1. Next year, Britain will decide whether to build a new generation of nuclear missile submarines.
Since 1998, the only nuclear weapons Britain has are Trident intercontinental ballistic missiles aboard four Vanguard-class submarines.
The submarines are expected to be retired in 2028. In 2007, MPs voted overwhelmingly (by 409 votes to 61) to replace them; a final decision will be taken in 2016.
2. The current submarines each carry up to 16 missiles, and each missile can carry up to 12 warheads.
The Royal Navy has a total of 58 missiles, less than the 64 which would be needed to fully kit out all four boats. However, since only one submarine is required to be on patrol at any given time, one boat is usually out of service for maintenance, and the missiles are shared between the remaining three active boats. The boats are based at the Royal Navy base in Faslane, Scotland.
There are also only believed to be 192 British warheads. Most Trident missiles are believed to be fitted with three warheads each, although some will have more and some will just have one. The warheads can be independently targeted.
3. Each warhead has a maximum explosive yield of 100 kilotons (the equivalent of 100,000 tons of TNT). That's about six times as powerful as the bomb which destroyed Hiroshima.
They can be programmed to detonate less powerfully: The website nuclearweaponsarchive.org suggests that the likely lower settings are 0.3kt and between 5 and 10kt.
4. A 100kt bomb detonated in the air over central London would probably kill about 250,000 people, almost instantly.
That's before taking into consideration the effects of radioactive fallout.
(Technical note: The 250,000 figure has been worked out using Nukemap, this population-measuring map, and a rule of thumb that the total dead caused by a nuclear explosion is roughly equivalent to the population inside the "5psi overpressure" radius.)
5. One Vanguard-class submarine, carrying 16 missiles with three warheads each, could therefore destroy 48 cities.
If those warheads were targeted at the 48 most populous cities in Russia, a conservative estimate for the number of dead – again ignoring fallout effects – would be 3 million.
6. At least one nuclear submarine is constantly on patrol.
Since 1969, there has always been one British nuclear-missile-armed submarine at sea. The Royal Navy describes this as the "continuous at-sea deterrent". The idea is that even if Britain is destroyed by an enemy's nuclear attack, the patrolling submarine will still be able to retaliate.
7. According to the Royal Navy, the Trident missile has a range of 4,000 nautical miles, or 7,500km.
That means that a submarine at its base in Faslane could hit targets in Nevada, or central India.
Within two minutes of launch the missile will be travelling at 6km a second, and can reach a target at maximum range in about 20 minutes.
8. The cost of replacing the submarines is expected to be up to £26 billion over the lifetime of the project.
That's taken from a 2006 government estimate, which put the cost at £15 to £20 billion – equivalent to £19.5 to £26 billion today. Greenpeace claims it will be more like £34 billion, including taxes and other costs. If Greenpeace is correct, the Trident replacement would amount to about 8% of the total MOD annual budget.
Jeremy Corbyn and Nicola Sturgeon have quoted a figure of £100 billion, but the MOD told BuzzFeed News that they "do not recognise" this figure, which is "plucked out of the air". They suggested it may be an estimate of the figure for replacing the whole deterrent, not just the submarines, and including yearly running costs. A parliamentary commission into the replacement of the whole deterrent estimated the total cost would be £57 billion in 2014 prices, taking into account inflation; it may be that the Corbyn/Sturgeon figure does not take inflation into account.
9. The Trident missiles are built, and owned, by the US, although the warheads and submarines are British-built.
The US weapons manufacturer Lockheed Martin builds the missiles, and provides the technical support to keep them operational. This has led to criticism that the independent nuclear deterrent is not truly independent. The US does not have to give permission to launch the missiles – despite an attempt in the 1960s by John F Kennedy to bring Britain into a "dual key" system which would require both countries to give permission before firing. The nuclear deterrent is therefore operationally independent, at least in theory, although the scenarios in which it would be used without the US being involved are unlikely.
10. Only the prime minister has the authority to order the launch of nuclear weapons.
If there has been an attack on the UK, but the government is still operating, the chief of the defence staff recommends to the PM that a retaliation be made. The PM (or, in the event of their death, an appointed second person, usually a high-ranking member of the cabinet) then gives authentication codes to the Royal Navy headquarters at Northwood; the Navy commander-in-chief and his or her second-in-command would broadcast the orders to the submarine on patrol. The submarine commander and his or her executive officer then both have to authorise the actual launch.
General Lord Guthrie, a former chief of the defence staff, told the BBC that there are safeguards against the possibility that a prime minister could "go mad" and launch missiles without good reason: "The chief of the defence staff, if he really did think the prime minister had gone mad, would make quite sure that that order was not obeyed... Prime ministers give direction, [but] it's not prime ministers who actually tell a sailor to press a button in the middle of the Atlantic."
11. When a new prime minister is elected, they give the commanders of each of the four submarines a sealed letter, known as the letter of last resort.
These letters contain orders of what to do in the event that the government has been destroyed, and the prime minister and the "second person" have been killed or incapacitated, in a nuclear attack on Britain.
When the prime minister leaves office, their orders are destroyed unopened. No one knows what Gordon Brown, Tony Blair, or any of their predecessors wrote in their letters of last resort, and what action would have been taken if there had been an attack.
12. There are believed to be four possible versions of the orders.
According to the BBC's documentary The Human Button, the four options are: To retaliate, with nuclear weapons, against the attacking state; not to retaliate; to use the commander's own judgment; or to place the submarine under the command of an ally, for instance the United States or Australia.
13. It has been claimed that one of the ways that a submarine commander will know if the British government has been destroyed is if BBC Radio 4 stops broadcasting for several days in a row.
According to Lord (Peter) Hennessy, author of The Secret State: Whitehall and the Cold War, 1945–1970, there are several tests, but one of them is to listen for the Today programme on Radio 4's frequencies. If it isn't heard for some days – Hennessy says three – it is taken to be evidence that the state has been destroyed and that the letters of last resort should be opened.