Sun in a Bottle - Charles Seife [3]
A chain reaction, if it gets big enough, can level a city. Every time a uranium nucleus splits, it releases energy. Like radium, a uranium atom loses mass when it splits. In a tiny instant, the mass is converted into energy, just as E = mc2 predicts. The more atoms that split in the chain reaction, the more energy is released. After forty rounds of splitting uranium atoms, the energy is roughly enough to light an incandescent lightbulb for about a second. After eighty rounds, a mere fraction of a second after the chain reaction begins, the result is more energetic than the explosion of ten thousand tons of TNT, roughly the size of the blast that eventually destroyed Hiroshima.
FISSION CHAIN REACTION: When a neutron strikes a U-235 nucleus, the nucleus splits, releasing more neutrons, which strike more nuclei, and so on.
In 1939, though, the idea of fission—and a chain reaction that would release a tremendous amount of energy—was just a theory. Before World War II began, scientists were uncertain whether the theory was right—and if so, how to turn that theory into the hard reality of a useful weapon. It took two years of cogitation and experimentation for the consensus to build: it was possible to build a powerful bomb out of uranium-235 or plutonium-239 (an atom created in the lab by bombarding uranium with neutrons). Nuclear theory progressed quite rapidly; by 1942, the physicist Enrico Fermi was busy building the first nuclear reactor in a squash court1 at the University of Chicago. Fermi’s project was a major step toward releasing the power of the atom—and eventually bringing the wrath of the sun upon the Earth.
The core of a nuclear reactor is little more than a controlled chain reaction: a pile of fissioning material that is not quite at the stage of entering a runaway explosion. Scientists arrange the pile so that the number of neutrons produced by splitting atoms is almost precisely the right amount to keep the reaction going without getting faster and faster; each generation of fission has roughly the same number of atoms fissioning as the last. In physics terms, the pile is kept right near critical condition. Scientists can manipulate the rate of the reaction by inserting or removing materials that absorb, reflect, or slow neutrons. Pull out a rod of neutron-absorbing material and more neutrons are available to split atoms and release more neutrons: the pile goes critical. Drop the rod back in and more neutrons are absorbed than released: the reaction sputters to a halt.
At 3:36 PM on December 2, 1942, Fermi and his colleagues pulled a neutron-absorbing rod out of a pile of graphite and uranium oxide. The radiation counters chattered. Fermi had created the first self-sustained nuclear reaction. The pile had gone beyond critical; more neutrons were being produced by each generation of fission than the last. The reactor was producing more and more and more energy. About a half hour later, Fermi ordered the control rods back into the pile, and the reaction stopped. At its peak the reactor was producing about half a watt of power, almost enough to light a dim Christmas-tree lightbulb. Nevertheless, the possibilities were enormous: Fermi’s reactor showed that nuclear power could, in theory, light up a city. Or destroy it.
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