Nuclear submarine power plant

[ckatt]

Does anyone know if the nuclear powerplants on submarines and aircraft carriers output AC or DC power? I've been searching on google for a few days and can't find an answer. If anyone has a suggestion of where I might search that would be appreciated too.

 

[Foxbat]

I used to work in a nuclear power station and it’s AC.

Because the principle is the same in a sub reactor (steam used to rotate a turbine which in turn rotates a generator) I’m pretty sure it will be AC there too. The rotational energy is passing through a series of coils (the stator) and it’s this movement that forms the alternating current waveform. AC is normally changed to DC using a wave rectifier.
Just think of a car alternator. It produces AC and has a built in rectifier that converts it to DC. Years ago, cars had a separate alternator and recifier but nowadays, the rectifier is built in to the alternator.

Of course, nuclear sub reactor technology is classified so we’ll never know for sure

 

[Robert Zwilling]

Here is an old article from 1996 that might give some clues Motor generators have been used as the main method of power conversion and power transfer between the ac system and the dc system on nuclear submarines

I think AC generators would be chosen over DC generators in nuclear subs. The power starts out as AC and is converted to DC as needed. There are probably needs for AC and DC and modified DC. Motor generators were used before solid state inverters became practical. I would suspect it is all solid state inverters now. Solid state inverters can be easily fixed by replacing modules instead of mechanical parts. Solid state inverters can also be controlled by computers to keep everything in line and that allows for a very wide variety of outputs.

 

[Wayne Mack]

Here are a couple of quick references. It sounds like the electrical systems are all DC as they are run off of battery power. Diesel or nuclear power plants are used to charge the batteries and do not directly run the electronics. One advantage is that the sub can continue to operate even if the power generation system fails.

Submarine Electrical Systems - Chapter 2

maritime.org

How Submarines Work

Not so long ago, the world's naval forces worked entirely above water. But all that changed with the addition of the submarine to the standard naval arsenal.

science.howstuffworks.com

Caveat: I know nothing about submarines and have no first hand knowledge. I just was feeling bored and ran a Google search.

 

[Foxbat]

I just had a brainstorm. I finally remembered that I had a copy of the Haynes manual for Astute Class Nuclear Submarine (the Uk's most advanced attack sub). The primary electrical system is two 440V three-phase AC turbo generators driven by steam from the reactor. Each of these AC generators provides power both to DC rectifiers and a 280V DC supply to charge the boat's main battery. It also has backups in the form of an AC and a DC diesel generator.

Obviously, this is UK only specifications and might be different in (say) the USN but I'd guess the fundamentals will be similar.

 

[ckatt]

Of course, nuclear sub reactor technology is classified so we’ll never know for sure

That's the problem right there. I'm familiar with how a generator works but was hoping to find a definitive answer on whether it was DC or AC. then if nobody knows I supposed I don't have to worry about readers calling me out for getting it wrong.

@Wayne Mack thanks for the links but I think I read those already. I've been searching for the answer for months now. Diesel subs are one thing but I can't find if the nuclear ones work with the same generators.

 

[ckatt]

I just had a brainstorm. I finally remembered that I had a copy of the Haynes manual for Astute Class Nuclear Submarine (the Uk's most advanced attack sub). The primary electrical system is two 440V three-phase AC turbo generators driven by steam from the reactor. Each of these AC generators provides power both to DC rectifiers and a 280V DC supply to charge the boat's main battery. It also has backups in the form of an AC and a DC diesel generator.

Obviously, this is UK only specifications and might be different in (say) the USN but I'd guess the fundamentals will be similar.

That's great to know.

Ultimately I guess what matters is if there is some reason why a nuclear sub couldn't use a DC generator or at least a reason why it would be impractical.
What I'm hoping makes sense is that the lights and computers could be running on DC but the propulsion system is AC so requires an inverter.
Not that any current boat has to run this way. I just need to know if it's plausible.

