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Comment by 01100011 | Hacker News Reader

Totally unrelated question but does anyone know if nuclear powered US Navy vessels are able to feed power to the grid while in port? I know it's technically possible, just not sure if anyone has ever implemented such a thing.

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Navy nuclear vessels have (relative to the grid) little real power generation capability and cannot handle grid reactive loading at all. The grid appears as an infinite reactive load to shipboard electrical switching equipment.

Sure, relative to the total grid capacity it's not much but during peak loads every bit helps.

I suppose you'd want to transition from ship to shore via a DC path and convert it back to synchronized AC which should avoid issues with reactive loads.

There's no mechanism for this specifically because shipboard MWe is so measly.

If the ship did have significant MWe capability you would have no way to get it shoreside, the shore power cables would burn. In availabilities where significant shore power is required (>800 Amps), sets of cables have to be welded directly to busbars because the sockets can't handle it.

So you've got 360kW (800A * 450V) that the average plant can deliver. The typical estimation is 10kW per home, so 36 homes from one naval plant.

A naval reactor produces most of its power in direct steam propulsion and relatively little (~1/3rd) converted to electricity. I think the largest vessels can generate just 125MW which isn't much at all. The only beneficial use of a navy vessel as mobile infrastructure that I can recall is when the USS Carl Vinson was used to produce drinking water for Haiti after their earthquake.

The largest nuclear vessel produces up to 160 MW, the smallest single nuclear reactor power plant about 500 MW (sites often have multiples), the largest nuclear reactor in the US is about 4 GW. An average wind turbine produces 2-3 MW.

CVN-78s reactor according to Wikipedia:

> It is estimated that the total thermal power output of the A1B will be around 700 MW

CVN-78 has two of them.

I think the discrepancy is not all the thermal output is converted to electricity:

> A1B reactors likely produce enough steam to generate 125 megawatts (168,000 hp) of electricity, plus 350,000 shaft horsepower (260 MW) to power the four propeller shafts.

Ah, yes. Most steam is not converted to electricity. Should have mentioned that.

My understanding is that they shut down the reactors in port and run off the grid.

This is my understanding as well. I believe that they may even shut down at sea and run in on batteries/generators, which doubles as a way to test those systems. Refueling is quite the ordeal, so there is an incentive to minimize fuel depletion.

Reactors may sometimes be shut down at sea to test backup generators and run reactor restarting drills, but it would not be done otherwise. Aircraft carrier propulsion comes from steam from the reactors - if the reactors are offline the ship cannot move. Moreover, the backup generators burn jet fuel, which is convenient because aircraft carriers already have a store of jet fuel for the planes and so don’t have to carry extra fuel for the backup generators, but it’s very expensive, and not something that would be done outside of an emergency or testing emergency preparedness.

Nuclear powered vessels do not spend any significant amount of time shutdown at sea, and there is no reason to. Electrical power is a very small fraction of their total MWh production, with almost all power going to propulsion.

When fuel lifetime becomes an issue for a nuclear naval vessel they will have propulsion limits in place that limit transit speeds to those which are most efficient for the propulsion turbines.

Sometimes, in relief missions like after big earthquakes.

More often they use the onboard desalination plants (also nuclear powered) to make lots of fresh water.

e.g. https://www.militarynews.com/norfolk-navy-flagship/oceana/ne...

I think they do while in port in Hawaii. Hawaiian electricity is largely diesel (other than nuclear coming from Navy ships).

Isn't Hawaii also transitioning a significant fraction of its generation to solar? Diesel can be ramped up and down easily to account for varying production from solar.

Yes, but I'm not sure what the current percentage is or how fast that change is happening.

Solar and Wind have really been exploding in the past few years. I fully expect this hand wringing over nuclear will end up being overtaken by events in the next 30 years or so. The real limitation at the moment is battery technology, there is a lot riding on finding cheap and efficient energy storage.

There is no need to "find" cheap energy storage. Many different methods are already known to store energy.

But building storage before you have enough renewables to charge it from would be stupid. So, the money goes to generating capacity, in the meantime. When storage is needed, it will be much cheaper than today, as cost is falling even faster than generating capacity.

The Hawaii government page [0] list a goal of getting to 100% renewables by 2045. If I am reading the report correctly, the 2020 number was already at 36% renewables.

[0]: https://energy.hawaii.gov/wp-content/uploads/2022/01/HSEO_20... ( pdf warning )

Russia did it https://thebarentsobserver.com/en/arctic-industry-and-energy...

quick google suggests it has been done in emergencies.

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