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Friend and CNS Scientist-in-Residence Ferenc Dalnoki-Veress has written a detailed primer on what might happen if North Korea detonates a hydrogen bomb over the waters of the Pacific Ocean. We’re pleased to bring it to you here.

Effects of a Potential Thermonuclear Test by North Korea in the Pacific

Ferenc Dalnoki-Veress

For more than fifty years, since the entry into force of the Partial Nuclear Test Ban Treaty (PTBT), almost all nuclear-weapon possessing states have observed a moratorium on nuclear tests conducted above ground. France stopped after 1974, and China after 1980, ending the era of atmospheric nuclear tests. This choice is principally meant to prevent the release of radioactive gases and particulates from bomb debris (known as fallout) into the environment, where it could contaminate large areas after it settles to ground level. As observed by the late Herbert York, the first director of Lawrence Livermore National Laboratory, the PTBT also legitimized nuclear testing and had the consequence of making the “the continuation of uninhibited weapons development politically respectable.” (Quoted in Fehner, Terrence R., and Francis George Gosling. Battlefield of the Cold War: The Nevada Test Site. US Department of Energy, 2006.)

Unhappily, the foreign minister of North Korea, Ri Yong Ho, stated recently in New York that North Korea may test a hydrogen bomb over the Pacific Ocean rather than underground, as they have done previously, ending the global moratorium that has existed since before many of us were born. He did not state explicitly that any such test would involve a warhead mounted on a missile, but this would be a reasonable assumption. If the warhead is a thermonuclear bomb, it may also have an additional jacket of uranium to enhance the yield while at the same time producing more radioactive fallout. What might it look like? What might be the results?

The least provocative version of such a test might be a high-altitude airburst detonation in the region near where the last two North Korean missiles that flew over Japan landed. The geographical location of the detonation is not critical so far out into the ocean, so accuracy of the missile that deliver the warhead is also not critical. (The North Korean missile launched over Japan on August 29 may have landed hundreds of km from where it was expected, based on an analysis conducted by Dr. Marco Langbroek.)

As many have observed, this would be akin to the “Frigate Bird” test of 1962 where the United States Navy tested a 600 kt nuclear weapon coupled to a Polaris SLBM at an altitude of 11,000 feet (3.4 km). The Navy wanted to quell Air Force critics who were not convinced by the “effectiveness of the system under ‘real’ operational conditions.” (See: Spinardi, Graham. From Polaris to Trident: The Development of US Fleet Ballistic Missile Technology. Cambridge University Press, 1994, p. 62.) The North Korean leadership undoubtedly feels pushed into a corner to respond to President Trump’s verbal attacks, but there may also be technical reasons to do a “Juche Bird” test to demonstrate the reliability of the coupled system of missile and bomb. Since a test is also a public demonstration, it may be that North Korea may issue a NOTAM (Notice to Airmen) and/or NTM (Notice to Mariners) over a large region to prevent contamination of nearby ships and air traffic, while at the same time drawing in aerial sampling aircraft to collect information on the nuclear device design. On the other hand, they have only done this for satellite launches, not for missile launches, and to give advance warning might make the missiles vulnerable to a sea-based intercept from U.S. Navy Aegis vessels. So we may not know when they test until they test.

Testing at high altitude

A high-altitude airburst would allow the prompt gamma rays and neutrons emitted by the bomb to be absorbed by the air rather than irradiating ocean water. If the test is conducted in or near water, it would risk the production of sodium-24 through neutron capture on sodium in the salt. Sodium-24 has a 15-hour half-life and emits fast electrons (beta particles), which may penetrate the skin and cause lymphatic-system cancer. Armed forces personnel were unfortunately exposed to this isotope after the 1946 “Crossroads Baker” test, because they were given permission to swim in the lagoon near to where the test took place without any precautions or sense of how dangerous the water may be. (Robbins, Anthony, Arjun Makhijani, and Katherine Yih. Radioactive Heaven and Earth. London: Zed (1991), p. 7.)

