Submarine Propulsion Questions Answered

[saltine]

Hi, I was looking at the following diagram and have some questions:

Is the purpose of the pumps to transport liquid of one temperature to another section of the loop?

Is it more efficient to use steam to turn turbine or to use water to turn a turbine? (Is hydroelectric more efficient, but given that the energy output of nuclear reaction is in the form of heat, which can make steam, therefore steam turbine is used?)

What is the function of the Pressurizer? Is it for measuring the pressure to control the control rods and the coolant pump?

- Thanks

 

[Dr.D]

The main coolant pump moves coolant in through the reactor. This fluid is radioactive.

The feedwater pump (labeled just pump) moves water from the condenser back to the steam generator. This water is not radioactive.

There is a third pump (dark blue sea water line) that moves coolant through the turbine condenser.

There would be no good way to use water to turn the turbine. What is available as energy is heat from the nuclear reactor. This can be used to boil water and make steam. The rest of the process is a matter of recovering that energy in mechanical form to turn the propeller.

The pressurizer I really cannot address.

 

[mgb_phys]

It looks like the pressurizer runs a smaller turbine/generator set to charge the battery without the main turbine.
Since a nuclear sub has no reason to run on batteries this is presumably mainly to run all the electrical systems

 

[Dr.D]

Don't think so, mgb_phys. That green line is an electrical connection between the Turbogenerator, the M/G set, the battery, the propulsion motor, and the several pumps, and the pressurizer (whatever that is).

Are you suggesting that there is a small turbogen set inside the pressurizer? That would be highly unlikely simply from a maintenance perspective. That is inside the radioactive boundary and it would be very difficult to maintain and entirely unnecessary to put such their. That would run counter to the entire design philosophy.

 

[mgb_phys]

No, I thought the green line was pressure to a turbine in the motor generator set.
I suppose you need to pre-pressurise the water before you can run the reactor. You need the water to be about 300C in the reactor - so if you want it to stay liquid

 

[saltine]

How does the coolant pipe prevent itself to become radioactive?

A question about steam turbines:

Do steam turbines work by using the pressurized steam to blow onto its fan blades? The layers of blades of the turbine allows the steam to go around multiple times before it loses the usable kinetic energy. The steam could condense into water in the turbine, therefore for both turbines, the steam enters from the top, and the exhaust drains at the bottom. Sea water condenses the exhaust and the pump circulates it back to the steam generator. If the steam at the exhaust is not removed, the pressure would build up and the turbine would stop.A question about the reaction itself:

Enriched Uranium 235 naturally decay and emits a neutron when it does so. In normal environment, it that neutron would just fly away without colliding into and breaking another Uranium atom. In side a reactor, the neutron that could naturally fly out is contained. It will therefore eventually hit the control rod or hit another Uranium atom. The control rods are lowered to a position where the reaction could continue at a slightly super-critical level so that the coolant would hit up at a controlled rate. The control rods are designed to be inserted from above, so that during a power failure, the control rods would fall and shut down the reaction.Is this understanding correct?

 

[mgb_phys]

How does the coolant pipe prevent itself to become radioactive?

It doesn't - the primary coolant circuit becomes radioactive, just like the reacto and is disposed of at the end of the life.

Do steam turbines work by using the pressurized steam to blow onto its fan blades? The layers of blades of the turbine allows the steam to go around multiple times before it loses the usable kinetic energy.

Yes, it uses the pressure of flowing steam (no different really from just blowing through it).

The steam could condense into water in the turbine, therefore for both turbines, the steam enters from the top, and the exhaust drains at the bottom.

Generaly that's bad, you use an external condensor to convert the steam back into water - this also creates a partial vacuum helping to pull the steam through.

Sea water condenses the exhaust and the pump circulates it back to the steam generator. If the steam at the exhaust is not removed, the pressure would build up and the turbine would stop.

Correct - it is the expansion of water into steam that creates the pressure an powers the turbine.

Enriched Uranium 235 naturally decay and emits a neutron when it does so. In normal environment, it that neutron would just fly away without colliding into and breaking another Uranium atom. In side a reactor, the neutron that could naturally fly out is contained. It will therefore eventually hit the control rod or hit another Uranium atom. The control rods are lowered to a position where the reaction could continue at a slightly super-critical level so that the coolant would hit up at a controlled rate. The control rods are designed to be inserted from above, so that during a power failure, the control rods would fall and shut down the reaction.

