The Mystery of Fogbank (2009)

The refurbishment of the aging W76s has taken much longer than was originally anticipated because once the engineers cracked open the old warheads they encountered a substance codenamed “Fogbank.” The Oak Ridge site sprang from the original Manhattan Project and became one of the seven facilities making up America’s nuclear weapons complex (an ominous-sounding phrase coined by the government, and gleefully seized on by the antinuclear left). Because Fogbank was manufactured at Oak Ridge, it most likely functions as an “interstage” in the warhead: a substance that surrounds the fission and fusion portions of the weapon and channels energy from one to the other.

Source: www.weeklystandard.com