The UK's fleet of Trident nuclear missile submarines is again in the news as the Lib Dem conference debate whether or not they think there should be a like-for-like replacement.
Existing Trident submarines will end their working lives in the 2020s.
What is Trident?
A sea-based nuclear weapons system. There are three parts to Trident - submarines, missiles and warheads, and although each component has years of use left, they cannot last indefinitely. The current generation of four submarines would begin to end their working lives some time in the 2020s.
Work on a replacement cannot be delayed because the submarines alone could take up to 17 years to develop.
Only one submarine is on patrol at any one time and it needs several days' notice to fire. Its warheads have been reduced to 48 and are no longer pre-targeted.
What is the case for UK nuclear weapons?
Prime Minister David Cameron has always maintained that the UK needs to keep its nuclear deterrent, and replacing Trident was a Conservative manifesto pledge in the last general election.
As part of the coalition agreement that brought the Conservatives and Liberal Democrats into power, they said Trident would be replaced, but the programme would be scrutinised to ensure "value for money".
The Treasury had previously been expected to pay for the replacement system, but Mr Osborne has now said the Ministry of Defence will have to foot all the bill, despite the MoD facing major cuts to its overall budget.
Despite being part of the government, the Lib Dems are continuing to make the case against renewing Trident. They are calling for cheaper, and possibly land-based alternatives, to be explored. Their annual conference is debating a motion on Wednesday calling for a full review of the idea of Trident replacement, saying it must be part of the strategic defence review.
There is also an argument that without a Trident replacement the UK's nuclear industry could be severely damaged. Some estimates say that up to 15,000 jobs may be lost - as well as considerable expertise - if a new batch of submarines is not commissioned.
How much will a replacement cost?
The government has put the bill at between £15bn and £20bn, but campaign group Greenpeace claims it will run to at least £34bn once extra costs like VAT are factored in.
TRIDENT MISSILE SYSTEM
Missile length: 44ft (13m)
Weight: 130,000lb (58,500kg)
Diameter: 74 inches (1.9m)
Range: More than 4,600 miles (7,400km)
Power plant: Three stage solid propellant rocket
Cost: £16.8m ($26.2m) per missile
Source: Federation of American Scientists
Officials have also warned that any decision to reduce the number of Trident submarines from four to three would not result in a 25% cut in costs. However, Defence Secretary Liam Fox did say when in opposition that cutting the fleet of any replacement to three submarines would provide better value for money for the taxpayer.
What are the arguments against a replacement?
The main one is that the old Cold War threat from the Soviet Union no longer exists and therefore the UK no longer needs nuclear weapons, or does not need a submarine-based system designed for the Cold War era. It is also said that nuclear weapons are useless in that they could never be used and would not combat the new threats from international terrorism.
The cost issue is also especially salient at a time of extensive government spending cuts.
Finally, there is the question of Britain's obligations under the Nuclear Non-Proliferation Treaty, and the point made by the Campaign for Nuclear Disarmament that if some states renew their arms it encourages proliferation elsewhere.
What are the alternatives?
Trident's ballistic missiles have a long range, of up to 7,500 miles. One alternative that has been suggested is using cruise missiles based on different submarines. However, cruise missiles have a far shorter range, of over 1,000 miles, and are slower and more vulnerable to being shot down.
Whether other potential alternatives would be much cheaper in practice is also disputed. If Britain chose a cruise missile system, it would probably have to develop its own missile programme, bearing all the research and development costs.
Others have suggested using a land-based delivery system, to avoid the cost of building new submarines. But that has been rejected in the past as too vulnerable to attack - and impractical on these crowded islands. A White Paper also concluded that the through-life costs of this option would be double those of the submarine option.
Some say it would be cheaper to launch missiles from a long-range aircraft. However, the shorter range would again be an issue - and the aircraft could be brought down. The White Paper also examined having a large surface ship that could launch Trident missiles, but judged that the vessel would be too easy to detect.
What about Trident's legality?
Some argue that under the Nuclear Non-Proliferation Treaty, Britain should not be re-arming. Critics also argue that using Trident would break international law since such a weapon would not be able to distinguish between combatants and civilians.
Trident submarines are based at Faslane on the River Clyde
Would a new warhead have to be tested?
Britain signed the Comprehensive Nuclear Test Ban Treaty in 1996 and is observing a freeze on tests though the treaty has not come into force yet. So the testing of new warheads by explosion is in effect banned.
Is Trident independent?
Former Prime Minister Gordon Brown has stressed Trident's independence, saying its firing does not require the permission, the satellites or the codes of any other country (ie the United States).
However, critics argue that Britain is technically so dependent on the US that in effect Trident is not an independent system. For example, the British Trident missiles are serviced at a port in the state of Georgia and warhead components are also made in the US.