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Fort Halstead: Buildings F16 and F17

A Grade II* Listed Building in Dunton Green, Kent

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Latitude: 51.3119 / 51°18'42"N

Longitude: 0.1496 / 0°8'58"E

OS Eastings: 549914

OS Northings: 159168

OS Grid: TQ499591

Mapcode National: GBR S8.0DZ

Mapcode Global: VHHPK.J9Y4

Entry Name: Fort Halstead: Buildings F16 and F17

Listing Date: 21 March 2013

Grade: II*

Source: Historic England

Source ID: 1412293

Location: Dunton Green, Sevenoaks, Kent, TN14

County: Kent

District: Sevenoaks

Civil Parish: Dunton Green

Built-Up Area: Fort Halstead

Traditional County: Kent

Lieutenancy Area (Ceremonial County): Kent

Church of England Parish: Halstead St Margaret

Church of England Diocese: Rochester

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Bomb Chamber (Building F16) and Detonation Chamber (Building F17), Fort Halstead, 1947.


The Fort Halstead site is located on a prominent hilltop, which is part of the North Downs, to the north-west of the village of Dunton Green. The site includes both the late C19 Mobilisation Centre ('Fort Halstead’) in the south of the site and a large number of buildings associated with the research facility to the north and north-east of the fort. Buildings F16 and F17 lie within the fort.

A reinforced rectangular concrete structure for the detonation of explosive devices with an E-shaped single armoured chamber. An external metal staircase attached to the south-east elevation provides access to the flat roof which has metal railings to the rear (north-east elevation) and sides (south-east and north-west elevations). The rear and side elevations have three horizontal rows of square metal plates with central bolts which are presumed to be fixings for the monitoring of any detonations. There are also electrical inlets and outlets in the rear elevation.

A bi-partite building of reinforced concrete and brick. To the south and west it is a single storey flat roofed structure housing the armoured laboratory (in reinforced concrete); to the south and east it is faced in red brick English bond, but the north elevation is in concrete (the west elevation was not inspected). The laboratory has a large and prominent T-shaped funnel to its flat roof. The main access is via folding wooden doors in the east elevation. A porch has been added to protect a south pedestrian doorway. At its north-east corner the building rises to two storeys, again in red brick English bond. All windows are metal-framed casements with concrete heads.

The interior was not inspected but it is known that the bursting chamber is located in the single storey part of the building. It is divided into two parts described as ‘large’ and ‘small’ on original plans. Each section has armoured glass observation windows and the whole is flanked by camera rooms to either side. Explosions were recorded using an angled mirror and a high speed camera. The two-storey ‘tower’ housed a photographic dark room on the ground floor and a control room above from which the trials were overseen.


Fort Halstead is one of fifteen late C19 mobilisation centres established to defend London in the event of invasion. Designed in 1894, it was probably constructed between 1895-7 and was intended to be a nodal point where volunteer forces could collect equipment and ammunition if the need arose.

During the First World War the fort was used as a defendable ammunition store forming part of the London anti-invasion stop-line. In 1937, after sixteen years of private ownership, the War Office bought the site to accommodate the Projectile Development Establishment as it provided a remote and contained site for rocket development building on earlier work by the Ballistics Branch at the Royal Arsenal in Woolwich. From the late 1930s the site expanded with a number of buildings constructed inside and outside the fort. After the end of the war, Fort Halstead became the top-secret High Explosives Research headquarters with the task of developing Britain’s first atomic bomb (the Mark 1 warhead which when assembled in its casing was known as ‘Blue Danube’) and this work was to dominate the work at Fort Halstead. Additional structures for this research were built in and around the fort, all within a secure fenced enclave. As was common to projects of the time different research establishments were responsible for developing different components of weapons systems. Although few records exist it is known that Fort Halstead personnel were responsible for developing both high explosive and electronic detonators for the atomic bomb. Britain exploded her first atomic bomb on the Mont Bello Islands, Australia on 3 October 1952.

Atomic weapons research and development continued at Fort Halstead until 1955 when staff transferred to the Atomic Weapons Research Establishment at Aldermaston (Berkshire). Fort Halstead has since continued as a government defence research establishment concentrating on explosives and other research.

Designed in July 1947 and with buildings F17 and F18 forms the key group of structures within the fort relating to atomic bomb research and development. The speed with which it was designed – only two months after the High Explosives Research Establishment was set up at Fort Halstead – is a testament to the importance of and need for this structure in the context of that body’s research and development.

This building was designed in August 1947.

Reasons for Listing

Buildings F16 and F17 at Fort Halstead, the Bomb Chamber and Detonation Chamber respectively both constructed in 1947, are listed at Grade II* for the following principal reasons:
* Rarity: these are unique buildings, specifically designed for the development of Britain’s first atomic bomb;
* Historic interest: both are vital buildings in our understanding of the nation’s atomic bomb research and development, a top-secret programme under the aegis of the High Explosives Research Establishment which through its work at Fort Halstead, and sister sites, was to prove one of Britain’s major scientific breakthroughs in the field of military armament;
* Design and form: the original function of both buildings is legible through their specialised form and both remain little altered from their original design.

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