Klaus Fuchs admitted spying for the Soviet Union
A history project has uncovered evidence that a Soviet agent leaked atomic weapons research from a top secret munitions factory in Flintshire.
According to local historian Colin Barber, Klaus Fuchs was "leaking like a sieve" during his time at the Rhydymwyn Valley Works near Mold in the 1940s.
The site, now a nature reserve, is said to have made 40,000 chemical weapons shells weekly during World War II.
Members of the public can explore the plant at an open day on Sunday.
Mr Barber's believes his research into the base, which was originally set up to make chemical weapons, proves conclusively that Fuchs was supplying the Soviets with classified information.
"All the documentation shows he was leaking information from day one of his time as a scientist in the UK," said Mr Barber.
"And that includes his time here at the Valley Works."
Fuchs, a scientist, left Nazi-ruled Germany because he was a communist and came to Britain in the 1930s. He earned his PhD in Physics at the University of Bristol and in 1942 was granted British citizenship and even signed the Official Secrets Act.
At Rhydymwyn, he worked for over a year on highly sensitive research into the manufacture of weapons-grade uranium, all the time passing those secrets to the Soviets, the historian claims.
After leaving Wales in 1943, Fuchs went to the United States to work on the Manhattan Project that ultimately led to the Hiroshima atom bomb.
But in 1950 he confessed that he had spied for the Soviet Union and was sentenced to 14 years in prison.
"He was a life-long Communist and considered it his duty to leak information to the Soviets, he felt they had a right to know about the research," said Mr Barber.
"That's why they were so much ahead of us. Because Fuchs worked in so many departments, he was able to let them have the results of that research. It saved them money and time."
Rhydymwyn has been open to the public as a nature reserve since 2003, and its history can be traced back to a foundry in 1747.
Hundreds of people spent WWII making chemical shells at the site
At the outbreak of war, the site was chosen to become a chemical plant because of its history of industry and the infrastructure of roads and a railway that had been created there.
At first the factory, so secret it did not figure on any local maps, was involved in making mustard bombs and smoke grenades for the Allied Forces.
It was not until 1942, as part of Britain's highly secretive 'Tube Alloys' project, that work into producing weapons-grade uranium with atomic potential, started there.
Many of the Valley Works scientists, including Fuchs, later went on to work in post-war atomic research at home and abroad.
"The experimentation work done here lead to Capenhurst, the uranium separation plant near Chester, and the creation of Britain's first atomic bomb exploded in 1952 off the Australian coast," said Mr Barber.
The plant will be open from 1000 BST and members of the public will be able to walk around the site before they are shown a film about the factory.
Mr Barber will also be available to answer questions about the plant.