Reply #45: They are fusion weapons

[krispos42]

A small fission bomb of a fixed yield initiates the nuclear fusion reaction. The fusion fuel is some mixture of deuterium with (I believe) some lithium isotope. The bomb must have some way of selecting how much fusion fuel gets injected into the fusion chamber just before the fission bomb goes off.

Probably, the lowest yield available is just the fission bomb's detonation. No fusion fuel is added. At the far end of the scale, injecting the entire load of fusion fuel give you the 150 kilotons.