles at military parades, as it did on April 15, the stars of the show tend to be the big missiles that are designed to reach Washington and New York. While several intercontinental ballistic missiles rolled down the streets of Pyongyang, conducting a flight test that proves one could fly that far, and land with accuracy, is so far only an aspiration.
Missing from the parade were the short- and medium-range missiles that have been successfully flight tested. American intelligence agencies believe some of those can carry operational nuclear arms. The critical one is the Nodong, which has a range of about 800 miles.
But the North Koreans are discovering — as the United States, the Soviet Union and China did before them — that it is far more complicated to design an intercontinental missile. With that weapons system, a warhead would move at four miles a second and re-enter the atmosphere in fiery heat — so, if badly engineered, it would burn up long before hitting a target. To reach their goal, North Korean weapons designers are looking to miniaturize their warheads, making them far lighter and more powerful.
The big effort these days is to merge two technologies: Get a missile that can cross the Pacific, and marry it to a warhead that can survive the ride. And this is why the United States is so desperate to stop the cycle of testing.
The cyber- and electronic warfare attacks that President Barack Obama ordered against the country’s missile fleet were intended to slow North Korea’s learning curve. The Musudan, which can travel 2,200 miles, has racked up an embarrassing failure rate of 88 percent — although how much of that is due to incompetence or outside meddling is not known. Until the North Koreans figure out what is going wrong, and how to fix it, they appear hesitant to test the KN-14 and the KN-08, both of which are designed to hit the continental United States.
Examining North Korea’s Missiles
At a recent military parade, North Korea displayed several missiles at a time of heightened tensions with the United States. Here's a closer look at what some of them are designed to do.
By MARK SCHEFFLER and DAPHNE RUSTOW on April 16, 2017.Photo by Wong Maye-E/Associated Press. Watch in Times Video »
The diplomatic pressure from China to stop a sixth nuclear test at the Punggye-ri test site is intended to keep the North Koreans from making advances in warhead miniaturization and the design of a hydrogen bomb. As Mr. Obama noted before he left office, even failures are important learning tools for the North Koreans, aiding the trial-and-error process of making new warheads.
How long will it take for the North Koreans to solve those problems? The best guesswork is around 2020 — while Mr. Trump is still in his first term.
A Freeze, to What End?
The strategy emerging from Mr. Trump’s national security team comes down to this: Apply overwhelming pressure on the North, both military and economic, to freeze its testing and reduce its stockpile. Then use that opening to negotiate, with the ultimate goal of getting the North Koreans to give up all their weapons.
Many experts, however, believe that is a fantasy, because Mr. Kim regards even a small arsenal as critical to his survival. The upside of the strategy, if it works, is that the “nuclear freeze” would delay for years the day the North can fit a small, reliable, well-tested weapon atop a large, reliable, well-tested missile. The downside is that it would leave the North Koreans with a small, potent arsenal — one the United States would be essentially acknowledging, if not accepting.
That is why it will be hard for Mr. Trump to fulfill his vow to “solve this problem.” And every day, there is the chance of miscalculation, or an accident.
At any moment, Dr. Hecker said on a call to reporters organized by the Union of Concerned Scientists, a live weapon could turn into an accidental nuclear detonation or some other catastrophe.
“I happen to believe,” he said, “the crisis is here now.”
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