By Jay T. Cullen

The purpose of this post is to conduct a thought experiment to arrive at (I hope) a useful estimate of how much radioactive contamination might occur if North Korea detonates a thermonuclear weapon in the lower atmosphere over the North Pacific Ocean.  There are a significant number of unknowns, not the least of which is the fundamental uncertainty as to whether the rogue nation has successfully tested a Teller-Ulam style thermonuclear weapon or not.  I explain my assumptions and compare the resulting global release of radioisotopes that represent a radiological health concern from such a test to the amounts recently released from the Fukushima Daiichi Nuclear Power Plant (FDNPP) disaster, the Chernobyl disaster and aggregate atmospheric weapons testing in the last century. I invite comments and an accounting of the approach used here and how it might be improved.

The estimates represent a worst case scenario I think as there are serious questions as to whether the North Koreans possess thermonuclear weapons similar to those tested by other countries in the past and whether or not they could deliver a weapon similar in yield to their most recent underground test to the atmosphere.  I find that a North Korean test over the Pacific would likely release:

• similar amounts of ⁹⁰Sr but much lower ¹³⁷Cs as the Fukushima Daiichi nuclear disaster to the environment
• more ¹³¹I than Fukushima but 2-3 fold less than Chernobyl
• release up to 2000-fold more Pu globally than did Fukushima but 10-100 fold less than Chernobyl

The health consequences of these releases are beyond the scope of this post but would depend on (among other things) the height that the weapon was detonated in the atmosphere, the location of the detonation over the Pacific and atmospheric circulation that would all dictate where the isotopes would be deposited. The deposition distribution would determine the dose experienced by living organisms through external and internal pathways.

As part of an effort to determine the sources that contribute to exposure of the public to ionizing radiation the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) published a report on the Sources and Effects of Ionizing Radiation in 2000. In this document the expert committee estimated the radionuclides produced and globally distributed by atmospheric nuclear tests in the 20th century which they reported in detail in Annex C.  I will rely on their approach to estimate thermonuclear fusion and fission radioisotope production by a putative North Korean thermonuclear airburst over the North Pacific.

We will consider isotopes produced by fission and fusion that are most likely to pose radiological health risks to living organisms.  For fission these isotopes are Strontium-90 (⁹⁰Sr, half-life 28.78 years), Cesium-137 (¹³⁷Cs, half-life 30.07 years), Iodine-131 (¹³¹I, half-life 8.02 days), Plutonium-239 (²³⁹Pu, half-life 24,110 years), Plutonium-240 (²⁴⁰Pu, half-life 6563 years and Plutonium-241 (²⁴¹Pu, half-life 14.35 years).  For fusion we will consider Tritium (³H, half-life 12.33 years) and Carbon-14 (¹⁴C, half-life 5730 years).

Estimating Release of Fission Products

To estimate the global release of the fission products (⁹⁰Sr, ¹³⁷Cs, ^239,240,241Pu) for a thermonuclear air burst we need to calculate something called Normalized Production which is the amount of radionuclide activity, measured in units of PetaBecquerel (10¹⁵ decay per second) per Megaton (10⁶ ton) of TNT equivalent that the explosion produces. Normalized Production (PBq/Mt) is unique for each radioisotope and can be calculated as follows:

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​where roughly 1.45×10²⁶ fissions occur per Mt of the explosion, which is multiplied by the fission yield of the isotope of interest, and then multiplied by the decay constant of the isotope to convert between number of atoms produced to their activity and finally divided by the number of seconds in a year to arrive at decays per second or Bq. As the fission yields for the Pu isotopes are not know very well we use the ratio of Sr/Pu produced during past thermonuclear tests and our Sr estimate to calculate production of Pu isotopes.

Estimating Release of Fusion Products

For the fusion products (³H, ¹⁴C) we rely on estimates of fusion yield and total production (see references M3 and U6 in Table 9 Annex C).

The calculated normalized production values for our isotopes of interest are as follows:

What is the likely yield of a North Korean Weapon?

This is a very difficult question to answer as there is little known about the type of weapon that the North Koreans possess and whether or not they would be able to deliver such a weapon into the atmosphere to high altitude.  We will assume for the purposes of this calculation that they do possess a Teller-Ulam style thermonuclear device and could weaponize such a device to bring it into the lower atmosphere.  Obviously, and hopefully, this may never come to pass. To determine isotope production from such an event we need to estimate the likely Mt yield of the device based on known tests conducted by the North Koreans.  The last significant test occurred on Sept. 3, 2017 as indicated by a significant seismic event of magnitude 5.6-6.4 that was detected world wide.  Based on the magnitude of the quake the world community has estimated the yield of the weapon to be between 0.05-0.25 Mt of TNT equivalent. We also need to make an assumption about how much of this energy results from fission versus fusion during the explosion.  This is highly uncertain.  Based on past Teller-Ulam style atmospheric tests the split between fission and fusion yield ranges from about 1:1 or 1:2.

Comparing Global Release from a North Korean Weapon to Other Sources

Based on the range of yield and the split between fission and fusion we calculate the following release of isotopes for a putative North Korean thermonuclear test in the atmosphere and compare this release to previous releases below:

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Please note that the graph above has a logarithmic scale on the y-axis. Focusing on the fission produced isotopes we see the following. The global release of ⁹⁰Sr from an air test by North Korea would be similar in scale to the amount released by the Fukushima disaster that started in 2011 but an order of magnitude less significant than releases to the environment by Chernobyl in 1986.  Similarly ¹³¹I would be similar or exceed releases from Fukushima but be 2 to 3-fold lower than releases from Chernobyl. Much less ¹³⁷Cs would be released by a weapons test than has been released by the nuclear power plant disasters.  Relative to the Fukushima Daiichi disaster the release of Pu isotopes by an air test of a thermonuclear weapon is substantial with approximately 2000-fold more ^239+240Pu and 300-fold more ²⁴¹Pu being released.  Chernobyl released 10-100 fold more Pu than a North Korean thermonuclear weapon would likely release.  Overall the impact of the test would be most significant  for ⁹⁰Sr, doubling the amount released by Fukushima, and much more so for the Pu isotopes.  Depending on the transport and deposition of the fallout the isotopic signature of a test might be detectable in ocean water and soils.

All of these release estimates pale in comparison to the global releases associated with past nuclear weapons tests conducted last century which are summarized in the table below:

For an explanation of the release calculations see Table 9 in Annex C of the 2000 UNSCEAR report.

Anomalies for all of the isotopes would be easily detectable in air and would be catalogued by the monitoring network of the Comprehensive Nuclear Test Ban Treaty Organization.