What techniques do scientists use to confirm the occurrence of a nuclear explosion?

Following North Korea's announcement of an underground nuclear test on Monday, initial reports from both U.S. and Chinese officials revealed no signs of a nuclear explosion in the air above the test site. However, a subsequent U.S. report suggested that preliminary evidence of a nuclear blast had been detected.

The absence of airborne particles typically associated with a nuclear event does not necessarily rule out the possibility of a blast. This lack of evidence could indicate several scenarios:

On Tuesday, just one day after the alleged test, the United States deployed a specialized aircraft to scan the skies above North Korea for radioactive traces indicative of a nuclear event. This aircraft, known as the WC-135 Constant Phoenix, is an "atmospheric collection aircraft" tasked with supporting the Limited Nuclear Test Ban Treaty of 1963. According to the U.S. Air Force, the plane is equipped with external devices to collect particulate matter on filter paper and a compressor system to gather air samples in holding spheres. Its onboard technology provides real-time analysis, allowing immediate detection of radioactive particles as the plane flies over a specific area.

So, what does the WC-135 search for during its atmospheric testing? It seeks ionizing radiation—specifically radioactive isotopes like various xenon isotopes, which are unique to nuclear events. These isotopes are produced during nuclear detonation as a result of fission reactions (see How Nuclear Bombs Work). While atmospheric explosions release these particles, underground and underwater tests also often leak them into the air. A fully contained nuclear explosion is exceptionally rare (see Is it possible to test a nuclear weapon without producing radioactive fallout?).

Although atmospheric testing cannot identify the precise location of a nuclear blast, it can confirm the occurrence if it detects the characteristic ratio of xenon isotopes in the air. This finding is considered definitive proof of a nuclear event.

Another way to detect a nuclear blast is through the use of a seismograph, a device that monitors Earth tremors to identify and analyze earthquake activity and other ground-shaking events. A global network of 500 seismograph stations is dedicated to reporting such incidents, including potential bomb blasts. According to NPR's "Detecting Underground Nuclear Blasts," the seismic activity recorded on Monday suggested a ground disturbance equivalent to a 4.2 magnitude earthquake, indicating a blast with a yield of about 1 kiloton, or the power of 1,000 tons of TNT.

Distinguishing between a seismic event caused by an earthquake and one caused by a bomb blast is relatively straightforward. Scientists analyze wave patterns to determine whether the event was an earthquake or an explosion. In simple terms, earthquakes begin with gradual shaking as tectonic plates shift, intensifying over time. In contrast, explosions start with an intense initial shock, followed by diminishing tremors. However, while seismographs can confirm a blast, they cannot determine whether it was nuclear or conventional. Additionally, nuclear blasts can be concealed, such as by detonating them in large underground cavities, which absorb much of the energy and reduce ground shaking. These limitations highlight the importance of atmospheric testing in the detection process.