North Korea’s Mystery Bomb

On January 6, the North Korean government announced that it had successfully carried out its first underground test of a hydrogen bomb. Until now, this claim has not been independently verified and many international experts have cast doubt on it. So what was the bomb that was tested? It appears to have produced a yield that was larger than that of any previous North Korean nuclear test. As I write, North Korea has not announced whether this was a plutonium or uranium device, but the information we have about its nuclear program offers some clues.

The major North Korean nuclear facility—the Yongbyon Nuclear Scientific Research Center—is located about sixty miles north of Pyongyang, the capital. On the site there is a nuclear reactor that produces plutonium and a reprocessing facility for recovering the plutonium from the reactor fuel elements. There is also a centrifuge facility for enriching uranium that operates with at least 2000 centrifuges that are more advanced than the ones in general use in Iran.

The existence of the centrifuge facility was revealed in November 2010. The American physicist Siegfried Hecker and his colleagues, John Lewis and Robert Carlin, were on an official visit. Hecker, who had been for many years the director of the Los Alamos laboratory, described the experience:

I was stunned by the sight of 2,000 centrifuges in two cascade halls and an ultramodern control room.…Although I and other nonproliferation experts had long believed that North Korea possessed a parallel uranium-enrichment program–and there was ample evidence for such a belief––I was amazed by its scale and sophistication. Instead of finding a few dozen first-generation centrifuges, we saw rows of advanced centrifuges, apparently fully operational. Our hosts told us that construction of the centrifuge facility began in April 2009 and was completed a few days before our arrival. That is not credible, however, given the requirements for specialty materials, as well as the difficulty of making the centrifuge cascades work smoothly.

Exactly how North Korea gathered both the materials needed and the plans to build such a facility is not clear. Some of the scientists who operate the Yongbyon facility were originally trained in Russia but the present generation appears to have been trained in North Korea. The construction of the first reactor began in 1980. The choice of its fuel—natural, unenriched uranium–was dictated by the limitations of the country: at the time there was no enriched uranium and no supply of heavy water. Using open-source information, the North Koreans copied the design of the first British plutonium producing reactors, which had been built in the 1960s with the intent of producing weapons-grade plutonium. These reactors, called Magnox reactors, were long ago abandoned in Britain but they continue to function in North Korea today. Magnox is an alloy of magnesium with a small amount of aluminum that is used to clad the fuel elements, which are made of unenriched uranium.

Administrators observe seismic waves from North Korea's nuclear test in a media briefing at the Korea Meteorological Administration in Seoul, South Korea, January 6, 2016
Administrators observe seismic waves from North Korea’s nuclear test in a media briefing at the Korea Meteorological Administration in Seoul, South Korea, January 6, 2016

All reactors need a “moderator” to slow the neutrons produced in fission. In the Magnox reactors graphite was used. It should be noted that reactors that use unenriched uranium for fuel are ideal for manufacturing plutonium. That is because this uranium is over 99 percent uranium 238 and the plutonium producing process begins when a uranium 238 nucleus absorbs neutron. The Arak reactor in Iran, which was to be moderated by heavy water, was also designed to use natural uranium fuel. Unlike reactors that are used primarily to generate electricity, these Magnox reactors are designed so that the fuel elements can be changed every few months—allowing the plutonium to be extracted. Leaving the fuel elements in too long produces unwanted isotopes, which make the extracted plutonium less suitable for weapons.

It is known that, beginning in the early 1990s, the Pakistani proliferator A.Q. Khan exchanged centrifuge technology for North Korean missiles, in a deal that likely involved the Pakistani government. The exchange was facilitated by the use of Pakistani military aircraft. Around 2000, some twenty-four Pakistani centrifuges were delivered to North Korea. These were presumably of the old type and do not explain how, by 2010, the North Koreans had an ultramodern facility with thousands of advanced centrifuges in operation. Constructing such a centrifuge requires highly specialized materials such as maraging steels (low-carbon steels made from alloys of several metals). Where did these come from?

Iran, which has more advanced centrifuges than the early Pakistan centrifuges, has been suggested, though it’s unclear what the quid pro quo would have been. That the North Korean centrifuges seen by Hecker and his colleagues appeared to be more advanced than the ones in general use in Iran may be explained by the fact that both the Iranians and the North Koreans received from Pakistan the same or similar versions of an older centrifuge design. Both reverse-engineered this design and both used this information to produce upgraded versions. We know that most of the enrichment done in Iran until now used versions of the older centrifuge design, with the newer ones not yet fully deployed. We do not know what stages were followed in North Korea, since Hecker was shown only the facility with the newer centrifuges.