There is also a moderator to slow the neutrons, most neutrons from U fission are too fast to efficently split another atom. Paradoxicaly low energy neutrons are more effective, since they spend more time passing near another nucleus - fast/high energy neutrons pass straight through another atom too fast to fission it

 

[russ_watters]

The pressurizer would simply be there to control the residual pressure in the system, since it is a closed system with no phase changes to absorb pressure. It is like the expansion tank on your water heater.

To amplify Dr. D's post #2, you must use steam for the turbine because if you don't you aren't utilizing any real thermodynamic work. All you would have is a pump driving a turbine - the added heat would be superfluous.

Remember, the reactor water loop is not doing anything other than transferring heat from the reactor to the power producing steam loop.

 

[saltine]

Would it also make the steam water radioactive? Or is it true that it is radioactive but at an acceptable level?

How does the reactor keep the neutron inside itself? What material makes up the inner wall of the reactor vessel? Do neutrons in fact bounce back at the inner wall to collide with another uranium. During the reaction, is the fuel still a solid ball?

What is the moderator? Is it a liquid that surronds the the fuel?

 

[Dr.D]

The water in the primary loop, the loop that passes through the reactor, becomes radioactive. It is at a dangerous level, and this is a hazardous area of the boat.

Most steam turbines have two types of blading. The very high pressure blades are impulse blades that work by a momentum exchange with the steam flow and that is usually no more than the first one or two stages. This is followed by a number of stage of blading that employs aerodynamic lift on the blades to develop torque.Typical steam turbine design actually expands the steam down to a partial vacuum state in order to extract maximum work from it.

Back to mgb_phys #5, the M/G set is simply a motor and a generator on a common shaft. This is standard navy terminology. It is often done to convert DC to AC or vice versa.

 

[gmax137]

The pressurizer is what controls the primary system pressure. It is just a tank, about half full of liquid and half steam. This allows the fluid in the primary system to change temperature - as it heats up or cools down it expands or contracts and the pressurizer absorbs this volume change. The liquid in the pressurizer is at saturation conditions so as its level changes and compresses the steam space, some steam will condense and the pressure remains constant (and likewise if the fluid cools and contracts, some of the liquid in the pressurizer will flash to steam). The green line going into the pressurizer in the drawing is an electrical cable, the 'box' inside represents electric immersion heaters that keep the liquid at saturation. The other line going in at the top is a spray line, if the pressure rises too much the sprays will come on to condense steam and lower the pressure. Notice that the spray line is taking water from the 'cold' side of the core. It is probably about 100 F lower than the pressurizer temperature.

Now, one more thing - when I called the pressurizer a 'tank' - don't get the wrong idea, it is a closed vessel made of very thick steel to withstand the pressure. I am not sure about the naval systems, but the systems in the commercial power plants run at 2250 psia, and the pressurizer is at 653 F.

 

[saltine]

If the M/G set is used to convert DC and AC, does it mean that, on the diagram, half of the green cables are running AC and the other half is running DC? I suppose the half with the battery is DC (?) Why does the M/G Set convert DC to AC to power two pumps, but not the pump that circulates sea water??

 

[gmax137]

Another point, not often appreciated. While it is true that the primary system liquid is 'radioactive' it isn't normally very radioactive. There are a few very short half-life isotopes circulating, and corrosion products can become activated as they pass through the core, but unless the fuel cladding has defects there isn't much activity in the fluid. There's a famous episode where someone (lewis strauss? or Glen Seaborg?) offered to drink some to show that it isn't as bad as some think. The really nasty stuff is the fission products, and they normally are trapped within the fuel itself.

 

[gmax137]

If the M/G set is used to convert DC and AC, does it mean that, on the diagram, half of the green cables are running AC and the other half is running DC? I suppose the half with the battery is DC (?) Why does the M/G Set convert DC to AC to power two pumps, but not the pump that circulates sea water??

You're right - it isn't very clear from this sketch which components run on AC and which are DC. In commercial designs, only small motors (like motors to open & close valves) are DC; everything else is AC. But I guess in a submarine you have different constraints - you have to operate for a time if the reactor trips and the only power available is that from the batteries.

