There must be a trick to pumping water into a reactor (I’ll explain)

[thenewmans]

In a nuclear reactor, the trick is to create high pressures that can turn a turbine. For that, water is boiled by the reactor. As the steam goes through the turbine, it decompresses and cools and eventually turns back into a liquid. So far so good. But the water that leaves the reactor has to be replaced. To do that, you have to pump it from low pressure into very high pressure. I would think you would have to put as much energy into compressing the water into the reactor as you get out of the turbine. Why is that not true? I’m sure the answer has something to do with liquid water being denser than steam. But I don’t yet see why. If that’s true though and liquid water is 1000 times denser than steam, then I guess it’s 1000 times harder to pump in steam. Is that true?

 

[Andrew Mason]

In a nuclear reactor, the trick is to create high pressures that can turn a turbine. For that, water is boiled by the reactor. As the steam goes through the turbine, it decompresses and cools and eventually turns back into a liquid. So far so good. But the water that leaves the reactor has to be replaced. To do that, you have to pump it from low pressure into very high pressure.

That part of a nuclear power plant works basically the same as any other thermal power plant. It takes heat from a heat source and generates work. It is not really much different than a steam locomotive.

The water in the reactor core itself acts as both a neutron moderator for the fission process and as a convection heat transport to take heat from the reactor. That water stays in the reactor. There is a second heat transport system that is simply steam circulated through a heat exchange mechanism to turn it into high pressure steam. The steam turns a turbine and cools as a result. The cooler steam is then circulated back into the heat exchanger to turn it into hotter steam again and the cycle repeats. It does not take work to put the steam back in the heat exchanger. You simply replace the steam that leaves (ie to turn the turbine) with cooler steam from the turbine.

I would think you would have to put as much energy into compressing the water into the reactor as you get out of the turbine. Why is that not true?

I don't follow you there. You heat water and turn it into steam. You then use the steam to do work. The cooler steam is then heated up and you start the process over.

AM

 

[thenewmans]

Water gets pumped in, It turns to steam, which creates pressure. So why doesn’t the steam push hard enough to prevent the pump from pumping in more water? If the pump pushes harder than the steam, doesn’t that require a lot of work? How do you get more work out of the steam than the amount of work going into the pump?

Perhaps this will help: Why does it go in one direction and not the other?

 

[AlephZero]

Perhaps this will help: Why does it go in one direction and not the other?

That is farly easy to achieve with a liquid like water, using a "positive displacement" pump. In simple terms, you trap a volume of water between two of the impeller vanes of the pump and the casing, so it has no option but to go in the direction the pump takes it.

It is much harder to do it with a gas, for example air being compressed in a jet engine before it gets to the combustion chamber. You have to make sure that whatever conditions the engine is running at, the compressor is always working well enough to stop the air flowing the wrong way. If the airflow is disrupted for some reason, the air can (and does) flow the wrong way, which can have serious consequences.

You are right, it does take a lot of energy (or work) to pump water against a high presssure gradient. That is exactly the same as pumping water to the top of a mountain. However, if the heat energy coming from the fuel is greater than the energy used by the pump, the complete system still generates power. Of course the more energy the pump uses, the less efficient the complete system will be in converting heat into useful energy (e.g. electricity).

 

[Andrew Mason]

Water gets pumped in, It turns to steam, which creates pressure. So why doesn’t the steam push hard enough to prevent the pump from pumping in more water? If the pump pushes harder than the steam, doesn’t that require a lot of work? How do you get more work out of the steam than the amount of work going into the pump?

You should study thermodynamics. I am sure it would clear up a lot of your questions.

Would you have the same questions about how a building's boiler system works? The boiler heats steam. The steam is pumped through pipes to radiators where it radiates heat. The cooled steam is pumped back to the boiler where it is reheated and sent back to the radiator.

The only significant difference is that in the power plant, the steam cools by doing work (turns a turbine) rather than by radiating heat into a room.

Perhaps this will help: Why does it go in one direction and not the other?

The steam is pumped in the direction that you want the steam to flow.

AM

 

[russ_watters]

Water gets pumped in, It turns to steam, which creates pressure. So why doesn’t the steam push hard enough to prevent the pump from pumping in more water?

No. A boiler increases the temperature and volume of the fluid, it does not increase the pressure. A boiler is just a coiled-up pipe with no obstructions to cause changes in pressure. All of the pressure increase comes from the pump.