Still, even if the test is sufficiently high to avoid this sort of interaction with water, the intense neutron flux from the rapidly rising fireball would interact with nitrogen in the air and produce the long-lived isotope carbon-14, adding to the inventory already in the air after decades of atmospheric tests. Carbon-14 will become carbon dioxide gas, will be taken up by plants, and will enter the food chain far from where the explosion took place. It would be difficult to make any causal connection between any single nuclear test and carbon-14 ingestion. However, any new atmospheric test will contribute to the current atmospheric load of carbon-14, whose long lifetime will make it dominant among the residual effects of atmospheric nuclear testing for thousands of years to come.

Fortunately, an airburst test would also be likely to occur low enough so that in the line-of-sight to the Earth’s surface, any electromagnetic disturbance (EMP) effect would occur over a small area and would not affect local air or ship traffic appreciably.

Fallout: early vs. delayed

Two types of nuclear fallout are of concern: early and delayed fallout. Early fallout consists of particulates that are large and heavy enough that they fall to the surface within 24 hours. This occurs with low-altitude “surface bursts,” which suck material from the ground or the sea up into the nuclear fireball, generating copious amounts of debris that returns to the surface as fallout, as well as low-altitude “airbursts,” where the particulates come from the debris of the bomb itself. Residents of Rongelap Atoll, downwind from the site of the 1954 “Castle Bravo” test, mistook the deadly fallout hours after the test for snow. All 23 passengers aboard a Japanese fishing vessel downwind of the test also got severe radiation poisoning; one died. This early fallout is the most dangerous because most of the radioactive isotopes involved decay quickly. Just 100 hours after a nuclear test, the radioactive dose from fallout is 1/100th of what it was after the first hour. (Craig, Paul P. The Nuclear Arms Race: Technology and Society. McGraw-Hill College, 1990, p. 312.)

Fortunately, a high-altitude airburst would not produce appreciable early fallout. Rather, the fallout expected from a high-altitude test would be delayed fallout only. These are radioactive particulates less than several microns in diameter. Depending on the altitude of detonation and the yield, this fine radioactive dust is injected into the troposphere or the stratosphere. If injected into the troposphere, the particulates will tend to descend to the ground over a period of months, washed out by snow or rain. Particulates that enter the stratosphere, on the other hand, are essentially above the weather. They travel thousands of miles, and can stay airborne for many months or even years until they descend into the troposphere.

The particulates in fallout can be composed of as many as 300 radioactive species known as isotopes. These have varying adverse effects on living things. Depending on the dose, the exposure could lead to severe radiation sickness leading to death, or could cause mutations to DNA that could lead to cancer later in life. Exposure to radiation in utero can result in death of the fetus, microcephaly and/or mental retardation, depending on the gestational age of the fetus at exposure. The effects of fallout do not distribute themselves evenly around the globe, but may be concentrated unpredictably. One such “hot spot” was found in 1953 in Albany, New York, thousands of miles away from the Nevada Test Site, when an intense thunderstorm washed out the particulates as the radioactive cloud passed by. It is unclear how many other places may have been similarly affected by nuclear testing.

The isotopes of greatest concern are iodine-131, strontium-90, cesium-137 and zirconium-95, which together contribute the majority of the radioactive dose. Iodine-131 has an eight-day half-life, and can be inhaled or settle on land where cows graze, contaminating their milk. Once ingested, I-131 concentrates in the thyroid and may cause thyroid cancer. Cesium-137 and strontium-90 have half-lives of approximately 30 years and remain present for about a century, potentially irradiating people through ingestion or external exposure. Strontium-90 is known as a bone-seeker, displacing calcium in bones, which makes it an especially dangerous source of internal contamination. Estimates of the additional deaths due to past atmospheric nuclear testing runs into the hundreds of thousands. (Robbins et al., Radioactive Heaven and Earth.)

What you don’t know can hurt you

It is unlikely that the adverse health effects of any single nuclear test conducted over the Pacific Ocean can be measured, but that does not mean that these effects do not exist and are not important. We are reminded of the late Harvard social scientist Daniel Yankelovich, who identified four fallacies that policy makers make when using quantitative estimates. His third fallacy is particularly relevant here: to “presume that what can’t be measured easily isn’t important” leads to underestimation of the gravity of the situation. An atmospheric nuclear test would be a blow to human welfare around the planet, as well as a departure from a norm that has been observed by all nuclear-weapon possessors for 37 years.