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Rebuilding Carriers

July 8, 2009: The USS Carl Vinson, a Nimitz class carrier, just completed its mid-life refueling and overhaul. This is a three year process, and cost $3 billion. Part of that involved reloading the nuclear reactors, which  requires dismantling part of the ship, as does many of the upgrades and replacements done to electrical and mechanical systems. In effect, the ship is largely rebuilt, enabling it to serve for another 25 years.

The Vinson entered service in 1982, and is scheduled to retire in 2032. But this isn't the only time a Nimitz undergoes scheduled shipyard maintenance. During a half century of service, there are three other trips to the dry dock, and a dozen other shorter stays in the shipyard. But the midlife work is the most expensive, and takes the longest. All this is necessary to keep the carrier up to date in terms of technology, and deal with wear and tear. Other warships undergo similar periods of maintenance, but the nuclear boats all require the mid-life refueling. However, a new generation of naval reactors do not require a complex midlife refueling. These reactors are already being used in the Seawolf and Virginia class subs, as well as the new Ford class carriers.

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Bob Roberts       7/8/2009 12:49:14 PM
Just how extensive are mid-life refueling and overhauls?  Are the vessels more or less gutted or what?
 
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bartrat       7/8/2009 6:21:19 PM
Question: It was stated in the article that some new US attack subs do not need mid life refueling, how is that possible?
Reactors consume fuel and eventually run out, is is an easier process to refuel them or some new technology that does not consume as much fuel?
 
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cwDeici       7/10/2009 1:43:42 PM
I would guess they found a way to make it last as long as they needed.
 
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bartrat       7/10/2009 4:06:06 PM

I would guess they found a way to make it last as long as they needed.
I am curious about the exact details and if anyone knows, please post.
It is possible the reactors contain extra fuel so midlife refueling not needed, but I would like to see some confirmation on that (assuming it is not classified).
 
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kirby1       7/10/2009 5:43:30 PM
Can't swear to it, but I'm sure that along with the refueling, there is serious maintenance and inspections going in the reactor when the ship puts in.
 
Experience and advances in technology has allowed the navy to build reactors that last longer then the ships that they propel. There are probably remotely mounted sensors that monitor certain perameters that used to require a tear down and inspection, I also wouldn't doubt it if the new ships happen to have autonomous tech that allows certain maintenance and inspection jobs to be done remotely, rather then having to have the whole reactor took apart. 
 
I'm pretty sure the carriers will still go in for a midlife overhaul. Got to upgrade the computers and replace the rusty steel. But unless the sensors show some sort of unexpected trend that can't be repaired remotely, there really won't be a need to disassemble the reactor.  
 
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JFKY    Half-life is half-life   7/10/2009 6:05:03 PM
It is possible the reactors contain extra fuel so midlife refueling not needed,
 
Not likely.  It's not like Gasoline, if you don't "burn" it you still have it.  Naval Reactors use Highly Enriched Uranium (HEU).  HEU is 90-plus% U-238.  Giving the maximum power output for the minimum volume.  Whether or not the Uranium is fissioned or not, it still decays.  So even if the reactor is relatively cold, after a number of years the HEU is below the critical level for power generation.  How many years that is, I haven't a clue...but like I say it's not like you can say, "Put an extra 1,000 pounds of HEU in there.  That way we won't have to refuel."  It STILL decays as a fuel/power source.
 
 
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bartrat       7/10/2009 9:26:06 PM
Kirby1 thanks for your speculation, I realized that mid life upgrades to other things are needed. But the article hinted at no new fuel needed for reactor.
 
As to U-238 half life is well over  a BILLION years, and U-235 the fissionable isotope it has a half life of over 703 million years so decay is insignificant in 50 year period.
I am still curious as the actual details.
 
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WarNerd       7/11/2009 6:11:07 AM

It is possible the reactors contain extra fuel so midlife refueling not needed,

Not likely.  It's not like Gasoline, if you don't "burn" it you still have it.  Naval Reactors use Highly Enriched Uranium (HEU).  HEU is 90-plus% U-238.  Giving the maximum power output for the minimum volume.  Whether or not the Uranium is fissioned or not, it still decays.  So even if the reactor is relatively cold, after a number of years the HEU is below the critical level for power generation.  How many years that is, I haven't a clue...but like I say it's not like you can say, "Put an extra 1,000 pounds of HEU in there.  That way we won't have to refuel."  It STILL decays as a fuel/power source.

While Navy reactors use more highly enriched fuel than civilian reactors (which use low enriched uranium), it is only in the range of 40% to 60% enriched.  There is no reason to go higher because of the need to provide sufficient surface area for heat transfer to keep the fuel rods from melting.
 
There is probably some extra fuel in the new reactors, but the main difference will be in how fuel pins of different composition are arranged, and may include additional elements to shield or slow the reaction in some parts of the core to better balance the utilization of the fuel.

Lastly, U-235 has a half-life of 703 million years.  It will eventually decay, but not by much in the span of the human race.
 
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benellim4       7/16/2009 7:09:37 PM
The USN has developed reactors that will last the life of a sub. How we do it is not a matter and what the limiting factors were that were overcome are not matters for open discussion. However, considering we have 50 years of experience to draw upon.
 
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