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Pyro Building P6 (Carillion Building 45) at Rhydymwyn Valley Site

A Grade II Listed Building in Cilcain, Flintshire

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Latitude: 53.19 / 53°11'23"N

Longitude: -3.1912 / 3°11'28"W

OS Eastings: 320508

OS Northings: 366538

OS Grid: SJ205665

Mapcode National: GBR 6X.35B2

Mapcode Global: WH774.Y2KT

Plus Code: 9C5R5RQ5+XG

Entry Name: Pyro Building P6 (Carillion Building 45) at Rhydymwyn Valley Site

Listing Date: 10 September 2008

Last Amended: 10 September 2008

Grade: II

Source: Cadw

Source ID: 87562

Building Class: Defence

Location: On very large site to the SW of the A 541 at Rhydymwyn. This building is the westernmost of the large Pyro Plant buildings, about 300 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, P6 (as well as P4 and P5) was intended to house plant for the production of Pyro mustard, a less stable variant. However, as the threat of invasion receded, none of the buildings was used for full production.
At P6 plant was not installed. From 1941-43, and again in 1946, it was used for experiments in the enrichment of uranium by gaseous diffusion in connection with the development of the atomic bomb. The project was code named "Tube Alloys". In 1943, following an agreement by Britain and the USA to concentrate research in America, development was stopped on this site, and most of the scientists from the British project were transferred to the USA. The building was adapted with the insertion of offices and laboratories in the E side.


A very large building of reinforced concrete frame construction with infill of red brickwork laid in English bond. Similar to Pyro buildings P4 and P5, but reversed by 180 degrees. Seven bays to long (east and west) elevations; on lower level of E elevation each bay formerly had 2 openings (ie 14 in all) now blocked.
The central section of 5 bays has segmental concrete vault over each bay. At each end a bay with flat roof. Six bays externally to short elevations Square concrete tower to S over second bay from E. Much alteration and blocking of original openings.


The interior is articulated by reinforced concrete posts; some circular openings to roof. Overhead rails for lifting gear. Building is divided longitudinally into 2 areas; to W an area of 2 wide bays, and to E an area of 3 more narrow bays; this area has been subdivided to make laboratory/ office areas. Marks in floor from bases of gaseous diffusion machinery. At S end of W area, small single-storey division probably for switch room. At upper level against N and S walls, steam air heating ducts.

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. This building has additional historic interest as the site of the experiments connected with the enrichment of uranium for weapons purposes. Group value with the other listed buildings on the site.

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