Technical Challenges to a Successful Nuclear Strike

From longtime tech correspondent Ralph Vartabedian, at LAT, "North Korea has made a nuclear weapon small enough to fit on a missile. How worried should the world be?":

Before the age of compact cars, laptop computers and pocket telephones, there were miniature nuclear warheads.

For as long as there have been engineers, they have been working on making complicated things smaller and better. 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 “Little Boy” bomb that the U.S. dropped on Hiroshima on Aug. 9, 1945, weighed as much as two 2017 Cadillac Escalade SUVs, about 9,700 pounds. Three days later, the “Fat Man” bomb, slightly heavier at 10,300 pounds, was dropped on Nagasaki.

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 one-tenth 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.

After the war, work progressed on smaller bombs. One of the crucial design steps was to create a small, precisely uniform air gap between the conventional explosives 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 unclear that Pyongyang has mastered that precise construction, said Jeffrey Lewis, a nuclear weapons analyst with the James Martin Center for Nonproliferation Studies in Monterey, Calif.

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 the North Koreans are also injecting tritium gas into the hollow sphere to get some fusion energy out of the bomb, as well, he said...

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