A question about nuclear powered ships

[eocoolj]

OK, so this may be a stupid question, but I figured you guys should know the answer to it. Do nuclear powered ships consume their nuclear "fuel" at a rate that is dependent on the speed of the ship, or is the nuclear reaction always going on at the same rate? In other words, is there any reason for a nuclear powered ship or sub to cruise more slowly to "conserve" fuel, or should they just go full tilt as much as possible, because the fuel is being used either way?

Thanks

[SnowLeopard]

OK, so this may be a stupid question, but I figured you guys should know the answer to it. Do nuclear powered ships consume their nuclear "fuel" at a rate that is dependent on the speed of the ship, or is the nuclear reaction always going on at the same rate? In other words, is there any reason for a nuclear powered ship or sub to cruise more slowly to "conserve" fuel, or should they just go full tilt as much as possible, because the fuel is being used either way?

Thanks

What's the difference between a bomb and reactor? In the layman world, it's the speed of the reaction.

As oppose to trying to answer those questions, it may be just better to explain reactors a little bit.

For the reactor to producing heat, enough neutrons need to be "flying" to be splitting atoms. Since the fuel is radioactive, there will always be something flying out of the atoms......but it may not be enough to cause sufficient heat generation for the coolant to be useful (ie, to carry heat to the heat exchanger).

In a reactor, there are at least two kinds of rods, fuel rods and control rods. The fuel rods have the radioactive elements. The control rods control how much the neutrons are flying. Pull the rods out of the reactor and they fly alot. Drop them back in and the reactor's activity drops.......sort of.

There are a lot of side items to this. Ie, if the reactor was really going, running wild, dropping in the control rods would slow down the reactions.....but there would still be a lot of residue heat to get rid of. But for now, let's not worry about that. Further, there are many different kinds of reactors, but here, let's just take it as it splits atoms, causes heat.

Okay, when it comes to marine propulsion and reactors, there are generally three things at least to look at. Method of coolant transfer, method of turbines, and method of shafting.

First of all, coolant can vary, but again, let's skip what the coolant is and look at how the coolant moves. One can either pump it or they can allow it to move on natural circulation. Ie, if it is hot, it expands, when it cools, it contracts, and this causes movement thru the system. In relation to your question, for natural circulation, how much activity that reactor is putting out could be a way to control how much energy it is passing on to the rest of the ship.

Now, this next paragraph is more of me looking at the question at hand. Whether pumped or natural, let's think about that coolant running from the reactor to the heat exchanger. Reactor is very hot all the time, coolant picks up the heat, takes it to the heat exchanger.....but how much heat does it transfer at that point? How hot is the coolant going back to the reactor? It's not going to be hotter than when it left...hopefully.....but how much cooler is it going to be? How much heat can it pick up from the reactor on the passes afterwards and will that be sufficient to keep the reactor and anything else that is in that question from getting too hot in the long run?

Secondly, there is the point of what kind of turbine is being used. The coolant from the reactor goes into a heat exchanger which produces heat to be used in the turbines. The ship may main engine turbines where reactor is primarily used to produce shaft horsepower to move the ship. Or a ship may have turbo-electro drive where the reactor powers a SSTG where steam is turned into eletricity and that drives an electric motor which turns the shaft.

Now, in addition to turning the shaft, there are a lot of other features that the reactor has to provide power to. Something just to keep in mind here.

Now we come to shafting, the props. We might have main reduction gears, a transmission system of a sort, in question since the shaft from the turbine moves much too fast to be of use. We might not have that and instead have some kind of shafts within shafts. We could be running off an electric motor as previously mentioned. Further, the props themselves could be a solid piece of metal or they could be controllable pitch where the blades can be turned.

Shifting for a moment to gas turbines, those do tend to run at very high speed all the time since gas turbines can be inefficient at slow speeds. Further, gas turbines tend not to have a reverse turbine. Ie, to go in reverse with a steam ship, one feeds the steam in thru a different port and turns the turbine in reverse. Can't do that with a gas turbine, so they have controllable pitch. Not only does this allow the pitch to be turned so the prop, instead of pulling forward thru the water is now pulling backward thru the water, but also, it allows the prop to decide how much power is delvered to the water even if the shaft is always turning at a constant speed.

