What geology reveals about North Korea’s nuclear weapons – and what it obscures

North Korea’s leader, Chairman Kim Jong Un, clearly is in no hurry to demilitarize his country. In the wake of two historic yet unproductive summits with President Trump, Kim made a state visit in April to Moscow, where he made clear that his country will not give up its nuclear weapons without international security guarantees. North Korea also tested what appeared to be short-range missiles on April 18 and May 4.

These tests are reminders that North Korea’s military forces, particularly its nuclear arsenal, pose a serious threat to the United States and its Asian allies. This reclusive nation is a high-priority U.S. intelligence target, but there are still large uncertainties about the power of its nuclear weapons. North Korean scientists work in isolation from the rest of the world, and defectors are far and few between.

My research focuses on improving techniques for estimating the yield, or size, of underground nuclear explosions by using physics-based simulations. Science and technology give us a lot of tools for assessing the nuclear capabilities of countries like North Korea, but it’s still difficult to track and accurately measure the size and power of their nuclear arsenals. Here’s a look at some of the challenges.

[embedded content] Experts say the US and North Korea are closer to nuclear war than many Americans believe.

A nation in the dark

For an isolated nation like North Korea, developing a functional nuclear weapons program is a historic feat. Just eight other sovereign states have accomplished this goal – the five declared nuclear weapons states (the United States, Russia, Britain, France and China) plus Israel, India and Pakistan.

North Korea has been developing nuclear weapons since the mid-1980s. Paradoxically, in 1985 it also joined the Treaty on the Non-Proliferation of Nuclear Weapons, or NPT, under which it pledged not to develop or acquire nuclear weapons. But by 2002, U.S. intelligence discovered evidence that North Korea was producing enriched uranium – a technological milestone that can yield explosive material to power nuclear weapons. In response the U.S. suspended fuel oil shipments to North Korea, which prompted the North to leave the NPT in 2003.

Then the North resumed a previously shuttered program to extract plutonium from spent uranium fuel. Plutonium-based nuclear weapons are more energy-dense than uranium-based designs, so they can be smaller and more mobile without sacrificing yield.

North Korea conducted its first nuclear test on Oct. 6, 2006. Many experts considered the test to be unsuccessful because the size of the explosion, as determined from seismograms, was relatively small. However, that conclusion was based on incomplete information. And the test still served as a powerful domestic propaganda tool and international display of might.

More tests, more uncertainty

Since 2006 North Korea has conducted five more nuclear tests, each one larger than the last. Scientists are still working to measure their yield accurately. This question is important, because it reveals how advanced the North Korean nuclear program is, which has implications for global security.

Estimates of the size of North Korea’s most recent test in September 2017 place it between 70 and 280 kilotons of TNT equivalent. For reference, that’s five to 20 times stronger than the bomb that was dropped on Hiroshima. In fact, the explosion was so strong that it caused the mountain under which it was detonated to collapse by several meters.