It appears that someone neglected to properly record how to make a super-secret, and highly toxic, substance called “Fogbank”, which is thought to be a, “thought by some weapons experts to be a foam used between the fission and fusion stages,” of the warhead.

Not only was the old facility to make Fogbank demolished, but, “Vital information on how Fogbank was actually made had somehow been mislaid”, or perhaps never recorded, because the stuff is so super-secret.

Un-dirtyword-believable.