How Do You Know When a Nuclear Weapon Test Has Happened?

After the first of two nuclear weapon tests by North Korea, in 2006, a monitoring station in Yellowknife, Canada, caught a whiff of xenon-133, a radioactive isotope of xenon, in the atmosphere. It had been released by the blast and had traveled more than 7,500 km. Being a noble gas, it hadn’t reacted to anything on the way.

The Yellowknife station is part of the International Monitoring System installed by the Comprehensive Test Ban Treaty Organisation (CTBTO), which ensures signatory countries’ compliance with the treaty that prohibits them from testing nuclear weapons. In early July 2014, the organisation signed a pact with an American company named NorthStar, which manufactures radioactive isotopes for medical diagnostics. The pact was signed because, as it turns out, radioxenon is also a by-product of the manufacture of substances used in medical diagnostics, and NorthStar was agreeing to deliberately limit its emission to help the IMS facilities focus on radioxenon from nuclear weapon tests alone.

Seventy years after the atomic bombings devastated the cities of Hiroshima and Nagasaki, xenon-133 and xenon-135, two radioactive isotopes of xenon, are reliable proxies for sniffing out when and where a secret nuclear weapon test could’ve happened. Here’s how it works.

The 18,000 sq. km. expanse of the Semipalatinsk Test Site (indicated in red), the Soviet Union’s test-bed of choice. It was shut on August 29, 1991. The date was chosen as the International Day against Nuclear Tests by the UN. Image: Wikimedia Commons

The 18,000 sq. km. expanse of the Semipalatinsk Test Site (indicated in red), the Soviet Union’s test-bed of choice. It was shut on August 29, 1991. The date was chosen as the International Day against Nuclear Tests by the UN. Image: Wikimedia Commons

What happened during the 2006 nuclear tests by North Korea?

North Korea’s first nuclear test was conducted in October 2006. While CTBTO-installed seismographs picked up the underground shockwaves, that a nuclear explosion occurred was further certified by the Yellowknife atmospheric radionuclide detection station in Canada two weeks after the blast. On the other hand, no radioxenon was detected after the May 2009 nuclear test also by North Korea.

How do xenon-133 and xenon-135 show up in nuclear detonations in the first place?

Xenon-135 is a decay product of iodine-135, which itself is an important fission product of uranium-235. Iodine-135 has a half-life of 6.7 hours and is also produced by the radioactive decay of tellurium-135. As a result, radioxenon is continuously produced during a fission reaction and can thus “poison” a reactor. This happens because xenon-135’s neutron-absorbing capacity is more than 5,000 times that of uranium-235. So during a fission reaction, the radioxenon can remove the slow-moving trigger neutrons from the fray, reducing the reaction rate. For some reason, this event is called a “poisoning”. The mismanagement of a similar “poisoning” event was what led to the 1986 Chernobyl disaster.

How much radioxenon is released from a nuclear weapon explosion?

When xenon-135 absorbs a neutron, it turns into xenon-136. When it doesn’t, it decays into cesium-135, a long-lived isotope (with a half-life of 2.3 million years).

According to a 2012 thesis submitted at the Third University of Rome, the energy from a fission blast is roughly split as:

Fallout from a nuclear blast

Fallout from a nuclear blast

The last 15% is split as 5% in the form of neutrons and gamma rays, and 10% in the form of beta and gamma radiation over time. A one-kiloton blast produces about 1.08-1.33 x 1016 becquerel of radioactive xenon-133. This number is important because it could inform scientists attempting to estimate radioxenon amounts based on observation records shared by its compliance monitoring facilities. A 2009 study in the Journal of Environmental Radioactivity estimated that 1.3 peta-becquerel of radioxenon escaped each year from nuclear reactors – one-tenth of a kiloton’s blast.

The CTBTO Public Information office adds:

The Comprehensive Nuclear-Test-Ban Treaty does not have any provisions for approving reactors or other nuclear facilities. Member states however work closely with us to help identify civilian sources and thus factor these in. Regarding the “pacts” with NorthStar and five other medical radioisotope producers: these are pledges on voluntary activities towards mitigating the impact of xenon emissions on observations at our stations.

What happens to the radioxenon after a nuclear weapon explosion?

