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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?"