N. Korea has nuclear weapon small enough to fit on a missile
Before the age of compact cars, laptop computers and pocket telephones, there were miniature nuclear warheads.
And for as long as there have been engineers, they have been working on making complicated things smaller and better — and weapons are no exception.
Now, North Korea apparently has figured out how to make a very big explosive small enough to sit atop one of its mobile-launched missiles, a development that could threaten much of the U.S., according to a U.S. intelligence report that surfaced this week.
North Korea is making progress, showing it can put together competent teams of scientists and solve technical problems, but it is far from proving that it is capable of launching a punishing nuclear strike on the U.S., according to U.S. weapons experts.
Making a miniature nuclear weapon that has a large explosive force involves a lot of scientific and engineering know-how.
The “Fat Man” bomb that the U.S. dropped on Nagasaki in World War II weighed as much as two 2017 Cadillac Escalade SUVs. Since then, the weight of U.S. atomic bombs has shrunk considerably, as scientists have refined the physics of the devices and streamlined how they are armed.
With the last generation of nuclear weapons designed in the 1980s, engineers at Los Alamos National Laboratory produced the W88, weighing only 800 pounds despite having an explosive force equal to 475,000 tons of TNT — in other words, less than onetenth the weight of the first atomic bomb, but 400 times more powerful.
What technical capability is necessary to build a missile-ready nuclear bomb?
The first step is understanding how to reduce the amount of conventional high explosives that surround a hollow pit of highly enriched uranium or plutonium. A nuclear detonation occurs when the high explosive implodes the hollow sphere of fissile material next to it to start an uncontrolled chain reaction.
The first implosion device was the bomb dropped on Nagasaki, Japan, in World War II, a bomb that weighed 9,700 pounds.
After the war, work progressed on smaller bombs. One of the critical design steps was to create a small, precisely uniform air gap between the conventional explosive lenses and the sphere of nuclear fuel, amplifying the force of the conventional explosion and reducing the amount needed to trigger a nuclear chain reaction.
It's not clear that Pyongyang has mastered that precise construction, said Jeffrey Lewis, a nuclear weapons analyst with the James Martin Center for Nonproliferation Studies in Monterey.
What Pyongyang has said so far is that its weapon is a “Korean-style mixed charge” device, indicating “they don't have a lot of plutonium so they are mixing it with uranium,” Lewis said.
It is possible they are also injecting tritium gas into the hollow sphere to get some fusion energy out of the bomb, as well, he said. “The concept is well-known, but you can't know without testing. But North Korea tests, so they would know,” he said.
What North Korea has is probably not a miniature atomic bomb but one that is merely compact, he said.
The biggest stride in miniaturization involved the hydrogen bomb design, pioneered by two Eastern European immigrants, Edward Teller and Stanislaw Ulam.
The Teller-Ulam configuration created two or three stages in a weapon, in which a fission trigger causes Xrays to compress a secondary stage of the weapon containing fusion fuel. And the secondary stage can trigger yet a third stage that contains more fission fuel.