Day out: Mold, Flintshire - dark doings in the valley

These abandoned buildings were once laboratories where some of the deadliest substances used in warfare were created. Now they've been reclaimed by nature.

Published: November 16th, 2016 at 11:53 am

It is forbidden to enter Building 45, where the gaseous diffusion experiments took place.


But from its threshold you can peer through the inky interior to a small square of fluourescent nuclear-green light – it’s another entrance, overgrown with a tangle of vegetation filtering the light.

Building 45 is part of the now disused Rhydymwyn Valley Works, near Mold in Flintshire, where mustard gas was manufactured and stored during WWII. Tunnels for storage tanks were dug into the limestone hill, the River Alyn was diverted, a toxic effluent disposal system was created, and buildings
to produce chemicals and assemble weapons were constructed. Over 2,000 people were employed at Rhydymwyn. Some worked in support services such as haulage or laundry, the rest were known as ‘toxic workers’.

Toxic workers handled both Runcol and Pyro mustard gas. Pyro was imported from Cheshire and put into munitions. The more powerful Runcol was produced on site in liquid form and designed to be sprayed from planes. Should chemical warfare have broken out, Churchill would have broken the Geneva Protocols prohibiting the use of chemical weapons – enough gas had been processed at Rhydymwyn to sustain Britain for three months of aerial combat. Churchill was also prepared in the event of a German invasion to spray the Kent coast with Runcol.

Equally sobering is Rhydymwyn’s other project, known by the code name ‘Tube Alloy’. It was in Building 45 that research into uranium enrichment took place – work that was moved to Manhattan and eventually resulted in
the atomic bomb.


By 2000, Rhydymwyn was closed for business and all contaminated structures were destroyed. Down in the storage tunnels today, only echoes creep through the cold air.

Above ground, the mosaic of rubble, avenues, grasslands and woodlands provides habitats for birds, frogs, lizards, snakes and butterflies. Ragwort and vetch grow rampant. Palmate, smooth and great-crested newts populate the pond built on the overflow and dingy skippers fly among the sward where Howitzers were once
charged with fuses.

Cloaked by foliage, the buildings loom like weird industrial theatres, each with a whispering gallery of acoustics. Feathers, pinecones and thistledown lie in the dust beneath crane gantrys that once shifted tanks of Runcol.

You can explore the buildings at will, except for Building 45, which is off limits. But that’s not due to nuclear secrets or lingering toxicity, but because a rare colony of lesser horseshoe bats have made it their home.

Rhydymwyn Valley Works is owned by DEFRA. To arrange a visit (usually Mon-Fri 8am-6pm and Sat-Sun 9am-5pm) phone site manager David Williams on 01352 741591 or email him at

Julie Brominicks is a Snowdonia-based landscape writer and walker.


Photos: John H. Luxton Photography


Julie Brominicks is a landscape and travel writer who lives off-grid in a caravan in a mossy Welsh valley.


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