Hecker estimates that North Korea currently has enough fissile material for eighteen bombs, with the capacity to produce six or seven a year. So far the North Koreans have tested four devices, the first three of which certainly used plutonium. The first test, in October 2006, produced a one kiloton explosion. If this was a failure—a “fizzle” to employ the term of art—it was still a very large explosion. The second test, in May 2009, produced a yield of four kilotons, while the third, in February 2013, produced a yield of seven kilotons. To put the yields of these tests in perspective, the Hiroshima bomb was equivalent to about fifteen kilotons of TNT. The North Korean tests are not yet this big but clearly their technology has been steadily improving.

The most recent test, on January 6, appears to have produced a yield of about ten kilotons. North Korean leader Kim Jon-Un described the tested device as a “hydrogen bomb.” Let us recall what this means. The North Korean devices that were tested previously used fission as their energy source. But a hydrogen bomb uses the fusion of light elements such as the isotopes of hydrogen as its energy source, or at least as one of its sources. Fission comes into play in two ways. A true hydrogen bomb uses a fission device as its trigger. This produces the temperatures and pressures needed to induce fusion, which produces very energetic neutrons that can induce more fission. The yield produced can be in the megaton range. Therefore it is extremely unlikely that the North Korean device was a true hydrogen bomb.

More likely, the North Korean bomb was what is known as a “boosted device.” It is initiated by a fission explosion, which causes fusion with the production of very energetic neutrons that cause more fission. (These “fission-fusion-fission” bombs are known as three-stage boosted devices.) This enhances the ratios of the yield to weight and volume of the device. The bombs can be made lighter, which makes them ideal for putting on missiles. There is a long history to this kind of weapon. On August 31, 1957, I witnessed in the Nevada desert the first test of a three-stage boosted device; the stages are fission-fusion-fission. “Smoky” had a yield of forty-four kilotons. It was the height of the Cold War and the weapons laboratories were designing and testing devices that could be carried in missiles.

There is much we don’t know about the North Korean device. Hecker has stated that we may never learn exactly what the North Koreans tested. He noted in an interview, “North Korea has now been in the nuclear testing business for almost ten years, so we can’t rule anything out for certain.” However, David Albright, who served as an inspector for the International Atomic Energy Agency in both Afghanistan and Iraq and who has studied nuclear proliferation closely, has noted some characteristics of this test that differentiate it from the previous one.

As Albright has noted, the January 6 test occurred about seven hundred to eight hundred meters below a mountain, as opposed to the three hundred fifty meters of the previous test. Thus one might conclude that the North Koreans were expecting a high yield. One way a nuclear test is usually discovered after the fact is by detecting the isotopes produced in the explosion once they seep into the atmosphere. The continued lack of such markers, long after the previous test had been detected in this way, suggests that the North Koreans have taken pains to reduce this leakage. Perhaps there will eventually be traces.

If the January 6 test had a fusion component, it would mean that the North Koreans have been able to produce both deuterium with a nucleus of one neutron and one proton and tritium with a nucleus of two neutrons and a proton. The most energetic fusion reaction involves the fusion of a triton and a deuteron to produce helium and an energetic neutron. All samples of natural water contain fractional amounts of heavy water. This is water in which ordinary hydrogen is replaced by heavy hydrogen-deuterons. This can be separated out by electrical means.

While the deuteron is stable, the triton is not and decays with a half-life of a little over twelve years, so about 5.5 percent of any sample is lost each year. Therefore it is not found naturally but has to be manufactured. This can be done in reactors. One method is to produce lithium 6, which then produces tritons when it is irradiated with neutrons. Albright has found evidence that the North Koreans have developed facilities for such a purpose.

In short it would appear as if North Korea is determined to produce fuel to be used in fusion-enhanced nuclear weapons. This is a very serious matter because these weapons can be made light enough to fit on rockets, which the North Koreans have in abundance. That is the real threat. North Korea has announced it is launching a space satellite this month; the same rocket technology can be used for long-range missiles.

Jeremy Bernstein’s books include Plutonium: A History of the World’s Most Dangerous Element , Nuclear Weapons: What You Need to Know and A Palette of Particles.
 His latest book is Nuclear Iran (October, 2014).

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