Electric motor on a nuke boat

[Josh]

I know at least some D/E submarines use a secondary creep motor as well as their main motor...Type XXI, Kilo. Is there any reason a nuke boat couldn't opperate a 'creep' electric motor and carry enough batteries to employ for short periods? My thinking is this would allow the nuke to spin down its generators and steam turbines to eliminate that as a source of tonal noise. A very modest electric motor could maintain 3-5 knots; on the kilo O think its only in the neighborhood of 300 hp (obviously something like a Virginia would need more). The nuke boat wouldn't have to completely rely on the battery for power, so it could use its regular steam trubines for generation and propulsion periodically and only would need enough battery power for a few hours to be effctive. The nuke could alternate between regular propulsion and electric to throw off a TMA analysis by an opposing sub.

 

This of course assumes that turbine noise is a major source of noise and that canceling this out would be of benefit. We have no idea if active and passive noise cancelling technologies already can mitigate this as a major component of noise, but lets just assume not.

[Archie Pellagio]

I thought they did?

But you still need to run the reactor.

[Mistral]

I always was under the impression that most of the noise in a nuclear sub came from the cooling plant, not the reactor per se.

[Guest JamesG123]

Yes, everything within the hull is pretty well muffled and isolated, the most noise comes from where it interfaces with the external environment.

[Archie Pellagio]

Only at faster speeds, below ~8-10kts the plant and associated cooling is louder on most modern boats than the shaft/prop.

[Mk 1]

Only at faster speeds, below ~8-10kts the plant and associated cooling is louder on most modern boats than the shaft/prop.

As I understand it, one simply can't shut down and re-start the reactor and the cooling system whenever the mood strikes. In fact, I think that once the reactor is shut down, it can only be re-started with the support of base facilities. (Though that may not be the case in all designs, I was told it was how they worked back in the day when I had an acquaintance with dolphin wings.)

 

So having a silent battery-operated motor gains you nothing of substance -- your reactor and the cooling pumps are still running, making as much noise as they make.

 

-Mark 1

[Archie Pellagio]

As I understand it, one simply can't shut down and re-start the reactor and the cooling system whenever the mood strikes. In fact, I think that once the reactor is shut down, it can only be re-started with the support of base facilities. (Though that may not be the case in all designs, I was told it was how they worked back in the day when I had an acquaintance with dolphin wings.)

 

So having a silent battery-operated motor gains you nothing of substance -- your reactor and the cooling pumps are still running, making as much noise as they make.

 

-Mark 1

 

Shutting off the reactor cooling process is not healthy for children or animals.

I believe the battery power option is more of a SHTF thing.

 

I think you misunderstood my point about speed - above the ~10kts mark (varies) the noise of the prop and associated machinery become the largest noise generator, below that, it is the actual plant machinery.

[JOE BRENNAN]

In fact, I think that once the reactor is shut down, it can only be re-started with the support of base facilities. (Though that may not be the case in all designs, I was told it was how they worked back in the day when I had an acquaintance with dolphin wings.)

Some types of nuclear reactor can't be restarted for a certain period once they are shut down because Xenon 135 then builds up from decay of heavier elements and the Xenon absorbs too many neutrons for the reaction to restart even with the control rods fully withdrawn. Eventually the Xenon further decays and the reactor core is once again reactive enough to begin a chain reaction with the rods withdrawn. However AFAIK USN reactors from the start have been designed to give a very high margin against 'Xenon dead time' in part by using highly enriched (minimum. 93%) Uranium fuel, and Xenon dead time mainly applies to land based plants using low enriched fuel, and perhaps those of other navies which use lower enriched fuel than the USN. Anyway the reactor still produces significant heat for many days after 'shut down', chain reaction halted (as in the obvious recent case of the Fukushima Dai Ichi reactors), which the cooling system must still carry off, but a lot less than when operating at full power, and less at low power than full power.

 

On the original point the French Barracuda class SSN's are to be equipped with a hybrid turbo-electric/geared turbine drive specifically to optimize noise at lower speeds using electric motor drive (as opposed to strictly emergency electric motor drives on other nuclear submarines, along with batteries, diesel generators and snorkels). The noise generated by the reactor's cooling system varies depending on the power setting. In many plants the primary coolant loop circulating pumps don't have to be run at all at low power (in some not at any power setting, ie fully natural circulation). And I would assume that the secondary circuit feed pumps (which reinject condensate into the secondary steam loop, that can't be done naturally) and main circulating pumps (which circulate seawater through the condenser) are arranged for variable speed operation depending on power output. Also, the main turbines, reduction gears and associated systems (lube oil, etc) make noise which is reduced or eliminated if instead only a smaller turbo alternator is run to power the electric motor. So, the French concept seems sound, though is presumably not the only satisfactory way to optimize low speed noise since it hasn't been adopted previously, despite much emphasis on low speed quiet especially in SSBN's (in USN SSN's, it seems the main design goal in recent decades is relative quiet at fairly high speeds, 20kts or more, not few kt creeping speeds like an SSBN on patrol; I don't know the specific speeds contemplated for the Barracuda's in turbo-electric v. geared turbine mode).

