taken from the Scottish Herald -
Q&A: what's the Trident row about?
The Conservatives and Labour are engaged in a bitter row around replacement of the Trident nuclear deterrent - but what is it, how much will it cost and what are the options?
What is Trident?
Trident is a complete system of submarine-based nuclear missiles. It is based on four boats and at any time at least one of them is at sea, on patrol, somewhere in the world. One of the four Vanguard-class submarines has been constantly on patrol since the system came online in 1994.
The system is operated by the Royal Navy and based at the Clyde Naval Base on the west coast of Scotland.
The name Trident comes from the American-built UGM-133 Trident II missiles which are carried by the boats.
Trident replaced a system of Polaris nuclear weapons based on Resolution-class submarines. These had operated since 1963 under an agreement with the United States. The Trident programme was announced as a replacement in 1980.
Why does it need replacing?
While the Trident missiles are expected to be useful for several more decades, the four submarines are coming to the end their life. The Vanguard-class was designed to operate for 25 years - taking the boats to the mid 2020s.
The life of the submarines can be extended by about five years with a refit but new submarines - either new Vanguard-class boats or a completely new design - will be needed to renew the Trident system.
Why can't Britain just disarm and give up its nuclear weapons?
The Government insists unilateral nuclear disarmament would leave Britain vulnerable in a dangerous world. The number of warheads has been reduced and is already the smallest stockpile of the five recognised nuclear powers - Britain, the United States, Russia, France and China.
Government policy is to put British disarmament up for negotiation when other powers have reduced stockpiles to similar levels.
How much will a like-for-like replacement cost?
A House of Commons Library note suggests that in 2013/14 prices, replacement for the whole Trident system would cost about £17.5 billion to £23.4 billion. Replacing the submarines would be £12.9 billion to £16.4 billion of that cost.
Once in place, the successor system is expected to cost around 5% of the annual Ministry of Defence budget - similar to today.
Around £750 million will be saved over the next five years by the decision to shrink Britain's stockpile of warheads.
Trident opponents, such as the CND, claim replacing the system will cost £100 billion over its lifetime.
Are there any alternative systems or options?
At the urging of the Liberal Democrats, the coalition commissioned a review of alternatives to a like-for-like replacement for the Trident system.
Produced by former Liberal Democrat defence minister Sir Nick Harvey, the review suggested abandoning continuous at-sea patrols and cutting the number of submarines from four to three. The Conservatives and Labour rejected this as a "part time" deterrent which would risk increasing international tensions every time a sub was put to sea.
Alternative platforms, including land-based nuclear weapons and missiles that can be fired from an aircraft, were also considered by the review. It found aircraft-based missiles could be more vulnerable to a pre-emptive strike and that it would take much longer to design and build the new weapon.
Land-based cruise missiles would limit Britain's ability to strike anywhere in the world without a third-party nation's help, as well as also being expensive to design and produce.
All alternatives to a submarine system would still need at least two new submarines to cover the capability gap between the end of the current Trident programme and the new alternative, significantly increasing the cost of all alternatives, the study found