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.
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. “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.
February 4, 2016, 5:08 pm