What is a nuclear submarine and why are they nuclear powered?

A nuclear submarine is one that is powered by a nuclear reactor. This does not mean that it carries nuclear weapons: some types of nuclear submarines carry nuclear weapons and some don’t.

The main advantage of using nuclear reactors to power submarines is that they are able to stay underwater for long periods without the need to refuel or surface for air.

How many nuclear submarines does the UK have?

The UK currently has 27 nuclear submarines. Of these, 11 are still in service with the Royal Navy and 16 have left Naval service.

What types of submarine are there?

The UK has two types of submarine: Fleet Submarines and Ballistic Submarines. Fleet Submarines (sometimes called attack submarines, hunter-killer submarines or SSNs) carry torpedoes and/or conventionally armed missiles, and are used for normal military operations. They do not carry nuclear weapons. Ballistic Submarines (sometimes called SSBNs) carry nuclear missiles, and so provide the UK’s strategic nuclear deterrent.

What is a nuclear reactor?

A nuclear reactor contains uranium fuel elements, and uses nuclear fission to provide energy to power the submarine. The entire process takes place inside a heavily shielded reactor compartment that protects the crew and the outside world from radiation.

The Royal Navy’s submarines all use a type of reactor called a Pressurised Water Reactor (PWR). The idea behind this is similar to a domestic central heating system: water travels around a loop of pipework, called the primary circuit. The water first passes through the reactor pressure vessel, where it is heated by the nuclear fuel elements. It then passes through a steam generator, where the heat is used to convert water in the steam generator into steam. This steam is then used to drive the submarine’s main turbine engines.

A reactor compartments is a cylinder that is typically around 700 tonnes in weight, 10 metres in diameter and 9 metres in length.

What is the service life of a nuclear submarine?

Most submarines have an operational service life of around 25 to 30 years. They come to the end of their life due to a combination of mechanical wear and technology becoming outdated.

What happens at the end of a submarine's operational service life?

At the end of their operational service life, submarines are prepared to be stored safely afloat. This includes adding material into the reactor that stops the nuclear reaction from occurring, removing flammable materials and preserving the submarine against corrosion. The submarine is then defuelled at the earliest opportunity. Defuelling is the process in which the uranium fuel elements are removed from the reactor compartment; they are then transported for storage at Sellafield in Cumbria.

This work is not part of the Submarine Dismantling Project, which will only start working on submarines after they have been defuelled. The defuelling work and the preparation of submarines for afloat storage has been carried out in Devonport Dockyard for many years, and will continue in the future.

What state are the laid up submarines in now?

All submarines presently stored afloat are well maintained to preserve them in a safe condition. They undergo annual surveys, six-yearly underwater surveys, 12-yearly dockings and continual internal monitoring to ensure that they remain safe.

Why can’t the submarines stay laid-up, as they are?

Afloat storage has proved to be a very safe arrangement for over 20 years. However, UK Government and MOD’s long-term nuclear decommissioning policies require that ‘decommissioning and disposal operations should be carried out as soon as reasonably practicable’. In addition, current afloat storage capacity is expected to run out before 2020, and the cost to the tax-payer of maintaining these submarines in a safe state is increasing significantly as they age.

What are the safety implications of the submarine dismantling process?

In an interview on BBC Radio Devon & BBC South West Spotlight Rear Admiral Simon Lister, Director Submarines said:

"Our primary aim for the Submarine Dismantling Project is to develop a safe method for dismantling submarines and we will not compromise the safety of either workers or the general public to do this."

"There is no reason to believe there will be any increased health risk."

"Will use specialist facilities and highly skilled workers."

"We will not compromise the safety of either local residents or the workers."

"Currently looking at techniques to minimise worker dose - such as remote operation."

"Nuclear work is always carried out to the highest standards of safety and is subject to stringent regulation from the Health and Safety Executive."

"We fully appreciate that the views of the public must be considered in this process, which is why the MOD will consult widely on the location for the initial dismantling of submarines, and on the location for the storage of the resultant radioactive waste."

Project Timeline to date

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