UK Weapons of Mass Destruction

One of the normally inaccessible features of the Rhydymwyn Valley Works, developed by ICI in 1939 to manufacture and store mustard gas, is the tunnel system. Three tunnels (central one in bottom photo) were driven about 600 feet into the side of the valley, through limestone, and connected by four cross-tunnels, the stores (below). The system was designed to enable the storage of 3,120 tons of mustard gas, both Runcol and Pyro.

The site's production facilities were closed at the end of the war, when most of the UK's chemical weapons stocks were simply dumped at sea. But the country's then 'strategic reserve' of mustard gas remained stored in the Rhydymwyn tunnel facility until its destruction in 1958-60.

The store was ventilated by means of two huge extractor fans, at the top of the chimneys at the ends of each of the north and south tunnels. Air was drawn into the tunnels, deflected into a void above a mild steel ceiling throughout the storage areas, down through vents in this, and drawn out through grille-covered floor ducts, and up the chimneys. The steel ceiling was carried on concrete corbels.