The (MoS) Ministry of Supply Factory, Valley was a Second World War site in Rhydymwyn, Flintshire, Wales. It was used for the storage and production of mustard gas and was later used in the development of the UK’s atomic bomb project code-named Tube Alloys. In July I was invited to go and have a look at the now disused tunnels, buildings and nature reserve.

The site occupies around 35 hectares of the Alyn Valley, to the south of the village of Rhydymwyn, once a district of Mould 3 miles away. The area was part of the Gwysaney Estate and was extensively mined for lead with a foundry nearby. Following the closure of the foundry land use on the site was largely agricultural. 

Chemical Weapons
In the late 1930s, the Chamberlain Government planned that the United Kingdom should be in a position at the beginning of any war to retaliate in kind if the Germans, as expected, used mustard gas. In April/June 1939 the Alyn Valley was surveyed by the Department of Industrial Planning on behalf of the Ministry of Supply (MoS), and Imperial Chemical Industries (ICI) who were tasked with managing this programme.

In 1939 the land was purchased by the Ministry of Supply and developed as a purpose-built storage facility and chemical weapons factory. The Treasury approved the sum of £546,000 for initial work on 27 August 1939, and work began in October 1939 on the storage tunnels in the limestone hillside. The factory, to be called M.S. Factory, Valley, opened in 1941. The government authorised the expenditure of over £3 million for the project and ICI’s construction fee was £80,000.

The Factory produced Runcol and Pyro variants of mustard; records reveal that only the purer and more stable Runcol was made in bulk. Two Runcol plants R3 and R4 were constructed to produce 100 tons per week followed by three Pyro factories P4, P5, and P6 with a potential production capability of 216 tons per week.

From 1940 to 1959, the site was involved in either the manufacturing, assembly or storage of chemical weapons, or mustard gas in bulk containers. And from 1947–1959 the tunnel complex held the majority of the country’s stock of mustard gas.

The Nuclear Connection
The MAUD Committee was a British scientific working group formed during the Second World War. It was established to perform the research required to determine if an atomic bomb was feasible. It was founded in response to the Frisch–Peierls memorandum, which was written in March 1940 by Rudolf Peierls and Otto Frisch, two physicists who were refugees from Nazi Germany working at the University of Birmingham under the direction of Mark Oliphant. 

The memorandum argued that a small sphere of pure Uranium-235 could have the explosive power of thousands of tons of TNT. On accepting the findings of the Maud Report in 1941, the government needed to verify that a cost-effective atomic bomb could be manufactured. This required verification that a gaseous diffusion process would work on an industrial scale to provide enough fissile material to manufacture a cost-effective and timely atomic bomb. The Tube Alloys project was formed.

Tunnel Preparation
Preparing to tour the tunnels
Side Tunnel
One of the side tunnels where mustard gas would have been stored
Viewing an opening through a bricked up tunnel section
Viewing an opening through a bricked up tunnel section
Side Tunnel
The view back to the entrance
Looking back to the entrance

Tube Alloys 
The Tube Alloys project was spread over a number of locations some of which were Oxford University’s Clarendon Labs, Birmingham University, ICI at Billingham, Liverpool University and Metropolitan Vickers at Trafford Park, Manchester

It was decided to build four prototype gaseous diffusion machines and the contract was awarded to Metropolitan Vickers at Trafford Park in Manchester at a cost of £150,000. This contract required the building of a single-cell unit, a double-cell unit and two ten-cell units. Each of the cells in the first stage units could weigh up to three tons. It was intended to run the two ten-cell units in series making a twenty-cell unit weighing about 60 tons.

Why Rhydymwyn? 
The physical size of the units and their great weight precluded the use of any buildings in the Clarendon. There were also no units of sufficient size on the Trafford Park Industrial Estate or particularly high where a 20-24 feet clearance was needed. ICI suggested that the redundant Pyro Building P6 at Rhydymwyn may fit the bill. 

A number of other sites were inspected before the virtues of P6 prevailed. Although constructed to manufacture Pyro mustard gas it was never fitted out with the manufacturing machinery. It was large enough, had all of the required services readily available, was accessible by all the research and manufacturing sites, and was located in a secure, guarded enclosure. 

Wallace Akers the leader of the Tube Alloys project telegrammed from the USA and instructed that all preparations should be made to make P6 ready for installation of the gaseous diffusion units.

Buikding P6
Ministry of Supply Factory, Valley, Rhydymwyn: Building P6
Building P6

During 1941 a great deal of work was carried out on the building to make it ready. This included an enhanced power supply, the filling of air channels, the installation of a new air conditioning system, and changing the internal layout to provide, offices, laboratories and outside storage.

In the early days, James Chadwick was involved before he went to the USA to be the leader of the British Contingent on the Manhattan Project. The Head of the technical section was Rudolf Peierls and his deputy was Klaus Fuchs. 

The initial run-up period of the units was dogged by bad engineering practices and delays caused by the need to locate rare materials and parts. This may also have been the result of skilled craftsmen being called up or a lack of appreciation of the fine tolerances necessary in such machines. 

During this period the experiments taking place in a redundant poison gas factory in a tiny village in north Wales were at the cutting edge of one of the most important developments in the whole of history.