So many systems, so many ways to do it, what's best? Essentially, that often depends on the military advantage one is trying to achieve. And in that area, there are a whole lot of other questions to consider, some of them which can also apply in the civilian world. Cavitation, noise, at what depth can the system be used and at what speed....etc.

Now, one of your questions: why not run the ship at full tilt all the time? Keep in mind that there are a lot of other things besides the reactor that makes the ship move and one might not want to put them at full tilt stress.

(all of this is written from memory from what I've learned of marine propulsion, nuclear engineering, ship construction, and nautical matters, starting over 35+ years ago and without referring to any of my texts)
_________________________________________
("Put weapons grade plutonium into a naval reactor? Instant catrosphic melt down."--Dr. Christimas Jones, (w,stte), "The World is Not Enough")

[eocoolj]

sorry for the delay, but I just wanted to say thank you for your very detailed response. It was excellent reading.

[maximusslade]

The short answer is yes, nuclear fuel is consumed at varying rates. This rate is not based directly upon speed but rather the amount of steam needing to be produced to push the ship through the water and provide electicity for the ship.

Not to discount any of what Snow said, as elaborate and detailed as it is, it is still a crude (and no offense Snow inaccurate) explaination of how nuclear power works. Having operated a plant, I sometimes do over simplify things sometimes i.e., hot rock make steam make thing go roundy roundy.

As for running at head flank all the time. It is possible to do. The Nautilus did it all the time when she was first commissioned, hence why she had to refuel after only a couple years. Granted reactor technology has advanced since then to make reactors last longer, but operationally commanders dont run balls to the wall because a reactor is no different than a tank of gasoline, the harder you push it the more fuel you use, and topping of the tank of a submarine or carrier is a tad expensive.

I would reeeeeaaaaaaaalllllllllllllly love to explain more how nuclear power works, but unfortunatly I can't due to much of its classification. Needless to say, when you understand how nuclear power works it can be very boring and SO FRIGGIN AWESOME at the same time.

You have any more questions, just let me know and I'll try to explain best I can.

[SnowLeopard]

.........Not to discount any of what Snow said, as elaborate and detailed as it is, it is still a crude (and no offense Snow inaccurate) explaination of how nuclear power works. Having operated a plant, I sometimes do over simplify things sometimes i.e., hot rock make steam make thing go roundy roundy........

I was beginning to wonder when we would hear from you. Having been there, done that, it would be easy for you. Me, well, it's like exactly what it's like, something of a spy's knowledge with knowledge of 30 years answering a question in 30 minutes to include not looking at references to see what was read 20-30 years ago was updated. I mean, how much can you learn when you've only been on one nuke boat for 3 days?

Of course, that is one side of the point I tried to keep in mind when I wrote the response, that his question was not just limited to under the sea. Or for that matter, the US fleet. And while it was pretty certain the asker was referring to reactor based system, part of my answer did draw, for a second, from the "poor man's nuclear sub" concept.

Plus, of course, I think we both know things that we would probably rather not talk about.......such as, for example, aspects of cavitation.

I recognize that you know more about it than I do, so please, go ahead.
____________________________________________
("If l were you, l wouldn't stick my finger - or anything else, for that matter - in there. At 200+ below zero, that liquid helium would break it off like an icicle."--007
"You really know far more about it than l do."--Francisco Scaramanga, (wtte), "The Man with the Golden Gun")

[Chogy]

I don't know jack about nuclear propulsion, but I was under the impression that over the lifetime of the fuel rods, their rate of expenditure is roughly proportional to the neutron flux, which is controlled (somewhat) via the control rods.

If a plant is at "idle" (minimum heat being created; 0 knots, essentially shut down) then the fuel will still deplete but at a vastly slower rate than that required to create steam pressures for flank speed.

I vaguely remember the Air Force had a concept for a nuclear-powered strategic bomber in the '50's, the NB-36. Of course, in theory, it could stay airborne indefinitely. But issues with shielding, weight, and the potential for leaks in the inevitable crash nixed the project. But still interesting to contemplate. A good read on the NB-36 project

Perhaps one of the board's nuclear engineers could comment.

