Nuclear reactor
A pile of fissionable material arranged so that neutrons from splitting atoms reliably split more atoms — a chain reaction held exactly at the edge of self-sustaining, so it neither dies out nor runs away. Control it and you have a steady, enormous source of heat.
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✦ え、本当に？
The first reactor ever to sustain a chain reaction, Chicago Pile-1, went critical at 3:53 p.m. on 2 December 1942 in a squash court under the stands of Stagg Field. It produced only about half a watt — not enough to light a torch bulb — and had no radiation shielding and no cooling. A man stood on the balcony with an axe, ready to cut a rope and drop an emergency rod; the success was reported to Washington in code: "The Italian navigator has landed in the New World."
これは何か
A nuclear reactor is a chain reaction kept on a leash. When a uranium-235 nucleus is struck by a neutron it splits, releasing a great deal of energy and, crucially, two or three fresh neutrons. If on average exactly one of those goes on to split another nucleus, the reaction sustains itself at a steady rate — "critical." Let that average climb above one and the reaction grows; drop it below one and it fades. A reactor is the apparatus that holds the number pinned at one, so the splitting continues in a controlled, self-feeding way, pouring out heat you can then use.
なぜ重要だったのか
Fission had been discovered in 1938, but a single split is a laboratory curiosity; the question was whether the spare neutrons could be made to carry the reaction forward on their own, without an accelerator forcing every collision. Chicago Pile-1 answered it: yes, a chain reaction could sustain and control itself. That single fact underlies both faces of the nuclear age — the bomb, where the chain is allowed to run away in microseconds, and the power reactor, where it is held steady for years. It also opened a source of energy of a wholly different order: the energy locked in the nucleus is millions of times denser than in any chemical fuel.
何を解き放ったのか
The controlled chain reaction became the heat source for nuclear power stations, which supply a large share of the world's low-carbon electricity, and for the reactors that drive submarines and ships. It also produced plutonium and the isotopes used in medicine and research. Every such machine, however large, is doing what the pile under Stagg Field did first: arranging matter so that fission feeds itself at a controllable pace — a fire that burns in the nucleus rather than the chemical bond.
実用最小限の形
Lumps of natural uranium spaced through a stack of graphite blocks, with cadmium rods that slide in to soak up neutrons — Fermi's pile, run so that on average each fission triggers exactly one more.
この項目は完全な記述を待っています——地図製作者たちが作業中です。グラフ上の位置はすでに検証済みです。
必要としたもの
解き放ったもの
最前線——その先はまだ記載されていません。
出典
- — Enrico Fermi, 'Experimental Production of a Divergent Chain Reaction,' *American Journal of Physics* (1952)
- — Richard Rhodes, *The Making of the Atomic Bomb* (1986)
- — Argonne National Laboratory / University of Chicago accounts of Chicago Pile-1 (2 December 1942)
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