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The renewal of Britain's Trident nuclear deterrent is causing controversy. But why can't these submarines just be kept going, without spending billions?
Billions will be spent on new subs
The government plans to spend up to £20bn designing and building a new class of submarines to carry the Trident missile system in a process that will take 17 years.
In the US, there are plans for the equivalent submarine, the Ohio class, to extend its life from 30 years to over 40 years.
For the UK's Vanguard-class Trident-carrying submarines no such plan is possible because of the way it was designed. Everything from the nuclear propulsion system to the electric cables in the sub have a lifespan that means no equivalent extension is possible.
The officials say if the government had wanted the Vanguard class to last 40 years they would've had to incorporate that into the original design, as was done with the Ohio class.
Some of the kit, like the generators that make the steam from the nuclear energy in order to power the turbines, would require an expensive refit to install and would extend the life of the submarine for only a short while.
The MoD says the lifespan of the submarines can be extended by five years to 30 years, but that any further would be risky. During this five-year period, the subs will have a much higher risk of going out of service for maintenance.
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An American physicist Prof Richard Garvin has told the Commons Defence Committee the decision to replace the Trident subs is "highly premature".
He believes they can "safely and economically be operated for 40 to 45 years rather than 30", with changes to water chemistry extending the lifespan of the steam generators.
But the MoD insists any extension in life beyond 30 years will require the replacement of "external hydraulic systems, elements of the control systems (plane and rudder), sonar systems, electrical systems (including main battery) and refurbishment or replacement of elements of the nuclear propulsions system".
As well as this, the "turbo generators, secondary propulsion gear and assemblies, deterrent missile hydraulics, hatches and mechanisms" would need looking at. Eventually the main engine and gearbox mechanism might need replacing. Much of this would involve cutting open the hull and would cost a lot of money.
Despite the controversy, Britain's Trident missile system will fundamentally remain the same. Although there will be a programme, as part of American efforts, to modernise the existing missiles, the government insists "there will be no enhancement of the capability of the missile in terms of its payload, range or accuracy". Any such improvements could have diplomatic repercussions.
So if the missiles are just being copied and rebuilt, or having outdated components replaced, why do the submarines need to be redesigned from scratch, instead of copied?
Duncan Lennox, editor of Jane's Strategic Weapons Systems journal, says this would be madness.
"The lifespan is fairly typical, with a nuclear reactor on them. It is an idea, but anybody would tell you that it wouldn't work because more modern technology is available. They were designed more than 20 years ago. They are very old in terms of the technology."
A new submarine will allow new computer systems and an improved nuclear reactor that will get more power for the amount of fuel. This could save money in the long-term as the chance of it needing an expensive refuelling will be reduced.
Opponents include famous faces
Safety standards have improved since the design of the Vanguard-class was started in 1980 and the MoD will want to see these improved standards incorporated into the new Trident-carrying subs.
But their lifespan will still be in the region of 25 years, meaning the same debate about renewal will have to be had in 2030 to 2035. A "non-submariner" might ask whether it would be a good idea to give the new boats a longer lifespan from the start.
One of the problems is the British submarines are built on a tightly-controlled cost. Rear Admiral
Andrew Mathews has said the Americans "built some fat into their design" for the Ohio.
And as for a massively improved lifespan there is also a question of the submarines being rendered a white elephant. To the officials it's rather like asking whether you could build a fridge that could last for 100 years. You might very well find an engineering solution to do it, but it would cost lots of extra money.
Also, despite the money you'd spent, you might find after 25 years there had been huge advances in refrigeration technology which you would find too expensive to install in your super-lifespan fridge.
In addition, the government might feel the political situation may have changed so much in 25 years that it really no longer needs a fridge.
The good thing, Mr Lennox says, is that Trident remains a missile system against which no nation has any defence.
"There are 25-30,000 nuclear warheads in the world. Trident is a sensible insurance policy at a relatively low cost."