Trident warhead refurbishment delayed by Fogbank problems

The refurbishment of W76 warheads used by UK and US Trident nuclear missiles has apparently been delayed because the US has "forgotten" how to make Fogbank, a mysterious material used in the manufacture of the warheads. Fogbank is classified naturally but is thought to be an aerogel or foam that acts as an interstage between the fisson and fusion stages of the bomb. A new facility had to be built in the US to make the material as the old facility had already been demolished. The Fogbank problems are said to have delayed the warhead refurbishment by a year and cost an extra $67 million.