 

[Foxbat]

I think you’re likely to have it the wrong way around. AC generally produces greater torque than DC so would most likely be used for propulsion. Also, consider a standard AC supply throughout the ship? This would allow standard entertainment devices (for crew morale who are submerged for months at a time) like Playstation or XBox without the need for conversion. Standard lighting and pumps also. Standardisation tends to keep down expense.

There will be bespoke equipment that runs only on DC and so would explain the need for that supply. Emergency lighting normally runs on DC (battery power) in order to provide illumination in the event of a complete power failure (unlikely with backup and DC diesel generators….but if trapped at the bottom of the sea, how long can you run these without polluting the atmosphere inside the sub?).

It might be worth visiting a forum frequented by navy personnel for better answers to your questions.

Edit: This is interesting. According to the book, I’m wrong about propulsion. It is powered by DC and there’s a very good reason that I had not considered. There may be times when the sub has to move in complete silence (perhaps whilst being hunted). When this happens, the reactor is dropped to minimum power and propulsion switches
from DC mains to battery power. Again, this is to maintain an extremely quiet profile. Obviously, this will be limited by battery power but probably handy with a foe up above.

Also, the DC turbine generators are reversible and, in the event of a power failure or reactor fault, can be turned by the batteries to produce AC for the boat’s essential systems like lighting or powering a pump to control a leak etc.

 

[Robert Zwilling]

I think it would be the main generator powered by the steam is generating ac and the ac is used to run the engines and is converted to dc as needed to run the equipment and charge the batteries. I suppose if you wanted really quiet operating conditions, the main ac generator would be off and everything would run off the batteries with inverters providing higher dc voltages and ac where needed.

Tried searching "is the steam powered ac generator in a nuclear submarine always running" and this article came up, Fast Attacks and Boomers, Submarines in the cold war In this article it says the submarine propellers are powered directly by steam from the reactor. It is a mechanical system. The newest design, Columbia Class, uses all electric motor propulsion system. The mechanical propulsion version uses 80 percent of the total power generated by the reactor, so switching to electric drive allows more reactor power towards other uses besides propulsion. The Columbia article doesn't say if the propulsion motors are AC or DC.

If the propulsion system uses 80 percent of the reactor power, I'm not sure that it would get very far running off the batteries for propulsion.

One of the uses of the generated electricity is to produce oxygen, something I hadn't thought about.

The newer submarines don't use propellers, which I didn't know, they use propulsers, which are quieter but resulted in a loss in surface maneuverability. The new Columbia class has a X shaped stern which makes them more maneuverable.

I don't think a DC generator would be a primary power device in a submarine. I think AC generators are more versatile and have a better contact system that requires less maintenance and are probably more reliable.

 

[Foxbat]

The Astute Class, like the Columbia Class, uses electric rather than mechanical propulsion. In fact, all the latest UK warships use electrical propulsion (Queen Elizabeth Class, Type 45 destroyer and the upcoming types 26, 31 and 32 frigates).

The DC generator is not the primary power device in normal circumstances but is driven via the AC generators.

 

[Foxbat]

No wonder these boats are called the most complicated machines ever built.
Note: this refersbto Astute. I have no idea if it is the same in all SSNs.

Here’s roughly what the book says on the reactor…..

While both DC (generated via AC) and batteries can be used to turn the shaft, the normal operation is via a steam turbine powered directly from reactor raised steam. This is controlled by a main gearbox assembly, which translates the turbine turning power to an output rotational speed within the usable range of the propulsor.

So, to recap. Propulsion normally provided by steam power can also be provided by DC (via AC generation) or battery. In the civil nuclear industry, this would be referred to as triple redundancy.

 

[Montero]

You might want to read
"On Her Majesty's Nuclear Service" by Eric Thomson - he is an engineer who served in submarines, starting with conventional and then onto nuclear and finished his career as the engineering officer in charge of nuclear power plants.
Will give you a lot of the feel and life of the matter as well as the technical detail.
It is also a wonderful read - very approachably written.