With conventional Nuclear Warheads, I recall my old Physics teacher saying something about a great deal of the potential energy in the Bomb isn't utilized, due to the Weapons relying on extreme compression on the raw materials to initiate and sustain the Nuclear Reaction. The explosion itself halts this process, at a point where something like 70% of the potential energy remains unused. The use of a dense alloy to prolong this pressure, and therefore the reaction, does have some merit.