1. Nukes: Spreading fast
2. Atomic bomb primer
3. Lazy man's atomic bomb?
4. Nuclear hound dog
The atomic bomb that leveled Hiroshima was this
primitive "gun type." An explosion drives a uranium "bullet" into
the second hunk of uranium, creating a critical mass and a fission
explosion. Gun-type bombs require more uranium, but less engineering,
than "implosion-type" bombs, so they're more likely to interest terror
groups. Courtesy The
Fission is the splitting of atoms into smaller
pieces. In a fission chain reaction, neutrons hit atomic nuclei,
releasing energy and neutrons, which strike other atoms, releasing
yet more energy and yet more neutrons. This chain reaction is the
heart of an atomic bomb. Original
Nuclear Regulatory Commission.
At its simplest, an atomic (fission) bomb does
one thing: It assembles a "critical mass" of fission fuel fast enough
to start a chain reaction: One liberated neutron strikes a uranium
nucleus, releasing energy and more neutrons. If the process occurs
in the eyeblink of time before the bomb blows itself apart, you have
the kind of fission weapon that destroyed Hiroshima and Nagasaki.
(Hydrogen bombs, AKA fusion bombs, are more powerful, but much more difficult to make. Thus the proliferation debate focuses on fission bombs.)
An atomic bomb requires two key ingredients: Expertise and bomb fuel.
Expertise includes understanding how to shape both the uranium core and the explosives that compress it quickly enough to start a chain reaction. Fission bombs are not simple: When the United States invented them during World War II, it gathered some of the smartest physicists in the world, holed them up in Los Alamos, N.M. for two years, and gave them everything they wanted in the way of equipment and support.
The Soviet Union took an easier route: It stole secrets from Los Alamos, then built its own weapons.
With each new entrant to the nuclear sweepstakes, expertise seems more available, as designs and knowledge continue creeping out of the nuclear shadows.
The general lay-out for an atomic bomb has been public for 25 years, and design issues are only a minor hurdle, says Matthew Bunn, a nuclear-proliferation expert at Harvard's Kennedy School of Government. "People who have studied what terrorists could do with relatively little knowledge have demonstrated this in experiments. They have hired people who don't know anything about the topic and asked them to design a bomb."
These successful experiments in do-it-yourself-nukes
show, he says, that "A reasonably well-organized and -financed group,
if it had the material, could make a crude explosive." No, the bomb
might not rate as "pride of Pantex,"
but potentially, Bunn says, "it could be put into a minivan and
have explosive power maybe as big as the bombs that obliterated
the Japanese cities."
The message is clear, adds Bunn, who has tracked
the nuclear genie for 20 years. The possession of fission fuel gives
proliferators a jump start: "Once you have the material, making
a bomb is not that hard."
That simple reality, Bunn says, explains the focus on fuel. But which fuel? The guts of a fission bomb can be either highly enriched uranium or plutonium. Plutonium doesn't exist naturally on Earth, and must be made in a nuclear reactor. Some of the uranium in the fuel turns into plutonium, which must be chemically separated from the uranium, then fashioned into a weapon. "If you go the plutonium route, you need to have a reactor, which is very hard to hide," says Bunn.
The reprocessing of fuel rods also releases
radioactive gases like the isotope krypton-85, which can be picked
up by radiation monitors.
If plutonium is difficult to make and use, making
highly enriched uranium is no simple trick, either. When mined from
the ground, natural uranium contains two major isotopes: U-235 and U-238. Bombs need a U-235 concentration
of least 20 percent, and preferably closer to 90 percent, but natural
uranium contains only 0.7 percent U-235.
Centrifuges spin uranium gas to separate
the rare, light isotope, U-235. Separation makes "highly enriched
uranium," the fuel for the simplest nuclear weapon. Centrifuge rotors
spin at about the speed of sound, demanding ultra-sophisticated
metallurgy and design. Courtesy The
Institute for Science and International Security.
Because an element's isotopes are chemically
identical, they can only be separated on the basis of different
density. Uranium separation was a critical challenge for the Manhattan
Project during World War II, and it remains a tough problem six
Today, one preferred separation technology revolves around hyper-speed centrifuges, which force denser atoms to the outside, leaving less-dense atoms near the center. Thousands of nuclear-fuel centrifuges operate in "cascades" to gradually concentrate U-235 in fuel for power reactors or bombs, which require much higher enrichment than power reactors.
Centrifuge technology was a specialty for A.Q. Khan, the Pakistani proliferator. He stole a design from a Dutch company in the 1970s, and decades later marketed designs and centrifuges for enriching uranium to customers in North Korea, Libya, Iran and perhaps elsewhere.
This satellite image is said to show the construction
of an Iranian uranium enrichment facility. The IAEA found the centrifuges
here to be sophisticated, the result of a large, expensive effort.
These large, (tm)protected underground buildings could house more
than 50,000 centrifuges. The facility could produce low-enriched
uranium for civilian purposes or highly enriched uranium for weapons.
Courtesy Digital Globe and The
Institute for Science and International Security.
Some of those centrifuges may be installed in the giant, underground buildings now being built in Iran.
If you want to make an atomic bomb, why not just
buy or steal highly enriched uranium?