Over the last two weeks, satellite photos of North Korea have shown evidence of tunnel-digging and what appear to be preparations for a nuclear test. On Monday, officials from the United States pledged to take some form of retaliatory "action" if North Korea pushes ahead and detonates one of the handful of nukes it's thought to possess. Does North Korea really need to test its nuclear weapons, or would the test be for show?
A North Korean nuclear test would have major political consequences, regardless of the data it produced. But the experiment might also serve as a proof-of-principle for an advanced weapon design. If Kim Jong-il were trying to make nuclear weapons that are small enough to be mounted on ballistic missiles, for example, a live test could be very important.
You can test a nuclear weapon in several ways without setting off a full-scale nuclear blast. A nuclear device that uses plutonium (like those thought to be in the North Korean arsenal) works by using a conventional blast to implode a shell of fissile plutonium. If the shell collapses symmetrically and at just the right moment, it will create a nuclear chain reaction. Early component tests of one of these weapons ensure that the conventional explosive will detonate at the right time, and that the electronic and mechanical components are functioning within acceptable bounds.
The next step is to perform what's called a "hydronuclear" test. The fissile plutonium in the device is generally replaced with a different isotope to create a less powerful bomb that shares many characteristics with the final product. When labs in the United States conduct hydronuclear tests, they use time-lapse X-rays to trace what happens to the plutonium shell after detonation.
But to prove that a complex weapon design works, you can conduct a live test. For underground tests, a long shaft of about 5 or 10 feet in diameter is dug either horizontally into the side of a mountain, or straight down into the ground for hundreds or thousands of feet. A crane deposits the nuclear weapon (which can weigh thousands of pounds) at the end of this shaft along with the testing equipment, and the shaft is filled in.
To gather data from an underground explosion, American testers have used long pipes that extended all the way from the surface to a tapered end close to the blast. When the bomb went off, a burst of X-rays and neutrons would shoot through the pipe in the split second before the tapered end melted and sealed shut, and scientists could record information about the weapon's explosive yield.
More information can be gleaned by measuring the seismic waves produced by the explosion, collecting nearby air samples, and by drilling down into the shaft afterward to recover radioactive samples.
The U.S. conducted the first nuclear weapons test on July 16, 1945; they tested the device that would be used a few weeks later at Nagasaki. The bomb dropped on Hiroshima was based on a much simpler design using highly enriched uranium, which didn't need to be tested.
Uranium weapons are easier to build, but they have two key disadvantages: Highly enriched uranium is more difficult to come by than plutonium, and uranium weapons can't be miniaturized for use with missiles. It takes sophisticated engineering to make nuclear warheads with plutonium that are small enough to mount on ballistic missiles. And to be confident in their use, you would need to have tested both the warhead and the missile itself.
Explainer thanks Robert Nelson of Princeton University, Daryl Kimball of the Arms Control Association, and Jim Walsh of Harvard University.