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Runcol Building R3 (Carillion Building 65) at Rhydymwyn Valley Site

A Grade II Listed Building in Cilcain, Flintshire

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Latitude: 53.1881 / 53°11'17"N

Longitude: -3.1894 / 3°11'21"W

OS Eastings: 320623

OS Northings: 366327

OS Grid: SJ206663

Mapcode National: GBR 6X.35RX

Mapcode Global: WH774.Z4D8

Plus Code: 9C5R5RQ6+66

Entry Name: Runcol Building R3 (Carillion Building 65) at Rhydymwyn Valley Site

Listing Date: 10 September 2008

Last Amended: 10 September 2008

Grade: II

Source: Cadw

Source ID: 87563

Building Class: Defence

Location: On very large site to the SW of the A 541 at Rhydymwyn. This building is to the SW of the large Pyro Plant building P4, and about 650 metres S of Nant Alyn Road.

County: Flintshire

Community: Cilcain

Community: Cilcain

Locality: Rhydymwyn

Traditional County: Flintshire

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For full history and analysis of the site and an explanation of the chemical processes see the Birmingham Archaeology report cited below.
The Ministry of Supply Valley Works, Rhydymwyn was built to produce mustard gas for military purposes, and to assemble, charge, and store chemical weapons. Important research took place here in the early development of atomic weapons.
In response to the use by Germany of poison gas in the First World War, Britain developed chemical warfare capacity. As war threatened again in the later 1930s, the Government, in association with industry, developed a series of new plants. Work at MS Valley began in 1939, and the site became operational in 1942. It continued to produce mustard gas until the end of the War. The works were run by ICI on behalf of the Ministry of Supply. After the War, stocks were gradually decommissioned.
The works was developed on a large site with distinct zones for production of mustard products, filling of shells and bombs, arming the weapons, and storage in tunnels in the hillside, as well as laboratories and administration areas.
Three plants were built to produce the Pyro variant of mustard gas and and two plants to produce the Runcol variant. Although two of the Pyro plants were commissioned they never went into full production. The third Pyro building was used for research in connection with the enrichment of uranium.
This building, R3, was built for the production of the Runcol variant of mustard
Another building, R4 to the same design formerly stood further north. Both buildings produced large quantites of Runcol.


Rectangular building with concrete frame with infill panels of red brickwork laid in English bond. Flat roof. Facing roadway, six bays; the first and fourth (from SE) are broader. At SE end, a doorway (narrowed) and at upper level a window with small-pane glazing. SW elevation has 2 low rectangular blocks which were pump room and syrup room; stairs between. NW end has at upper level a window (reduced in size).


Divided into 2 areas. Longer SE area (reaction section) has raised platform on SW side which formerly bore production equipment. At NW end, doorway through to acid generator section. Overhead crane and some internal details remain.

Reasons for Listing

As part of a group of buildings which along with the Scheduled buildings in the Danger Area and the Scheduled storage tunnels [FL210(FLT)] illustrate the industrial processes behind chemical weapons production to an extent unique in the UK. Group value with the other listed buildings on the site.

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