1800 RPM vs 3600 RPM

[bonzoboy]

I think most nuclear reactors spin their steam turbines at 1800 RPM, or 1/2 the speed of those used on fossil plants. Any idea why the slower rotational speed on the nukes?

 

[electricpete]

I don't know exactly what drives turbine design, but I can tell you two differences between nukes and fossil plants:

1 - nukes almost always have lower steam pressures. Steam temperatures and associated pressures are limited by material considerations on the primary side. There is no superheating (except when steam pressure is reduced).

2 - nukes tend to be larger (more MW). Usually but not always.

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[TheBlacksmith]

A lot of it is size. The larger generators obviously have to flow more steam (pressure does have a hand in this, but a lot of it is just sheer size), which means larger blades and larger diameter wheels, so the speed is lower due to centrifugal loading. Many smaller nuclear turbines, such as shipboard ones, turn faster.

 

[bonzoboy]

So it sounds like the nuke units are just physically larger than fossil steam units, so they would spin slower?

 

[rmw]

It is also a moisture issue. Since the inlet pressures are lower than a corresponding fossil unit for it's MW'age, the moisture in the steam is prevalent in more stages of the machine. To run the machine at 3600 or 3000 RPM would accelerate the wear due to moisture erosion.

rmw

 

[woolybacker]

the steam pressure and temperature in a nuclear reactor are governed by the thickness of the fuel cladding and nuclear physics - the thicker the material the more neutrons are captured in the material and they are not used in the fission process hence less fissioning and less heat produced - Most nuclear units rely on steam flow more than pressure - a typical 500Mw large scale fossil fuel plant would need a steam flow of about 2000lbs/hr to turn the turbine - a nuke plant of the same size would need about 8000lbs/hr - to get the most effiecency out of the steam the blades have to be much bigger and of course we then have the addittional stresses that come with bigger blades hence lower rpm - Incidentally most nuclear units will be superheated on the LP stages - LP inlet pressures are usually around 80 psia and temp around 230 deg C - the HP inlet pressures are usually in the 600 psia range with a steam temp of around 300 C - so lower pressure - bigger wheels - bigger wheels - slower rpm

 

[electricpete]

I was thinking about the analogy to pumps.

In the pump world, high-flow / low-Dp pumps tend to be low speed. Low flow, high-dp pumps tend to be high speed. It has something to do with keeping "specific speed" in an efficient range.

I don't know if the same applies directly to turbines, but it seems to fit. The nukes have lower dp and for higher power need higher flow. So the logic used for pumps seems to apply to turbines in this case, although I don't know if the reasons are the same.

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