 

[Dr.D]

I think you are asking too much of this simple diagram. The green line is electrical -- all of the electrical, AC, DC, high power, low power, -- all of it. This is not a wiring diagram fo the boat.

gmax137, what you said about the pressuizer confirms what I was thinking but was hesitant to say about it. This is essentially the steam drum of a conventional steam plant.

 

[saltine]

I think that is ture. But the diagram does show some weird details like the clutch and the thrust block. But perhaps there is a reason for the sea water pump to run on DC?

Does anyone know a more detailed diagram?

 

[Dr.D]

Speed control.

Interesting that you should remark that the thrust block is a "weird detail." I found when I worked for the Navy that there were a remarkable number of engineers that did not understand that this is where the driving force is applied to the ship, and they found this concept very difficult to grasp.

Similarly the clutch. You don't necessarily want to have to stop the turbine rotation (heat warping problems, etc) just because you want zero screw rotations for a while.

 

[saltine]

I am reading something and it says that the seawater pipe is large, and if the pump fails, the seawater could get inside the submarine and make it sink. So perhaps if the motor general fails, the seawater pump can at least run on DC and the submarine won't sink.

 

[Dr.D]

You must be reading Tom Clancy.

 

[saltine]

That was actually The idiot's guide on Submarine" on google:

http://books.google.com/books?id=aI...NyzCg&sa=X&oi=book_result&ct=result&resnum=6"

I thought those guides only talk about computers.

 

[Dr.D]

Simply losing a pump motor would not cause a loss of piping integrity. The problem would be if the piping burst, but that should not happen as long as the vessel is above crush depth.

Submarines, especially nuclear subs, are extremely complex systems with safeties upon safeties built into them. They worry at great length about every possible sort of accident that could happen, much more than you will find written up in a book you can access through Google.

 

[saltine]

Everything is so important

 

[russ_watters]

gmax137, what you said about the pressuizer confirms what I was thinking but was hesitant to say about it. This is essentially the steam drum of a conventional steam plant.

That isn't what gmax said and it isn't what the pressurizer does. A pressurizer regulates pressure (like an expansion tank, like i said), a steam drum does not. A pressurizer is not part of the primary loop cycle, but is an external attachment - a steam drum is part of the path of the steam in a boiler.

 

[Dr.D]

I stand corrected.

 

[mgb_phys]

Do you have to manually pressurize the primary circuit in a PWR or do you rely on the water boiling when it starts up to self pressurize?

 

[russ_watters]

That, I don't know for sure, but I can't imagine it would be possible to keep the pressure stable without an active automatic control system. That is unlike an expansion tank on a hot water system, which just keeps the pressure below a critical limit, but doesn't specifically control it to a certain pressure.

 

[FredGarvin]

My guess is that expansion tanks don't require electricity to run. To me it looks more along the lines of a regulator that maintains the delta P across the reactor to maintain the same flow rate. I have never seen anything like this though. It's actually more along the lines of a I/P regulator that will adjust as the fuel rods are moved in and out.

 

[saltine]

Hi, I have a question:

Given the diagram in the first post, is there any detail you would add to it, although it might have less to do with nuclear powered propulsion?

How do people move big equipment inside a submarine? Say a piece of pipe is broken, how would people take it out and bring in a new section?

 

[Dr.D]

Saltine, the short answer to your question is, "with great difficulty." There is very little provision for moving large equipment around in a sub, and every situation requires its own evaluation/solution. Smaller things like rack mounted electronics, etc. can be carried out without too much trouble, but big stuff is a problem.

The only openings through the hull are the torpedo tubes, the missile tubes, and the hatches, (and sea water piping), so the places where you can pass things in and out are pretty limited.

If you ever have a chance to visit the Museum of Science and Industry in Chicago, they have the U-505 from WW II on display there and you can walk through it. It is amazingly cramped! Todays nukes are "much bigger" but still very close quarters.

 

[mgb_phys]

Most modern nuclear submarines are designed not to refuel. The reactor contains enough fuel for the expected life of the vessel. Then it is decomissioned and dismantled along with the rest of the boat.