Upgrading the Royal Navy’s nuclear submarine support facilities

Plans to convert another dry dock in Devonport to refit the Dreadnought class submarines were recently revealed in an outline planning application. Here we examine the context and reasons for the upgrade.

Number 10 Dock is the biggest dry dock at Devonport and Babcock Marine, who operate the yard, have plans for a complete refurbishment to create a second facility certified to take the largest nuclear submarines. Stringent modern regulations require that the nuclear facilities must have redundant systems and be able to withstand earthquakes, high tides and high winds. The requirement to withstand a severe earthquake (Considered likely to happen, just once in 10,000 years in Plymouth) is particularly demanding from an engineering perspective. Very robust structures are needed and systems such as cooling water and electrical power need to have multiple backups in the event of failure.


Between 1999 and 2002 the adjacent number 9 Dock was refurbished to conduct refits and refuelling of the Vanguard-class submarines. The original dock floor was removed and a new floor with integral drainage system was constructed and fitted with a cradle to secure the submarine. The old dock was considerably narrowed by lining with counterfort walls constructed on top of the new dock floor. A new dockside edge structure (cope) with service subways to carry piping and cables was constructed on top of the counterforts. The cope was secured by more than seventy, 760-millimetre diameter steel piles anchored in 12-metre sockets anchored in the rock. The dock entrance is sealed by very large multi-cellular caissons and seismically-qualified dockside cranes have been installed.

To enable nuclear refuelling, a new Reactor Access House (RAH) was built that moves on rails to be aligned over the reactor compartment. Spent fuel can be raised up into the RAH and new fuel rods lowered into place. At the head of the dock, a Primary Circuit Decontamination and Alternative Core Removal Cooling (PCD/ACRC) system building was constructed. The PCD/ACRC building contains the plant used to cool the reactor, apply chemical decontamination and inject or remove boronated water reactivity suppressant. The building’s equipment and plant is connected by over 20 km of pipework and 150 km of electrical cable in 92 rooms.

CGI showing preliminary design proposal for the refurbishment of 10 Dock and the new support building to be constructed on the west side. (Image: Arcadis Consulting UK )

The new development at 10 Dock will have similarities with the 9 Dock upgrade project but does not have the cost of complexity of the RAH and PCD/ACRC as it is not intended to be used for nuclear refuelling or de-refuelling operations. The 2.61-hectare dock will be considerably narrowed and shortened by the reinforcement of the East and West walls and construction of a new headwall (the white areas on the mockup). The dock will be served by electrical, water and waste pipework accommodated in subway structures in the new walls. A new Water Retaining Boundary (WRB) will be built to protect the dock from tidal surges and possible future water level rise associated with climate change. Two obsolete support buildings, N125 and N093 on the west side of the dock will be demolished and replaced with a single building containing offices, production facilities and staff amenities.

The phased project is planned to begin in 2021 and Babcock estimates the peak construction time is likely to be between late 2022 and early 2025. New jobs will be created and up to 650 workers will be employed on site. It should be noted that the design is still under development and may be refined further, the proposals made public so far are for the purposes of environmental assessment.

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9 and 10 Dock was were constructed between 1896 and 1907 to accommodate the revolutionary new class of Dreadnought battleships. As battleships rapidly developed, acquiring heavier guns and armour they grew in displacement, considerably beyond what the architects of the dry docks had originally envisaged. The 38,000-ton battleships Nelson and Rodney, built during the 1920s, had a beam of over 32 meters, wider than any previous capital ships. Between 1936-39, Number 10 Dock at Devonport was been enlarged and could accommodate any ship in the navy, except HMS Hood.

10 Dock continued to be used for dry-docking the RN’s largest ships into the modern era, most recently the refit of HMS Albion completed between 2014-17. With the loss of HMS Ocean, the LPDs are the only large capital ships left (apart from the QEC carriers that are too large for any of the dry docks in Portsmouth or Devonport). Assuming HMS Bulwark is eventually re-activated, she can be accommodated in the slightly smaller number 8 Dock, which will be the last remaining big dry dock at Devonport. The other large vessels of the Naval Service belonging to the RFA are maintained away from the naval bases, at facilities in Falmouth and Birkenhead.


  • 5 Basin looking South. The Submarine Refit Complex (SRC) in the foreground and the larger SSBN dry docks at the top. (Photo: Andy Amor)

  • The size of the 10 Dock (centre) is apparent in this view. Assuming the proposed development proceeds, it will look more like the narrower 9 Dock on the left. HMS Vanguard can just be made out under the scaffolding and covers. (Google, Map data ©2020)

  • The scale of work to rebuild 9 Dock to meet modern nuclear safety standards can be appreciated in this photo from November 2000.

  • 74 docking cradle blocks attach to plinths on the dock floor to support the submarine. The massively strengthened dock walls are clearly visible this view of the work in 9 Dock nearing completion, mid 2001. (Photo: Babcock)

  • 9 Dock with HMS Vanguard in refit (Bottom). 10 dock (centre), 11 and 12 dock (top).  (Photo: Andy Amor, 2019)

  • HMS Albion enters 10 Dock to begin major refit, October 2014.

  • Another era – March 1964. 4 aircraft carriers in Devonport including the mighty HMS Ark Royal (IV) in 10 Dock, plus HMS Eagle, Hermes and Bulwark alongside. RFA Resurgent is in 9 Dock. The minesweepers in the basin at the top of the image are where the Submarine Refit Complex was constructed in the 1970s.

  • The previous HMS Vanguard, The Royal Navy’s last battleship (45,200 tons) in 10 Dock, Devonport. Light cruiser HMS Newfoundland is in the adjacent Number 9 Dock. (Painting by Charles Eddowes Turner, 1947 held by the National Maritime Museum)

Regulations have been considerably tightened since 10 Dock was last used for nuclear submarine maintenance necessitating this major works project which will eventually see Devonport have 4 nuclear-certified dry docks. 9 Dock will be in continual use completing the delayed refit and refuel of HMS Vanguard, followed by Long Overhaul Periods (LOP) for HMS Victorious, Vigilant and Vengeance. Fortunately, it would appear that refuelling the 3 younger boats can be avoided but each LOP will take around 3 years and it will be the early 2030s before they are completed. When the next-generation SSBN, HMS Dreadnought, arrives in the early 2030s, Number 9 dock will be needed to de-fuel the decommissioned HMS Vanguard and the other boats as they are replaced. Assuming space can be found, the decommissioned Vanguards maybe stored afloat for sometime after being de-fuelled but eventually they will need to be dismantled. 9 Dock is the only facility large enough and equipped for the work.

With 9 Dock occupied by the Vanguard boats well into the future, another option for maintaining the Dreadnought class will be needed and this is the primary driver behind the project to convert 10 Dock, although maintaining the SSNs is also part of the consideration. The colossal delays to the Astute class construction and the delay in starting the Dreadnought programme means that the work on building the Astute replacement, known as SSN(R), cannot start in Barrow until the late 2030s. It is therefore likely the first 3 boats HMS Astute, Ambush and Artful may undergo lengthy life extension refits and possibly refuelling. This could only be done in 15 Dock at Devonport so another option for SSN dry-docking will be needed. (Number 14 dock has been designated to begin dismantling old SSNs from 2023, a project that could be ongoing for up to 40 years.)

Number 10 dock will relieve the pressure on the limited nuclear submarine support facilities by providing another site for more routine SSBN and SSN maintenance. Although the plans are at an early stage it looks likely they will be approved and work could begin next year. This project further cements Devonport’s future as a submarine refit centre and its crucial role in supporting the nuclear deterrent.