Nuclear weapons can be detonated in four ‘places’: underground, in the atmosphere, in space, and underwater. When a nuclear weapon goes off in the atmosphere, it’s assumed that all the noble gas isotopes are released into the atmosphere and not deposited on the ground below. In an undersea blast, it’s thought that xenon-135 rises up through the water and enters the atmosphere. Space-based blasts can be ignored for now, leaving the underground explosion which is also the trickiest to study.

Four ‘places’ nuclear weapons can be tested in. Image: Wikimedia Commons

Four ‘places’ nuclear weapons can be tested in. Image: Wikimedia Commons

From the Third University of Rome thesis,

The released energy leads to an increase in pressure and temperature, which are transmitted into the surrounding, and an isotropic shock wave is created. Milliseconds after the detonation the outward directed pressure and shock wave create an underground cavity. The inside wall of the cavity is coated with fused earth, but since the inside of the cavity is pressurised the surrounding walls are likely to get fractured. When gas is leaking through these cracks, the pressure decreases and rubble can fall down. By repeating this the cavity can migrate towards the surface. The shifting of earth material downwards can cause a crater to be formed on the surface. The amount of gas that leaks through the earth to the surface can vary widely. It can hardly be predicted,  because it depends on many parameters such as the rock composition and depth, yield and kind of explosion.

Based on these conditions and calculations on detonations at the Nevada Test Site, xenon-135 is thought to leak to the surface in meaningful quantities in about three hours after an underground blast. At the same time, it’s not unlikely that the detonators have taken precautions and sealed off the site to prevent gases from escaping. This is what is thought to have happened after the 2009 North Korea test.

How is a radioxenon correlated with a nuclear blast?

The CTBTO has installed a worldwide ‘International Monitoring Station’ that’s actually a composite of 337 monitoring facilities. Out of them, 80 are on the lookout for radionuclides, and of them, 40 can detect radioxenon. The map below shows their locations. When a nuclear weapon goes off and radioxenon gets into the atmosphere, the stations closest to the blast, as well as those in the way of the winds that carry it, can pick it out.

The 80 radionuclide monitoring stations around the world operated by the CTBTO. Source: CTBTO

Why choose to track radioxenon and not anything else?

Xenon-135 has a half-life of 9.14 hours, which means it’s around long enough for it to be spotted in the atmosphere, but not long enough so that its presence masks more recent emissions. Second, it’s noble, i.e. chemically inert, and doesn’t react with any other substances and become transformed. Third, xenon-133 and xenon-135 are produced in significant quantities – some 5-7% of total emissions – in the aftermath of a nuclear explosion. Fourth, xenon-133 and xenon-135 are not produced naturally.

How is radioxenon formed during the production of medical isotopes?

Technetium-99 is an isotope used ubiquitously in medical diagnostics, and it is formed by the radioactive decay of molybdenum-99. The way to get molybdenum-99 is through an arduous irradiation and filtration of uranium-235.

First, uranium is sandwiched between two aluminium plates and bombarded with neutrons at something like 10 trillion to 500 trillion neutrons per squared centimetre per second for two or more days. Then, the uranium is left to cool for a day to remove short-lived isotopes that the irradiation would’ve yielded. Next, the important fission products are removed by dissolving the uranium in hot acid – and this is when radioxenon is produced, being subsequently isolated and released into the atmosphere. Once they’re out of the way, the molybdenum-99 can be filtered out.

Is there any way to say where some radioxenon has come from – a nuclear blast or a medical isotope purification facility?

Yes. A note from the CTBTO Public Information office states:

It depends on which isotopes are detected and how many samples catch the same release. Isotopic activity ratios (in addition to the mentioned Xe-133 and Xe-135, we also measure Xe-133m and Xe-131m) help to characterise the source and can be used to determine a hypothetical explosion time. Multiple detections and even the close-by non-detections are used to combine the geographic regions from which the air arrived and collected in the different samples to determine a possible source region that is consistent with all observations.

A combination of these findings with the known locations of legitimate sources and with explosion-like events detected by the waveform sensors results in a powerful decision-support mechanism to determine the character of an event and to distinguish between legitimate known sources and possible nuclear explosions.

Note: This article was updated on August 11, 2015, at 21.46 pm to include the CTBTO’s clarifications for the third and last questions.

  • The Wire

    Thank you. Your clarifications have been incorporated into the piece.