Considering it's only in one book this'll be tricky... Let's see.... From what I remember, it's multiple nuclear devices encased in an incredibly dense alloy. When detonated, the alloy is able to contain the blast for a fraction of a second, increasing the pressure of the force released into the petatons.

Interesting concept, unfortunately, there doesn't seem to be any real science behind it.

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.

No, but at that kind of atomic level, things turn into a primordial soup. Who knows what could happen. Or I'm talking out of my ass.