What is Trident? Assessing Britain's nuclear sub fleet

Staff, News Statesman, Apr 27

Britain's fleet of nuclear submarines is called Trident. There are four submarines (called Vanguard-class submarines), each with the capability of carrying 16 nuclear missiles. It's the missiles themselves that are actually called Trident (Trident II D-5 ballistic nuclear missiles). Each missile has the ability of delivering eight warheads. The missiles are capable of reaching targets 7,500 miles away.
Like a relay system, at any one time there is always one submarine constantly patrolling the seas, while one undergoes maintenance. The other two are used for training in carrying out manoeuvres.
Trident is based in the river Clyde in Scotland.
The current generation of Trident submarines are coming to the end of their working lives. Construction of the first submarine of a new fleet (known as the Successor) will need to begin in 2016 to be operational by 2028, with the current fleet being phased out by 2032. So the big vote in parliament on whether to renew Trident, or to disarm, will be next year.
Trident has always been politically contentious, with some arguing that it is a wasteful expense and a pointless throwback to the Cold War era. Lib Dems have traditionally opposed it (thought they've diluted their stance on it during this parliament), and some left-wing Labour MPs are also in opposition. The CND and other such groups oppose it both ideologically and because of the cost. Greenpeace puts the cost of replacement at £34bn, though the government estimate is £20bn.
The SNP, Greens and Plaid Cymru are firmly in favour of scrapping Trident. The SNP has managed to nudge the issue up the political agenda, by insisting it would be a "red line" in negotiations with Labour to form a government.
However, unilateral disarmament is a bold stance and there is no appetite for it from a government that has been criticised for drastic defence spending cuts. Most Labour and Conservative MPs would vote in favour of renewing Trident, so it is unlikely the SNP will succeed in its aim.