Nuclear submarines? Nuclear aircraft carriers? Are they really nuclear powered?

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Re: Nuclear submarines? Nuclear aircraft carriers?

Postby MartinL » 24 Feb 2012 14:57

FirstClassSkeptic wrote:It would seem to me that a nuclear sub would leave a hellava thermal signature in its wake. I looked it up here on the internet, and the technical guys have an explanation for why it doesn't. Or why it can't be detected. So, someone else must have raised the question. Of course, the someone else wouldn't consider that the nuclear sub wasn't nuclear.

Let's say that a certain submarine has a 100 MW reactor. Plotting it on a specific entropy vs temperature diagram reveals that at least 60% of the heat is rejected to the sea. Based on my experience, one can expect a ten degree increase from the inlet to the condenser to the outlet. This is a very big increase in temperature in the ocean. However the sub is moving through the water and the wake mixes the heated seawater reducing its signature. There are also natural thermal gradients in the ocean which submarines will find and hide under. These thermal gradient confuse sonar making it harder to find the target.

Then there is the problem of depth. The heated water from the sub is under tens or hundreds of feet of water. The thermal signature will not be seen from above the water unless it heats the water at the surface as far as I know. Detecting a sub by it's heat signature works best when it is on the surface for some reason. Detecting magnetic anomalies and using sonar to hear the sub works much better.
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Re: Nuclear submarines? Nuclear aircraft carriers?

Postby MartinL » 24 Feb 2012 15:17

BNSF9647 wrote:Yeah exactly. Why that roundabout way just to generate steam? not to mention so called reactors poor thermal efficiency. Which is documented per say to be only around 27%. So how do they come up with this fantasy number of 25-30 years between refuels? fuel consumption is directly related to thermal efficiency. Hell EMD's 710G (710 cubic inches per cylinder) medium speed Diesel Engine achieves around a 40% thermal efficiency rating, 13% better than these false reactors. That's why I believe these so called reactors are just giant wet cell batteries or capacitors.


A submarine reactor has poor thermal efficiency due to the lower peak core operating temperature compared to fossil fuel plants. Even though the efficiency is poor, the fuel load is large. So just as a large truck gets less mileage than an economy car, if it has a much larger fuel tank, the range is greater.

Putting a large amount of uranium pellets into a fuel plate is a challenge. The reactivity of the core increases with the fuel load, so other neutron absorbing materials called poisons (neutron absorbers) are loaded into the fuel also. As fuel absorbs neutrons and fissions, the poisons also absorb some neutrons (called burning) keeping the reactivity within a level that is controllable by the reactor rods.

All nuclear powered ships have the capability of receiving power from a shore facility for use while shutdown. But this power is only enough for hotel loads and starting the reactor to go to sea. It is not enough for normal underway ship operations. The energy stored in diesel generators (fuel tanks) and the battery on subs is a tiny fraction of the power available from the reactor.

It is not the efficiency that makes nuclear powered ships viable, it is the ability to run at high power without replenishing the fuel for the propulsion plant (carriers) and the ability to do without outside air or fuel supplies (submarines). Diesel and fossil fuel powered boilers need periodic fill-ups while the nuclear reactor runs for thousands of full power hours or years. Air independent propulsion for subs and battery power subs can only run at low speeds underwater for a few days or a few weeks at a time. A nuc sub can run for weeks or months at a time at high power underwater completely independent of outside services or support.

If there was a giant battery or capacitor running the sub, I would have seen it. But the only battery I have seen on a nuke sub is far too small to run it more than a few hours. The core of a sub reactor is far smaller than the battery, but the energy it obtains from fission far exceeds the comparative puny amount of chemical energy stored in a battery.

This stuff is easy to see. Join the Navy or get a job at the shipyard. Any American in decent health, the right age and without a criminal record can get in. The Navy has a high turnover rate; they need lots of people all the time.
Last edited by MartinL on 25 Feb 2012 02:28, edited 1 time in total.
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Re: Nuclear submarines? Nuclear aircraft carriers?

Postby MartinL » 24 Feb 2012 15:21

FirstClassSkeptic wrote:As for three decades between refuels, these submarine guys here are saying the sub only stays under about three weeks.


Who is saying this? I saw no such claim on this forum. Can you quote the post where the claim was made? Thanks.
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Re: Nuclear submarines? Nuclear aircraft carriers?

Postby Exorcist » 24 Feb 2012 18:57

MartinL

Is there information available concerning the depth at which Trident Missiles are launched? I'm referring to the depth measured from the top of the missile containment tube to the ocean surface. From videos I've seen, supposedly shot with underwater cameras, this distance would appear to be about 60 feet.
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Re: Nuclear submarines? Nuclear aircraft carriers?

