Transitional Design
The Mark 15 is widely described as a transitional design between fission and thermonuclear weapons. The Mark 15 was a staged weapon (see Teller-Ulam design), using radiation implosion from a fission nuclear primary (Cobra) to implode a secondary stage. Unlike most modern thermonuclear bombs, the Mark 15 used a secondary which was primarily HEU (highly enriched uranium), which generated most of its energy from nuclear fission reactions once the primary imploded it. There was a thermonuclear core which underwent fusion reactions, but most of the energy came from the HEU fissioning. The HEU fission was enhanced by fusion stage neutrons, but would have generated a very significant fission yield by itself.
Some later bombs used depleted uranium fusion stage tampers, and neutrons from the fusion would fission some of the tamper, but the primary energy release (50% or more) was from the fusion reaction.
The HEU secondary tamper concept may have been used in the most modern nuclear weapons, where compact size and weight were highly valued, including the W88 and W87 Mod 1 weapons.
Read more about this topic: Mark 15 Nuclear Bomb
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