The U-Bomb

Asked a reporter at President Eisenhower's press conference last week: Was the great thermonuclear explosion in mid-Pacific last year a "bargain basement U-bomb''--a sort of "super H-bomb with a jacket of natural-state uranium that gave it greater power at less cost?" The President replied that he did not think he should attempt to answer the question (and the White House clipped both question and reply out of the television coverage), passed the matter to AEC Chairman Strauss, who refused to comment. But the whole exchange whetted new curiosity about the U-bomb, the latest addition to the world's popular atomic vocabulary.

The U-bomb, say the educated guessers, gets much of its energy from uranium 238, the plentiful isotope of uranium that used to be considered inert and nonfissionable. In theory, such an explosion is entirely possible. So are many other new reactions. Man's armory of nuclear ingredients is growing like a mushroom cloud.

Fission to Fusion. At the end of World War II, only two ingredients were in the nuclear picture. They were uranium 235 and plutonium, both of which are fissionable, i.e., the addition of a single neutron to the atomic nucleus splits the nucleus, with a vast release of energy. Later a third nuclear ingredient, fissionable U-233, was made out of nonfissionable thorium.

About five years ago came the "fusion" reaction. In this, an isotope of hydrogen (either deuterium, H2, or tritium, H3) was forced by extreme high temperature to "fuse" into helium with an enormous release of energy. The scientists got the required high temperature by exploding a conventional fission bomb as a detonator. With this development of fusion--which has never been officially described--the number of reactive nuclear ingredients rose to at least six.

Temperature Does It. As far as nuclear research is concerned, the tremendous heat of the new-style explosion is more important than the H-bomb itself. The hotter the reaction, the faster atomic particles move. In the hottest reactions they may move at such speed that they shatter normally stable atoms. If these atoms are large ones, e.g., U-238 or thorium, their splitting releases still more energy. This process, now called thermofission, was described in the 1945 Smyth Report.

Some physicists are now guessing that the bomb exploded in the Marshall Islands in Operation Castle on March 1, 1954 was a thermofission device. They theorize that it had an old-style atom bomb (U-235 or plutonium) as a detonator at its center. Around this was hydrogen-containing material. Outside this, in turn, was a layer of 11-238. The extreme high temperature of fusion caused the U-238 to explode by thermofission.

Supporting this theory are a few public hints. One is that the AEC has always spoken of the Castle bomb as "thermonuclear," not as a hydrogen bomb. Another is that the explosion created an enormous amount of radioactive residue, which the AEC has officially described, as consisting of "fission products." Traces of fission products were, in fact, discovered by Japanese scientists who analyzed the fallout aboard the 'hapless fishing boat The Fortunate Dragon (TIME, March 29, 1954). If most of the bomb's energy had come from the fusion of hydrogen or other light elements, its residue would not have contained such large amounts of the fission products that are formed by exploding uranium.

Shipload Lots. A literal three-layer bomb would be awkward to build. It is more than likely that the nuclear weaponeers know how to combine their ingredients in many subtle ways. They have probably discovered that complex mixtures, rather than layers, are more effective or reactive.

If U-238 has indeed been made to explode, the weaponmakers have a great plenty of it to work with. The stuff was formerly considered an almost valueless residue left over from the manufacture of plutonium or the separation of fissionable U-235. It may need hardly any treatment to qualify in shipload lots as a nuclear explosive.

None of this is cheery news. It may be the origin of a wry saying of the nuclear weaponeers: "Thermonuclear bombs come in three sizes: little ones, medium-sized ones, and where's everybody?"

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