If the pump pushes harder than the steam, doesn’t that require a lot of work? How do you get more work out of the steam than the amount of work going into the pump?

This is the key concept of engineering thermodynamics. It may take some effort to put together all the pieces, but you can try reading the wiki on the Rankine cycle:
http://en.wikipedia.org/wiki/Rankine_cycle#The_four_processes_in_the_Rankine_Cycle

You can see in the description of the processes and in the t-s diagram that the work out for the turbine is greater than the work in for the pump. This is because as the steam condenses, it releases energy that the turbine absorbs. Basically, you add energy in two steps (once with a pump and once with the boiler) and take it out in one step (with a turbine). The pump increases the pressure, the boiler boils whereas the turbine decreases the pressure and condenses.

 

[QuantumPion]

It's a lot easier to pump water than steam, because water is incompressible.

 

[uart]

Water gets pumped in, It turns to steam, which creates pressure. So why doesn’t the steam push hard enough to prevent the pump from pumping in more water? If the pump pushes harder than the steam, doesn’t that require a lot of work? How do you get more work out of the steam than the amount of work going into the pump?

Perhaps this will help: Why does it go in one direction and not the other?

Hi thenewmans, the simple answer is that you're pumping a lot less volume (per second) with the water than the volume of stream that subsequently turns the turbine. The power is related to the product of pressure and volume/sec.

 

[Astronuc]

Perhaps this will help: Why does it go in one direction and not the other?

Momentum.

 

[Stretto]

Water doesn't leave the reactor. It is generally a closed system. You heat up the water, it expands, you use a pressure gradient device(such as a value) to allow some of the hot water to escape into a chamber of lower pressure. The water then expands into a gas and cools(ideal gas law). This drives a turbine in the process. One then simply uses a pump to pump in the differential change. That is, when some of the water left the high pressure side the pressure reduced slightly, this allows you to put back in some of the water.

In any case you are running off the power generated by the nuclear reaction which is independent of the turbine. If you had enough room you could create a pumpless system(basically using waters own inertia to create a pressure differential).

Note that you don't technically need pumps and valves. The mere fact that when the water enters a chamber of lower pressure will cause it to expand is enough to get the water moving. It more about geometry and thermodynamics than anything.

For example, suppose you have a very very large tank that is filled up to some point. At the top you have a very small valve that has a pipe that connects it to the bottom of the tank. In the pipe is a heat source. As the water heats up, and because it is incompressible, it will move in all directions both up and down the pipe. If the resistance of the water is large enough(could be done through geometry) then some water will go through the top valve(sort of leak out). As long as the temperature and pressure differences are great enough the water that leaks out will evaporate and expand which could be used to drive a turbine. (eventually we will reach an equilibrium if the tank is not cooled in some way to allow the cycle to repeat)

There are much easier ways to accomplish this type of stuff though such as a simple heat exchanger in which you have a vast reservoir of water. Basically the nuclear reaction heats up water in a closed system which in turn heats up water in an open system causing some of it to boil which drives a turbine.

Also, think of it this way. You could use a thermocouple to drive the pump in which the energy is from the nuclear reactor and has nothing to do with the turbine... Since the nuclear reaction generates so much more energy than we use to simply move the water it virtually costs nothing.

 

[Phrak]

Water doesn't leave the reactor. It is generally a closed system. You heat up the water, it expands, you use a pressure gradient device(such as a value) to allow some of the hot water to escape into a chamber of lower pressure.

The original poster is not actually asking about the circulatory system unless it directly involves the turbines. In which case it does leave the reactor.

In any case, the OP was interested in the energy balance involved in removing pressure from steam, thus a drop in pressure, then returning the same mass of water to the pressure source from which it came, without requiring the same amount of energy, extracted from the turbines, to do so.

i.e.: To paraphrase: "How can you get net work out of a turbine + return pump, given a heat source in a closed circulatory system?"

I'm sure I saw the answer posted above, yesterday. However, the short answer is that the energy required to move a volume of stuff to a higher pressure is equal to the pressure times the volume of stuff. This is energy supplied by the water return pump.

The energy available to the turbine is the volume of steam multiplied by its pressure drop.

Because the volume of water is less than steam there is a net energy gain. That is, the pump doesn't require as much energy as the turbine puts out.