Postby FirstClassSkeptic » 24 Feb 2012 19:07

MartinL wrote: Using a jet or rocket powered with gasoline will not be very efficient for an automobile either. Using heat to generate a mechanical means of traction or thrust is the most efficient at low speeds. It is only when you need to move more than 300 miles per hour is jet propulsion preferred..



Wouldn't you like your sub to move 300 mph?

Let's see; if I put a propeller on an automobile, it wouldn't work very well, so it wouldn't work very well on a ship either. That's your logic.

https://www.google.com/patents/US5545063 ... on&f=false
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Re: Nuclear submarines? Nuclear aircraft carriers?

Postby FirstClassSkeptic » 24 Feb 2012 19:13

MartinL wrote:
FirstClassSkeptic wrote:As for three decades between refuels, these submarine guys here are saying the sub only stays under about three weeks.


Who is saying this? I saw no such claim on this forum. Can you quote the post where the claim was made? Thanks.


I think I was quoting a US Navy website. But now, I can't find my post.
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Re: Nuclear submarines? Nuclear aircraft carriers?

Postby FirstClassSkeptic » 25 Feb 2012 01:49

ewing2001 wrote:https://en.wikipedia.org/wiki/Neutron_poison#Accumulating_fission_product_poisons ; -]

Image

https://www.panoramio.com/photo/67393129


How did that get there?
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Re: Nuclear submarines? Nuclear aircraft carriers?

Postby MartinL » 25 Feb 2012 01:58

FirstClassSkeptic wrote:Wouldn't you like your sub to move 300 mph?

Let's see; if I put a propeller on an automobile, it wouldn't work very well, so it wouldn't work very well on a ship either. That's your logic.


I did not suggest putting a propeller on a car. To move a sub at 300 mph underwater with a screw would take larger engines than can be fit into the hull. If we use the rule of thumb that to double the speed we need 8 times the power, then to increase speed from say 30 mph using a 20000 hp engine to 300 mph would require engines 1000 times more powerful.
Last edited by MartinL on 25 Feb 2012 02:26, edited 1 time in total.
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Re: Nuclear submarines? Nuclear aircraft carriers?

Postby ewing2001 » 25 Feb 2012 02:08

i did the bit silly'ish edit stunt at wikipedia. of course it might be removed in a few hours by their bot array ; i expected an explanation on the topic over there by our "Radiological Control Officer" (Code 105.3) *here, but he seems to ignore it ;

Image

btw, "Supervisories" in these kind of "jobs" do make $50,000.00 - $75,000.00 a year ;
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Re: Nuclear submarines? Nuclear aircraft carriers?

Postby MartinL » 25 Feb 2012 02:19

One of the reasons a reactor can run so long between refueling is that in addition to fissionable fuel, burnable poisons are loaded into the fuel matrix. These poisons "burn" or absorb neutrons thus reducing the reactivity of the core to a level that allows the control rods to keep it shutdown when inserted or operate at power when partially or nearly completely withdrawn.

Here is an example using data I made up that loosely replicates what the Navy uses.

Start with 100 kg of U-235. It is enriched to maximize the amount of 235 and minimize the amount of 238. It is made into uranium oxide pellets to provide space for fission product gases. The pellets are inserted into a metal plate then clad with another thin layer of metal. The plates are welded into fuel assemblies that have flow channels for coolant water.

Assumptions; 10% of the fuel is used by end of core life, each fission results in a mass defect that produces 200 Mev of energy, and the propulsion plant delivers about 10% of the power to the screw for pushing the boat.

100 kg of U-235 is 425 moles = 5.2x10^26 Mev or 8.3x10^13 joules = 3.1x10^7 horsepower-hours. 10% efficiency means 3.1x10^6 HP-Hrs. Divide by 10,000 HP (engine size) to get 3100 effective full power hours. If the sub is running an average of 25% power (they like to run quiet most of the time between high speed runs) this extends the time to 12400 hours or 516 days. If the operating tempo of the sub is 50%, then the fuel load will last nearly 3 years.

This is just a simple calculation, but it illustrates how a nuclear reactor can greatly extend the range of the ship.
Last edited by MartinL on 25 Feb 2012 17:36, edited 1 time in total.
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Re: Nuclear submarines? Nuclear aircraft carriers?

Postby MartinL » 25 Feb 2012 02:25

Exorcist wrote:Is there information available concerning the depth at which Trident Missiles are launched? I'm referring to the depth measured from the top of the missile containment tube to the ocean surface. From videos I've seen, supposedly shot with underwater cameras, this distance would appear to be about 60 feet.


60 feet sounds reasonable, but I am not sure. I have never had access to that information other than the unclassified info available to everyone else. I was never stationed on an SSBN.
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