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Assume there is a nuclear reactor consuming nuclear energy of one mega watt. The energy is converted to electricity by a generator with some energy loss.

For the sake of simplicity, let's assume the generated electricity is about 0.9 mega watts. If the load only consumes 0.1 mega watts in total, for example, where does 0.8 mega watts go? I don't think there is a giant battery to keep this unused energy.

Note that I have no idea how the electric power is generated and distributed. I only know that nuclear reaction releases energy to heat water, which in turn, rotates the generator to produce electricity. The power is distributed like "live" video streaming (with negligible delay). Please correct my understanding.

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Although your question seems to have changed since I began writing this reply, I think it still applies:

The answer is that the generator only generates 0.1MW. Regardless of how much steam you supply to it, the generator doesn't actually generate more power than the load needs.

You have a nuclear reactor which is generating steam. The steam drives a steam turbine which drives the generator which, for the sake of discussion, we usually say is spinning at 3600 RPM. If you attach a 0.1MW load to the generator -- which is not really the proper way to describe it -- then you draw 0.1MW from the generator.

The reactor may have produced enough steam to provide 0.9MW, but if you don't have a 0.9MW load on the generator then the turbine will not consume that "extra" steam. It will only consume steam at a rate which will drive that generator to provide that 0.1MW.

If you increase the load on the generator then the turbine will consume steam at an increased rate, up to that 0.9MW capability of the reactor.

That's a pretty loosey-goosey explanation but should get the picture across. Just remember that the 1MW represents how much steam the reactor can provide to drive the generator, not how much energy the turbine is actually consuming or how much power the generator is actually producing.

If you have a 0.1MW load and you are producing the maximum amount of steam all the time then your reactor will soon explode from the pressure.

As an addendum I want to say that the purpose of the steam is to keep the generator spinning at 3600 (or 3000) RPM. When the load increases it drags the RPMs of the generator down and the turbine must increase its steam consumption to maintain the generator's speed of 3600 (or 3000) RPM. When the load decreases then the turbine must reduce its consumption of steam in order to keep the generator from spinning faster than 3600 (or 3000) RPM. If the turbine can't maintain the generator speed within its limits then it "trips", the regulator bypasses, and the turbine cycles down to idle until the operators can spin it back up and re-sync it with the grid.

The generator only produces enough electricity to satisfy the load, and the turbine only consumes enough steam to satisfy the demands of the generator.

Hope that helps.

(Thank you for the correction Solomon Slow.)

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  • $\begingroup$ Could you elaborate more about why the frequency is kept 3600 (3000) rpm? Switching power supply with higher frequency seems to be better because direct current power supply does not need to use bigger capacitors to reduce the ripple. What is grid and its purpose? $\endgroup$ Nov 9, 2018 at 15:34
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    $\begingroup$ The power grid is the national power supply into which we plug our devices. Our power grid is roughly 120V at 60Hz. 60Hz = 3600RPM for the generator -- 3-phase alternator, actually. Some parts of the U.S. still use 50Hz, which is 3000 RPM. Voltage can vary, depending on your location and the load on the system, down to 110-ish volts or lower. Higher frequencies are all well and good, but you still need a spinning magnet to produce it. $\endgroup$
    – Ian Moote
    Nov 9, 2018 at 15:38
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    $\begingroup$ I doubt that any generator spins faster than half that speed because virtuall all electrical generators have at least two poles. I've seen generators in a hydroelectric power station that have 24 poles, and turn at a mere 150 RPM. $\endgroup$ Nov 9, 2018 at 15:49
  • $\begingroup$ Re, "When the load decreases then the turbine must reduce its consumption of steam..." Meanwhile, adjusting the reactor power output is a slow process, so (I'm guessing here) they just divert the extra, un-needed steam around the turbine and straight to the cooling tower or cooling pond where the excess heat is wasted. $\endgroup$ Nov 9, 2018 at 16:00
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    $\begingroup$ +1, nice answer. It is indeed interesting to keep in mind that electricity is not just an energy form one can "have in a cup" and from which you just take what you need. In reality, you produce it when you need it. Storage does not usually happen as electricity (but as thermal, chemical, gravitational etc. energy) (except for in certain cases such as with supercapacitor components) and waste does not happen as electricity (but rather as unused initial thermal energy in steam, as Joule heating in components and cables along the way etc.) $\endgroup$
    – Steeven
    Nov 9, 2018 at 16:24
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So if I take an alternator and make a water wheel, it will just spin freely, not generating any electricity until an electrical load is put on it. Then the electrical load will add some resistance to the wheel which the flow of water will have to overcome, thus generating exactly the amount of electricity consumed by the load or less if the force of the water is not strong enough.

In your case the water is steam created from the heat generated by the fuel rods. Steam rises and creates a lot of pressure so no gravity needed, you just need to cool the steam down so it turns back to water and cycle thru again.

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