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Ordinary-high temperatures, attainable by chemical means, are not nearly high enough, but the center of an exploding uranium-fission bomb (more than 50,000,000° C.) is as hot or hotter than the interior of the sun. Before the first atom bomb exploded, physicists were speculating as to whether atom bombs might serve as "detonators" to start fusion explosions.
The temperature of an atom bomb at the instant of explosion is fabulously high, but as the fireball expands, it cools off rapidly. If it cools too fast, any fusion reaction that it has started will die out. But if the high temperature lasts long enough, it will "ignite" the light elements. Then the fusion reaction will continue, generating energy to keep the materials hot until a large part of the light-element charge has been fired.
In the early postwar period, the prospects for fusion did not look very good. The available light elements—lithium, ordinary hydrogen and deuterium (heavy hydrogen)—seemed to require more heat than could be provided by the first atom bombs. The third hydrogen isotope, tritium, looked more promising. A mixture of tritium (H³) and deuterium (H²) will ignite at a comparatively low temperature, turning into helium (He4) and a free neutron, and giving a big yield of energy.
The disadvantage of tritium is that it does not exist in nature. It has to be made at fantastic cost in nuclear reactors. Optimistic physicists hoped that a small priming of tritium would ignite large amounts of light elements that are not so hard to come by. Pessimists feared that too much tritium would be required. They pointed out that each atom of tritium manufactured in a nuclear reactor costs about one atom of U-235 or plutonium, which could be used to better advantage, they thought, in old-style fission bombs.
The optimists won the argument, and a tritium-production program got under way. The great Savannah River plant (cost: $1.5 billion) was largely built for this purpose. As things finally turned out, it may not have been necessary.
GUESSING GAME During the last month or so, there has been a storm of guessing about how hydrogen bombs are made. Every non-insider's guess is surely wrong in some particular. In the early days of nuclear energy, only two main ingredients, U-235 and plutonium, were available to the bombmakers, and both behaved about the same. Now the situation is more complicated. Many light isotopes are suitable for fusion, and under the conditions in an exploding bomb, they may react with one another in many different ways. They also react with the products, e.g., neutrons, given off by the fission detonator, and with materials in the casing of the bomb. As the temperature changes, their behavior changes too. So a diagram describing the behavior of a fusion bomb can give only a few of the possible ingredients and tell only a few of the ways in which they may react.