The Valley Works of Rhydymwyn, near Mold, was founded in 1939 as a facility for the production and storage of mustard gas by ICI’s Special Products Division. It takes its name from the Alyn Valley. The Alyn was culverted and, back in the days before we pretended to give a damn about the environment, used to discharge toxic waste into the estuary of the River Dee.
The site was split into four main areas: mustard gas production, bulk storage in three tunnels driven into the limestone flanks of the valley, the filling of shells, and the fitting of these with explosives and fuses (one of the magazines is pictured above). The facility provided for the production of over 300 tons per week.
Mustard gas production stopped at the end of WWII. From 1947 to 1959 the tunnels were home to the UK's strategic stocks. Ultimately the material was dumped at sea or burned on site, which remained as a buffer depot until the early 1990s. Some remediation work was undertaken in 2003, but the site undoubtedly remains toxic, and is thus securely fenced and guarded.
There were originally 200 or so buildings. About 50 remain. One, building P6, was converted in 1942 to house equipment used to test how feasible it might be to separate the isotope U-235 on an industrial scale. This was for Tube Alloys, the codename for part of Britain's nuclear programme. This ended in 1945 when America constructed and used the world's first nuclear weapon, humankind's next generation of WMD.