A Gravel Gertie is a structure specially designed for use when handling nuclear weapons. It is not designed to contain the force of a nuclear explosion, but rather to reduce the damage and contamination caused by a non-nuclear explosion – for example if the explosive lenses used in a compression-type thermonuclear weapon detonated prematurely during inspection or maintenance.
A Gravel Gertie has thick reinforced walls, but is “open” at the top with the roof being a seven-metre layer of gravel, held back by a thick waterproof membrane. In the event of an explosion expanding gas vents out through the gravel, but this gravel also acts to trap radioactive contaminants. In tests at Sandia National Laboratories a Gravel Gertie reduced airborne contamination after an explosion by a factor of ten.
Four Gravel Gerties are visible in this aerial map of the Royal Ordnance Factory in Burghfield where the UK’s nuclear weapons are assembled; ROF Burghfield is part of the UK’s Atomic Weapons Establishment.