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Whether one considers the Iran nuclear deal, North Korea's H-bomb, Russia's arsenal or America's military establishment, it's clear we all need to know something about nuclear weapons technology.

North Korea, for example, began testing nuclear "fission" bombs in 2006. These are based on two kinds of atoms, uranium and plutonium. Uranium occurs naturally worldwide but plutonium doesn't occur naturally and is created only as a byproduct of nuclear reactors.

The minimum amount needed to sustain the explosion, called the "critical mass," is 110 pounds of uranium or 35 pounds of plutonium. As in all chemical and nuclear reactions, the explosion converts a small amount of matter to nonmaterial forms of energy. Einstein's famous equation E=mc2 applies here, and tells us that 0.7 grams -- one-third of a dime's weight -- vanished when the Hiroshima bomb exploded. The resulting energy release was equivalent to 15,000 tons of exploding TNT.

Natural uranium comes in two varieties called U-238 and U-235, but only U-235 will "fission," meaning the central nucleus of the atom can be split, releasing energy. Natural uranium is 99 percent pure U-238 and only 1 percent U-235, but a bomb must be made of at least 90 percent pure U-235. The gigantic task of converting natural uranium to bomb-grade uranium is called "enrichment." This requires a large factory and is the key technology for uranium bombs.

The main requirement for building plutonium bombs is possession of a nuclear reactor. Thus there is a dangerous connection between civilian energy production and military weapons. Most reactors derive their energy from uranium fission, using natural or slightly-enriched uranium. As an important example, the fuel for India's first nuclear weapon was plutonium created by a Canadian-supplied reactor fueled by natural uranium. So enrichment technology isn't required for a plutonium bomb, and any country with a power reactor can build one.

To derive large-scale energy, reactors and bombs create a "chain reaction" involving the nuclei of their uranium or plutonium atoms. When a nucleus fissions, it releases energy along with a few fast-moving nuclear particles called "neutrons." These neutrons then enter other nuclei in nearby atoms, which causes these nuclei to fission, which releases more energy and more neutrons, and so forth until a large number of nuclei have split. To start the reaction, a neutron source such as beryllium is used.

North Korea learned from Pakistan how to enrich uranium. The technique involves fast-spinning "centrifuges" within which the heavier U-238 atoms migrate outward from the spin axis while the lighter U-235 atoms migrate inward, partially separating the two varieties. North Korea's Yongbyon enrichment facility contains thousands of such centrifuges. North Korea also creates bomb-grade plutonium at Yongbyon, from a reactor fueled with natural uranium. The resulting estimated 50 uranium -- or plutonium -- bombs, each with the explosive power that destroyed Hiroshima, could be launched toward South Korea, Japan or Guam.

There's another type of nuclear bomb, one not based on fission. It's based on the method stars use to get their energy: the "fusion" of the nuclei of two hydrogen atoms. Because of the way nuclear forces work, one can obtain "useful" energy from either the fission of heavy nuclei such as uranium and plutonium, or from the fusion of light nuclei such as hydrogen.

Hydrogen bombs aren't easy to build, because the electric forces between the two hydrogen nuclei are strongly repellent, and this force must be overcome in order to get the nuclei sufficiently close together for their nuclear forces to grab each other. The way to do this is to heat the hydrogen because higher temperatures mean the atoms are moving faster, fast enough to smash into each other hard enough to fuse together.

The idea behind H-bomb design is to use a "small" fission explosion as the trigger. Fission happens to create high-energy X-rays, and these in turn heat a nearby container of hydrogen to fusion temperatures. So H-bombs are two-stage fission-fusion devices.

North Korea has mastered this process, as demonstrated by their underground H-bomb test last September. Lacking international chemical sensors at the test site, it's difficult to verify that this was actually a fusion, rather than pure fission, bomb. But the explosion's energy release can be estimated from its seismic (earthquake) wave: about 250,000 tons of TNT-equivalent, 12 times the energy of the Hiroshima bomb. It's unlikely that a pure fission bomb could be this powerful. Furthermore, North Korean publicity photos show a bomb casing that certainly appears designed for a two-stage fission-fusion bomb.

In my next column in three weeks, I'll address what dangers this poses for America.

Commentary on 08/21/2018

Print Headline: Getting to know U

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