The Quebec Agreement 
In August 1943 Great Britain, the USA and Canada signed the Quebec Agreement which committed the future development of an atomic bomb to take place in North America out of the range of German bombs and closer to the greater resources available there. This resulted in the British contingent of 23 scientists (mainly of foreign birth and education) travelling to North America to help with the Manhattan Project.

There were two types of atomic bombs possible, one by the use of Plutonium and the other by Uranium and both were being developed in North America. There were also four methods of separating fissile material from Uranium and all four were being developed in the USA. The British concentrated on the gaseous diffusion method to develop a Uranium bomb.

There was a massive effort in the USA to build a gaseous diffusion plant, which was located at Oak Ridge, Tennessee in a bespoke building called K-25 which covered about 100 acres. 

In December 1943 Peierls and Fuchs travelled to New York and went to a meeting with the Manhattan Project leaders on the gaseous diffusion project. They were in difficulty with the initial design and welcomed their input. Fuchs and Peierls worked as consultants for the Kellex Corporation at Columbia University designing the K-25 plant at Oak Ridge.

Building P6 doorway
P6 Interior
P6 Interior
The Interior of Building P6
The Interior of Building P6

In January Peierls left to go to Los Alamos to run the Theoretical Section and lead the British contingent. Fuchs stayed at Kellex until the Spring when he went to Los Alamos to lead the section working on the lens system for the Plutonium Bomb and he is credited with the design of the initiator for that device. 

Experiments continued until 1945 when the equipment was moved to Didcot and Harwell. The results of the experiments led to the building of the gaseous diffusion factory at Capenhurst, Cheshire. Building P6 is now a Grade II listed building and is of international importance; for a brief period, it was at the leading edge of nuclear physics.

The Cold War
As a result of Great Britain’s previous WWII experiences of disruption to supply, transport and communications, the government decided to set up a system of food and raw material stockpiles to counter the threats of a nuclear war. 

These stores were mostly based on the reuse of existing government-owned sites and buildings; and the former MoS Factory, Valley was adapted to become one of these storage sites.

Camouflage paint schemes can still be seen on many of the buildings on the site

The Rhydymwyn Valley Nature Reserve
M.S. Factory, Valley has not been used since the mid-1990s. In the preceding postwar period many of the buildings were still in use, mainly as a buffer storage depot, but some were demolished because they were considered dangerous. The site has become a Nature Reserve and a Visitor Centre was built on the site of the old gatehouse.

The western side of the site is woodland wild garlic, snowdrops, bluebells and orchids. The River Alyn flows in from the northwest corner of the site and follows the western side of the valley. The river originally meandered through the centre of the valley but it was diverted as part of the early construction works. The site is now home to 7 species of reptile, 8 species of fish, 17 species of butterfly/moth and 8 species of bat and 67 bird species have nested or been observed on the site.

The Tour
The site is open for visitors all year round, however, tours of the tunnels are restricted to small windows each year. The RVHS undertook Tunnel Tours at the Rhydymwyn Valley site on the 16th of July when I attended.

Approximately 120 people were taken through in groups of 20. The tour includes a talk on the history and construction of the tunnels. All hard hats, hi-vis jackets and headlamps are provided at the start of the tour, along with safety information.

The Tunnel tour costs £15, the website says it’s £5 but I suspect they haven’t updated the website.

The Society is planning tours in 2024 but hasn’t yet announced any dates. If you would like to tour the tunnels I would contact the History Society for further information – details to the right. 

The tour itself is more suited to people who are physically fit and able to walk for an hour or so with no issues. The tunnel systems have uneven surfaces with 1 to 2 inches of standing water in places. The tunnels are also quite cold with an average temperature of only 6ºc all year round. It is recommended that stout/waterproof footwear is worn along with clothing suitable for low temperatures. Wellington boots are probably the best option as it was muddy and water did splash as we walked the tunnel systems.

It also has to be noted that anyone who is pregnant isn’t allowed to tour the tunnel systems due to the higher levels of Radon Gas in the tunnels.

The tour of the surface buildings doesn’t require any safety wear but the surfaces are generally uneven. There was a mix of tarmac, gravel and what appeared to be a strange hard bitumen-looking uneven surface on many of the paths. Much of the paths are also covered in overgrown vegetation and entering buildings often requires walking through waist-high vegetation and ground-covering tendrils.

A big thank you to the guides of the History Society and to Phil from Subterranea Britannica for the very interesting tour of the Rhydymwyn Valley Works.


Contact Details
Rhydymwyn Valley History Society

Telephone 01352 781129

Visitor Centre
17 Nant Alyn Road

The Visitor Centre can be found at the end of Nant Alyn Road in Rhydymwyn. 

Opening Times
The Rhydymwyn Valley Site is open to members for managed access:
Mon – Fri 8am – 6pm
Sat – Sun 9am – 5pm

By Car
From the main A541 Mold-Denbigh road, turn into Nant Alyn Road, opposite the Rhydymwyn Service Station.
Postcode for SatNav: CH7 5HQ

By Bus
Route 14 or 14C
Mold – Rhydymwyn
The Visitor Centre is a 5-minute bus journey from Mold bus station.
Please check timetables before travelling.