Why Does A Nuclear Reactor Need A Body Of Water For Cooling?

[morangta]

Homework Statement

Why Does A Nuclear Reactor Need A Body Of Water For Cooling?

If steam is needed to run the generator turbine, why not send the water/steam exiting the turbine back into the reactor for additional heating without cooling the water/steam first?

Homework Equations

don't know

The Attempt at a Solution

Is this a thermodynamic law issue?

 

[rock.freak667]

A nuclear plant uses the nuclear reactions to produce the heat to produce steam in the Rankine cycle.

So the steam/water mixture from the turbine is cooled back to water so that it can be pumped back to the steam generator (boiler in essence) to become steam again.

Usually the designers try to maximize the pressure difference between the turbine and the condenser in order to get the maximum work output from the turbine.

Now this is in the simple case.

At other times, there is the rankine cycle with reheat. In this, there are two turbines, one that operates at a high pressure and one that operates at a low pressure.

The output from the high pressure turbine is reheated usually by the boiler again and then that is sent to the low pressure turbine, which also produces a work output.

This increases the efficiency of the overall cycle. But you will still need to turn the steam/water output from the turbine back to saturated liquid in order to pump it back to the steam generator.

In some other cases, the exhaust from the turbine is used to increase the temperature of the water so that it takes less energy to reheat it.



As to "why do reactors need water for cooling": The reaction may produce heat to turn water into steam, however, the heat produced is more than used. Some energy goes into heating the water while the rest is residual. If this heat becomes too much, it can melt the fuel rods and cause a meltdown I believe.

 

[rock.freak667]

Thanks for the fast reply.

That's a pretty technical explanation for someone (me) who doesn't know thermodynamics anymore. Actually, never really did understand it.

Will go as far as I can understanding the explanation with wikipedia and then will get back to you if I have more questions.

Right now, my understanding from your answer is that the cooling is provides higher efficiency. The reactor would work w/o water cooling, it just wouldn't work as efficiently.

Am someone who's always wanted US to switch to nuclear power but was concerned about environmental damage of locating at a water source.

Tnx again.

The reactor itself, where the fuel rods are needs cooling since the casing the fuel rods are in needs cooling, else it would melt! Which is the current case for Japan's nuclear plants. They tried using normal sea water, however, I think that would make things a bit worse (I don't know if it worked from I've read).

Depending on the type of fuel, if it is not cooled and the casing starts to melt, you can get a violent reaction. For example, I think hydrogen gas can be produced but you'd need to vent it to control the pressure, if not the boiler could explode.


EDIT: You can actually read about why you need coolant in this article

http://www.scientificamerican.com/article.cfm?id=nuclear-energy-primer

(Which I oddly found right on the front of the homepage of PF! )

 

[epenguin]

For example, I think hydrogen gas can be produced but you'd need to vent it to control the pressure, if not the boiler could explode.

The explosions were said in several TV transmissions I saw to be consistent with hydrogen.

What reaction produces the hydrogen?

 

[rock.freak667]

From the article

At present, three of the reactors at Fukushima Daiichi station are seriously crippled. Units 1 and 3 have experienced explosions that destroyed exterior walls, apparently from buildups of hydrogen gas produced by the zirconium in the fuel rods reacting with coolant water at extremely high temperatures, but the interior containment vessels there thus far seem to be intact.

The hydrogen was vented away to control the pressure, where they vented it contained enough oxygen for combustion. That is what caused the explosion.