The only British ammunitions site to remain a secret during World War II could be declared an ancient monument.
The Rhydymywn Valley works, near Mold, Flintshire, produced hundreds of tonnes of mustard gas shells.

Its scientists were also sent to work on the Manhattan Project in the US, which built the first atomic bomb.

Cadw, which looks after historic sites for the Welsh Assembly Government, says it is considering whether parts of the plant should now be preserved.

Inspectors have visited the site and say some parts of the Valley works could be given full ancient monument status, while other parts could be made protected Grade II listed buildings.

It would mean that Cadw must be consulted before any changes are made to the structures on the site by its current owners, the Department for Environment, Food and Rural Affairs (Defra).

Any parts declared ancient monuments would be taken over by Cadw, and maintained indefinitely for the public to visit.

"This would be fantastic news for Rhydymwyn Valley," said Clive Barber, who acts as the site's historian.

"This is a truly terrific site, with amazing stories to tell."

Cadw has identified what are known as the Pyro and Runcol buildings to be given Grade II listed status. They were both originally built to produce mustard gas, though only the Runcol building was ever used.

But in 1942, scientists began work in Pyro building six, on a secret project called "Tube Alloys", carrying out experiments to enrich radioactive uranium.

The expertise developed there was later called on when its researchers were sent to America to work with Robert Oppenheimer on the Manhattan Project, which built the world's first atomic bomb.

"From this little building came the genesis for the atomic bomb, nuclear submarines, and much of what the nuclear industry is today," said Mr Barber.

"We would love the opportunity to turn Pyro six into a museum so we let people know just what an important role it played in our history."

Cadw said the proposals are still at an early stage and it will be many months before the full plans for the site are finalised.

"When that is done, we will then consult the local authority and the Royal Commission and inform the site's owners, Defra," said a spokesman.

The site of the mile-long Valley Works was acquired by the Ministry of Supply in 1938 so that a purpose-built overground and underground chemical weapons production and storage plant could be constructed.

The tunnelling process at the site involved many 100’s of miners. At its peak, 1,756 people were employed here and by November 1942, workers were manufacturing 40,000 25lb Mustard shells every week. By the time chemical weapons production had ceased at the site in April 1945, 15,477 tons of Mustard had been produced and 4.7 million chemical munitions had been assembled.

From 1942 to 1944 work into ways of producing the enriched uranium necessary for the atomic bomb project was also based at the site, involving some of the key scientists who went on to be involved in the Manhattan Project; the American Government’s race to build an atomic bomb before Germany and Japan could do so.

In 1952, the UK adopted a nuclear deterrence defence policy and the chemical stockpile was disposed of between 1959, when the site was decommissioned, through to the late 1960s. The Valley Works also has another distinction. It was the only UK wartime installation never located by German intelligence.

Prior to ownership of the site being transferred to DEFRA in 1994, the Valley Works was controlled by the Ministry of Public
Buildings & Works, later to become the Property Services Agency (PSA) and was used by the Ministry of Agriculture, Fisheries & Food (MAFF) to store foodstuffs. Generators and mobile bakeries were also stored there as a precaution against national emergencies.

"The Home Defence Committee decided on 19th March that the NATO Wartime Civil Agencies should be accommodated in the Valley Tunnels, together with a Sub-Regional Headquarters.

A bid has since been received from the Treasury for the Bank of England to be allotted 30,000 square feet of Valley Tunnel for the deposit of bullion (Codeword MALLARD).

I understand that the total area of Valley Tunnels is 75,000 square feet, of which the NATO Agencies are expected to require about 40,000 square feet and the Sub-Regional Headquarters 20,000 square feet, which would not leave room for the Bank of England, thougg it might just be possible if these figures prove adjustable.

I would be very grateful if you would let me know whether you see any possibility of being able to fit into Valley Tunnels a sealed off area of 30,000 square feet for the Banke of England inaddition to the NATO Agencies and the Sub-Regional Headquarters.

I am hoping the Home Office can at the same time state whether, if it does come to the point of choosing between the Sub-Regional Headquarters and the Bank of England, they could in fact put their Sub-Regional Headquarters elsewhere.

If they say no, or if your work has already gone too far to allow easily for change, we shall have to find a different hole in the ground for the Bank of England, and I will be glad of any suggestions."