# Astraea (nuclear warhead)

The Astraea A21 is a British nuclear warhead planned to replace the Holbrook warheads on Royal Navy submarines from the 2030s. The new warhead will be carried by the upcoming Dreadnought-class submarines on Trident D5 missiles. It will share the Mark 7 re-entry body used on the planned American W93 warhead.

## Development

Development of a replacement nuclear warhead for the Trident nuclear programme was confirmed on 25 February 2020, by the defence secretary, Ben Wallace. The warhead will be designed, developed, and manufactured in the UK by the Atomic Weapons Establishment (AWE), nationalised in 2021. The UK is participating in the W93 programme as observers, and supporting the UK is one of the specified aims of the project.
The UK's stockpile of warheads was planned to decrease from a maximum of 225 to 180 by the mid-2020s, but was instead increased to 260 in the 2021 Integrated Review. The UK also stopped providing information on the numbers of operational and deployed warheads or missiles, under its policy of deliberate ambiguity.
Since 2016, the US and UK have collaborated on a "Joint Technology Demonstrator" to develop tests for warhead safety and manufacturing technologies. The "Replacement Warhead" programme's acquisition cycle is reported to consist of 6 phases covering both production and disposal of military equipment. The British government spent £214m on the project until the end of financial year 2020–21. It was confirmed to have entered the "concept" phase in March 2023, setting out requirements and the options available.
Development will involve the use of the AWE's Orion laser, which uses 10 long-pulse beams and 2 petawatt short-pulse beams, to simulate conditions at detonation up to 10 million °C. Computer modelling will involve AWE's 7.42 petaflop Cray Shasta supercomputer, named Vulcan. Hydrodynamics and radiographic studies will use a joint French-UK facility at Valduc in France, called EPURE, under Project Teutates, set up under the Lancaster House treaties.
AWE planned a 'Mensa' nuclear warhead assembly and disassembly facility in Burghfield, as well as 'Pegasus' enriched uranium and 'Aurora' plutonium manufacturing facilities at Aldermaston. In August 2024, the Aurora project was in the early design phase and was forecasted to cost £2.3 billion and Pegasus was to cost £1.7 billion (from an original cost of £634 million). In March 2023, Mensa was to cost £2.16 billion. The projects had been subject to delays and cost overruns, with Mensa running 7 years late, and £1.2 billion over-budget, while Pegasus was paused in 2018. Pegasus was restarted in 2021 and completion of the storage facility was to be by 2025 and the manufacturing facility by 2030. Mensa was expected to be completed by 2024.
AWE later merged the Aurora, Pegasus and other smaller projects into a single Future Materials Campus (FMC), also based at Aldermaston, planned for procurement in 2025. It is thought to also be aimed at addressing the skills shortages in the nuclear sector.
Under the UK's obligations under the Comprehensive Nuclear-Test-Ban Treaty, it will be the first British warhead deployed without live tests.
It was confirmed to be named Astraea, after the Ancient Greek goddess of justice, and designated as the A21/Mk7 in a Defence Nuclear Enterprise command paper in March 2024.

## Design

The Mark 7 aeroshell and some other non-nuclear components will be shared with the US under existing treaty arrangements. There will be collaboration with the US to ensure that the warhead remains compatible with the Trident missile system. The Mark 7 aeroshell will require specific dimensions and weight distribution of the warhead.
It is reported as being likely to mirror the American W93's yield, in between the W76-1 (90 kt) and the W88 (475 kt). This would mean a significant increase in yield over the current 100 kt Holbrook warheads. Technologies being developed in the Joint Technology Demonstrator, and insensitive high explosives are seen to be likely to incorporated into the warhead.
It is likely to be designed to resist radiation and electromagnetic interference, possibly including cold X-rays. Countermeasures to missile defence systems are likely to be built into the aeroshell.