On Sunday, North Korea claimed it had completed its sixth nuclear test – a hydrogen bomb.
This test was performed underground by the notoriously secretive regime. So, how can the international community know the state news agency was telling the truth?
Nuclear weapons either produce energy through nuclear fission (fission bombs) or a combination of fission and fusion (thermonuclear or hydrogen bombs). In both cases, nuclear reactions with neutrons cause the uranium or plutonium fuel to fission into two smaller nuclei, called fission fragments. These fragments are radioactive, and can be detected by their characteristic decay radiation.
If we detect these fission fragments, we know that a nuclear explosion occurred. And that’s where “sniffer” planes come in.
Enter ‘sniffer’ planes
Since 1947, the United States Air Force has operated a nuclear explosions detection unit.
The current fleet uses the WC-135 Constant Phoenix. The aircraft fly through clouds of radioactive debris to collect air samples and catch dust. By measuring their decay, fission fragments can be detected in minute quantities.
The crew are kept safe using filters to scrub cabin air. Radiation levels are monitored using personal measuring devices for each crew member.
On the ground, the Comprehensive Test Ban Treaty Organisation (CTBTO) operates 80 ground-based monitoring stations across the globe that constantly monitor the air for fission products that have dispersed through the atmosphere.
What can fission fragments tell us?
When a nuclear test occurs underground, the fission fragments are trapped except for noble gasses.
Because noble gasses don’t react chemically (except in extreme cases), they diffuse through the rock and eventually escape, ready to be detected.
In particular, some radioactive isotopes of the chemical element xenon are useful due to the fact these isotopes of xenon don’t appear in the atmosphere naturally, have decay times that are neither too long nor too short, and are produced in large quantities in a nuclear explosion. If you see these isotopes, you know a nuclear test occurred.
Something happened during this test that has people excited — there was an additional magnitude 4.1 tremor around eight minutes after the initial tremor, according to the United States Geological Survey. Among other things, this may indicate that the tunnel containing the bomb collapsed. If this happened, then other fission products and other radioactive isotopes could escape as dust particles.
This might have been accidental or deliberate (to provide proof to international viewers), but in either case, we may learn a lot, depending on how fast the sniffer planes arrived and how much dust was released.
For example, by looking at the probability of seeing fission fragments with different masses, the composition of the fission fuel could be determined. We could also learn about the composition of the rest of the bomb. These facts are things that nuclear states keep very secret.
Crucially, by looking for isotopes that could only be produced in a high intensity high energy neutron flux, we could suggest whether or not the bomb was indeed a hydrogen bomb.
What can’t they tell us?
The amount of information a sniffer plane can determine depends on how much material was released from the test site, how quickly it was released (due to nuclear decay) and how rapidly the sniffer plane got into place.
But fission fragment measurements probably can’t tell us whether the bomb tested was small enough to fit on an Intercontinental Ballistic Missile (ICBM). After all, it’s easy enough for North Korea to show a casing in a staged photograph and blow up something else.
Whether or not North Korea has a thermonuclear device that is capable of being mounted to an ICBM is a question weighing heavily on the minds of the international community.
Sniffer planes and the CTBTO network will be wringing all of the data they can out of the debris in the atmosphere to help the world understand the nuclear threat from North Korea.