[Dreadnought]

OK, so this may be a stupid question, but I figured you guys should know the answer to it. Do nuclear powered ships consume their nuclear "fuel" at a rate that is dependent on the speed of the ship, or is the nuclear reaction always going on at the same rate? In other words, is there any reason for a nuclear powered ship or sub to cruise more slowly to "conserve" fuel, or should they just go full tilt as much as possible, because the fuel is being used either way?

Thanks

In short, The reaction of the rods and how many rods are submerged you can consider as the gas pedal as it boils the radioactive steam to push the turbines. Rods are constantly being changed in and out dependant on the speed requirement and the life span of the rod. The older rods are kept submerged as they can still give off energy. Its all monitored by special electronics suite giving the carrier a twenty year span on her fueling rods before replacemnt needs to happen.

A quick reference

Aircraft Carriers, Inside and Out - Google Book Search

[maximusslade]

In short, The reaction of the rods and how many rods are submerged you can consider as the gas pedal as it boils the radioactive steam to push the turbines. Rods are constantly being changed in and out dependant on the speed requirement and the life span of the rod. The older rods are kept submerged as they can still give off energy. Its all monitored by special electronics suite giving the carrier a twenty year span on her fueling rods before replacemnt needs to happen.

WHOA!!!!!!!!!!!!!!!!!!!!!!!! In a pressurized water reactor (which naval plants are) reactor power is not based upon the height of control rods. Chogy is correct in that rods control the neutron flux inside a core but it does not directly control power. If a ship wants to go faster it DOES NOT move the rods.

Nuclear reactors fall into one of two categories. Positive reactivity reactors and negitive reactivity reactors. The way this is determined is by which type of moderator is used. Moderators are the materials used to control a nuclear chain reaction. A positive reactivity reactor is a plant where temperature goes up therefore reactivity goes up therefore power goes up which increases temperture and so on. This is an inherantly unstable configuration which requires the used of special safety features to maintain reactor safety.

Chernobyl was a positive reactivity reactor that used graphite control rods (IIRC) as its moderator. What went wrong at Chernobyl was the safeties used to control the reaction were turned off during some testing. We all know how that ended up.

Now a negitive reactivity reactor is a reactor in which if temperature goes up power goes down to to decreased reacitivity. Again, pressurized water reactors, where water is used as the moderator, fall under this category. In these inherantly stable powerplants power is not determined by anything in the core at all but rather by the heat exhangers. It is basic college level thermodynamics.

Now everything I have described thus far is stuff I learned in college. Anything more and I run the risk of saying something I should not. But I must say I do enjoy the mental exercise. This is a whole lot of stuff I havent had to think about in years :)

[Dreadnought]

WHOA!!!!!!!!!!!!!!!!!!!!!!!! In a pressurized water reactor (which naval plants are) reactor power is not based upon the height of control rods. Chogy is correct in that rods control the neutron flux inside a core but it does not directly control power. If a ship wants to go faster it DOES NOT move the rods.

Nuclear reactors fall into one of two categories. Positive reactivity reactors and negitive reactivity reactors. The way this is determined is by which type of moderator is used. Moderators are the materials used to control a nuclear chain reaction. A positive reactivity reactor is a plant where temperature goes up therefore reactivity goes up therefore power goes up which increases temperture and so on. This is an inherantly unstable configuration which requires the used of special safety features to maintain reactor safety.

Chernobyl was a positive reactivity reactor that used graphite control rods (IIRC) as its moderator. What went wrong at Chernobyl was the safeties used to control the reaction were turned off during some testing. We all know how that ended up.

Now a negitive reactivity reactor is a reactor in which if temperature goes up power goes down to to decreased reacitivity. Again, pressurized water reactors, where water is used as the moderator, fall under this category. In these inherantly stable powerplants power is not determined by anything in the core at all but rather by the heat exhangers. It is basic college level thermodynamics.

Now everything I have described thus far is stuff I learned in college. Anything more and I run the risk of saying something I should not. But I must say I do enjoy the mental exercise. This is a whole lot of stuff I havent had to think about in years :)

*Sorry Max if the wording didnt come out rite for the short version thats why I provided the link below it for a more indepth answer to the question posted. My bust.:)