Boracitol
Boracitol is a specialized explosive used in forming the compression wave in the implosion system of a fission device — i.e., a nuclear weapon whose reaction is initiated by extreme compression of plutonium. Implosion systems must form precise compression waves, which involves layering of shaped castings of explosives of different detonation velocities, as well as different densities — wave shaping also is affected by engineered "voids" of actual empty space or filled with plastic foams.
Baracitol, probably not used in recent weapons, is a "slow" explosive with a detonation velocity of 4,860 m/sec, light cast density of 1.55 g/cm3. It is composed of 60% boric acid and 40% TNT. Current weapons also require the explosives to be insensitive high explosives, which baracitol is not.