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 Globe Project.

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 graphic: U.S. Nuclear Regulatory Commission.

(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.

Nuclear know-how
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."

Fission fuel: Playing hard to get?
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.

Enter the spin doctors
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 decades later.

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?

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