 

Joe

[JWB]

(in some not at any power setting, ie fully natural circulation)

 

Do you have a source for this? I know that USN reactors utilise "percolation" for low power but have never seen that at max output.

[JOE BRENNAN]

Do you have a source for this? I know that USN reactors utilise "percolation" for low power but have never seen that at max output.

The first NCR (natural circulation reactor) in USN was S5G fitted to the protoype 'quiet' sub Narwhal in the 1960's, but the S8G type on the Ohio class is also natural circulation. There are primary loop circulation pumps but they aren't needed at normal operating speeds. A specific citation is Friedman "US Submarines Since 1945" p. 127 but this is widely reported. Such a reactor package, with steam generator located above it, must be taller in order to get enough convective 'pull' to cool sufficiently at any sizeable fraction of full power, and thus means less power within a given hull diameter (eg. S5G in Narwhal v conventional S5W in similar size Sturgeon class) or taller hull for similar power (eg. S8G in Ohio's v similar power S6G in smaller Los Angeles class). So it's a specialized design characteristic with a definite cost.

 

Joe

[hojutsuka]

The first NCR (natural circulation reactor) in USN was S5G fitted to the protoype 'quiet' sub Narwhal in the 1960's, but the S8G type on the Ohio class is also natural circulation. There are primary loop circulation pumps but they aren't needed at normal operating speeds. A specific citation is Friedman "US Submarines Since 1945" p. 127 but this is widely reported. Such a reactor package, with steam generator located above it, must be taller in order to get enough convective 'pull' to cool sufficiently at any sizeable fraction of full power, and thus means less power within a given hull diameter (eg. S5G in Narwhal v conventional S5W in similar size Sturgeon class) or taller hull for similar power (eg. S8G in Ohio's v similar power S6G in smaller Los Angeles class). So it's a specialized design characteristic with a definite cost.

 

Joe

[picky mode on]

 

The Narwhal had a bigger hull (beam 37' 7") than the Sturgeon (beam 31' 8¹/8"), and the S5G probably produced slightly more power than the S5W (Friedman says of S5G that it had about half the power of Los Angeles, so about 17,500hp, and the S5W is generally thought to be about 15,000hp).

 

[picky mode off]

 

Hojutsuka

[JWB]

Such a reactor package, with steam generator located above it, must be taller in order to get enough convective 'pull' to cool sufficiently at any sizeable fraction of full power

 

The higher up the condenser above the reactor core, the greater the percentage of max power can be achieved without the use of pumps. IIRC the condenser in the Ohio class is in the sail?

[JWB]

I was incorrect about the location of the condensor. It is in the engine room near the stern.

[Josh]

I was not envisioning a complete shut down, I was envisioning reducing power to a low level and ideally using free circulation. I've no idea if the current reactor generation allows this, but it was done for Ohio as noted.

 

Also as noted power generation and propulsion steam turbine noise is, in open sources, generally considered the loudest noise source at slow speed and more over as a narrow frequency source of noise is easier to detect that the broad band flow noise, at least at slow speeds. The steam plant noise of the reactor cooking should also be more broad band, as opposed to say the tight 60 hz pulse that AC power is always going to be (50 for you furriners )

 

So just running the nuke plant in neutral and keeping it cool with out using the turbines it seems to me would make for a more quiet profile, even though rafting, noise cancellation, 'anechoic' coatings, and other things we don't know about probably go a long way to muffling this source of noise.

[JOE BRENNAN]

The higher up the condenser above the reactor core, the greater the percentage of max power can be achieved without the use of pumps. IIRC the condenser in the Ohio class is in the sail?

The higher up the steam generator is. It basically doesn't matter where the condenser is.

 

The primary coolant water gets heated in the core, then travels to the steam generator where it transfers heat, across the metal tubes of the steam generator, to the secondary coolant water which is turned into steam (in a PWR as we're speaking of, the primary cooling water is kept at temp/pressure combination where it remains liquid, while the secondary loop is at a lower pressure, though still several 10's of atmospheres, where the water flashes to steam at a temp near that of the primary coolant). If the steam generator is far enough above the core, the convective force will be sufficient to make the hot primary coolant rise from the core and sink back once it gives off heat to the secondary coolant, without using pumps. This is nothing strange, but also how water circulates in a fossil fueled boiler from the 'mud drum' at the bottom through the tubes around the furnace to the steam drum and back down though the 'down comers'; forced circulation with pumps in fossil fueled boilers is not unheard of, but not common.

 

The secondary loop steam expands through the turbines to near absolute zero pressure (almost total vacuum) in the condenser where it then condenses in contact (again not direct physical but through metal tubes) with seawater. Then the condensate must be brought back up above the steam generator's pressure to reinject it into the steam generator. There's no way to do that without pumps, and the condenser's position is trivial wrt that large pressure change. The universal arrangement in steam turbine surface ships is for the condenser to be directly attached to the casing of the low pressure turbine underneath it via a thermal expansion joint, and from schematics I've seen of nuclear subs it's the same; there's no reason I know of it would be different.

 

Joe