British Navy starts modernisation of HMS Astute nuclear attack submarine to restore full combat readiness.

On June 30, 2025, the British Royal Navy’s nuclear-powered attack submarine HMS Astute (S119) arrived at HMNB Devonport to begin her Mid-Life Re-Validation Period (MLRP) following 15 years of continuous front-line service. This marks the longest uninterrupted first commission in the history of the Royal Navy’s submarine force.
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Although much of her activity remained classified, the HMS Astute regularly conducted patrols in UK waters and NATO maritime zones, including a reported surfacing near the Russian Navy's intelligence-gathering ship Yantar in November 2024. (Picture source: British MoD)

The Mid-Life Re-Validation Period (MLRP), to be conducted by Babcock in the No. 14 dry dock, is a multi-million-pound overhaul that will update and extend the submarine’s service life. Although all Astute-class submarines are based at HM Naval Base Clyde in Scotland, Devonport remains the Royal Navy’s designated facility for deep maintenance and refits, including those of the Vanguard-class strategic deterrent submarines. HMS Astute is the first boat of her class to undergo this phase, which will establish the technical baseline for similar life-extension work across the class.

The MLRP encompasses comprehensive maintenance of the hull structure, refurbishment of mechanical systems, and modernisation of core combat and navigation technologies. HMS Astute is powered by a Rolls-Royce PWR2 pressurised water reactor (Core H variant), which does not require refuelling over the boat’s projected 25-year service life, allowing the current MLRP to proceed without reactor replacement. The reactor drives a steam turbine system connected to a pump-jet propulsor, supplemented by two MTU 600 kW diesel generators, an auxiliary propeller, and an emergency drive motor. The submarine’s submerged speed is rated at approximately 30 knots, with a test depth exceeding 300 meters. Her endurance is limited to 90 days, based primarily on food storage for the 98 crew members, though she can theoretically remain submerged indefinitely due to onboard systems that purify water and air.

The HMS Astute is being upgraded with enhancements to the Thales Sonar 2076 suite, which integrates passive and active components, including bow, intercept, flank, and towed array sensors. The platform also carries the Atlas DESO 25 echosounder for seabed mapping and two Thales CM010 optronic masts, which replace conventional periscopes and feature low-light and thermal imaging sensors. The communications and data systems, including Raytheon’s Successor IFF, are also undergoing improvements to support network-enabled operations and future interoperability under the AUKUS framework. HMS Astute has six 533 mm torpedo tubes and internal stowage for up to 38 weapons, typically including Spearfish heavyweight torpedoes and Tomahawk Block IV cruise missiles, which have a range of approximately 1,600 kilometres. From 2024, the Royal Navy is progressively upgrading these missiles to the Block V standard.

Throughout her first commission, HMS Astute participated in a number of deployments, exercises, and missions. She was the first Astute-class boat to pass through the Suez Canal, the first to conduct extended weapons and sonar trials in the United States at the Atlantic Undersea Test and Evaluation Center (AUTEC), and the first to visit Australia as part of the 2021 Carrier Strike Group deployment. She also took part in Operation HIGHMAST in 2025, joining HMS Queen Elizabeth and other NATO units off Portugal. Although much of her activity remained classified, Astute regularly conducted patrols in UK waters and NATO maritime zones, including a reported surfacing near the Russian Navy's intelligence-gathering ship Yantar in November 2024 to deter suspected surveillance of undersea cables. Prior to entering dry dock, Astute hosted a short demonstration voyage out of Plymouth for friends, families, and VIPs, including a dive and meals prepared by the crew.

HMS Astute experienced multiple operational and technical challenges during her first commission. Early in her service life, she ran aground off the Isle of Skye in 2010 due to navigational error, prompting a command change. A separate incident occurred in 2011 during a port visit to Southampton when a guard changeover led to a fatal shooting inside the control room, resulting in the death of Lt Cdr Ian Molyneux. The assailant, Able Seaman Ryan Donovan, was sentenced to life imprisonment. The submarine has also experienced several technical setbacks, including corrosion, leakage, and substandard parts in her reactor and mechanical systems, as documented in internal Ministry of Defence reports. Among these issues were a leak in the primary circuit, faulty pipe components, and performance limitations in her optronic periscopes during rough sea states. Additionally, the submarine experienced difficulties reaching its planned top speed in early trials, though this performance requirement was confirmed as met in 2015.

Astute is the first of a planned seven boats in her class, all constructed by BAE Systems Submarines in Barrow-in-Furness. The boats measure 97 meters in length, with an 11.3-meter beam and a submerged displacement between 7,400 and 7,800 tonnes. They are designed to be operated by a crew of 98, with accommodation for up to 109, including dedicated bunks for all crew members, a first in Royal Navy attack submarines. The submarine’s quieting features include a hull covered by over 39,000 acoustic tiles to minimise sonar detection. HMS Agamemnon is scheduled to join the fleet later in 2025, while HMS Achilles, the final unit (originally named Agincourt), is currently projected for entry into service in 2028. The delays in construction, compounded by early cost overruns and design changes, were acknowledged by both the National Audit Office and the Defence Select Committee in previous evaluations.

The Royal Navy has already initiated a successor programme under the SSNR designation, aimed at replacing the Astute class beginning in the late 2030s. In 2021, the Ministry of Defence awarded £170 million in early design contracts to BAE Systems and Rolls-Royce. These efforts are now integrated with the AUKUS trilateral partnership between the UK, Australia, and the United States. Lessons drawn from the Astute programme, particularly its maintenance challenges, are expected to influence both domestic and export-oriented submarine projects. The MLRP currently underway on HMS Astute will contribute valuable insights into platform sustainment, reactor servicing without refuelling, and integration of new combat and communication systems in nuclear-powered attack submarines. As the first complete mid-life cycle refit for the class, it also marks a crucial point of transition for the Royal Navy’s undersea fleet and its long-term posture in both the North Atlantic and Indo-Pacific regions.