What is a U bomb?

Answer 1

A U-Bomb was believed to be an Atomic (fission explosion) or Hydrogen (fusion explosion) Bomb with a Uranium Casing. The casing was supposedly made of U-238, residual uranium material leftover from processing that was non-fissionable (couldn't be made to explode like a U-235 bomb).

In a fission bomb, a U-235 (Uranium) or Plutonium (Uranium enriched through processing to increase the yield) nucleus is split with a neutron resulting in the release of nuclear energy as an explosion. In a fusion bomb, an isotope of hydrogen (either deuterium (H2) or tritium (H3)) is fused into helium by extreme temperature, which is generated by a fission explosion as the detonating device.

Since U-238 was essentially worthless, finding a use for it that would enhance either types of nuclear explosion would have been a major step, and it's thought that the U-Bomb was exactly that - a regular fission or fusion bomb with a casing made of U-238, which, though not able to be detonated like U-235, would explode in the high temperatures generated by a fission or fusion explosion, thereby enhancing the nuclear yield (intensity) of the explosion itself. For obvious security reasons, its existence was neither confirmed nor denied by the military.

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Today, the design is part of some high-yield nuclear warheads, although the bombs would produce extreme amounts of radioactive fallout. The largest nuclear weapon ever detonated, the 50-megaton Soviet "Tsar Bomba", could theoretically have yielded 100 megatons or more because it used this design.