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While not directly a physics question, I can't think of forum better capable of answering my question.

In discussions over Japan's nuclear reactor situation the observation was made that reactors there and in the United States are built on shorelines or close to large bodies of water. Is this purely for the "backup backup backup" cases where immediate water is necessary or are there more reasons justifying these locations?

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A nuclear power plant consumes massive amount of waters just for its cooling needs and also for steam generation which then drives turbines that generate electricity. Its cheaper and easier if you don't have to transport thousands of gallons of water inland on a regular basis. – user346 Mar 21 '11 at 17:58
ALL combustion/nuclear power plants need to dispose of waste heat somehow. – nibot Mar 21 '11 at 18:27
Im more curious about; Why build nuclear reactors exactly where 3 tectonic plates meet? – Holowitz Mar 21 '11 at 18:59
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Look at a map of Japan. EVERYTHING is near water and near faultlines... – jwenting Mar 21 '11 at 21:36

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In addition to the above it might be noticed that coal fired power stations are often inland despite their similar requirement for lots of water. The difference being in the weight of fuel required to run the power station.

A significant desirable fact of nuclear power is the very high energy density of the fuel compared to coal. So we have more flexibility in where it is economic to build, so the second factor of water supply becomes dominant and siting by a coast helps with this expense.

Also I guess that in the worst case scenario there is a ~50% chance any radioactive cloud will blow away from civilian populations.

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Did You "compare" those power plants? Inland is not the question, but the reliable water supply! – Georg Mar 21 '11 at 18:56
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In the UK all Large coal fired power stations are built near to (ex) coal fields. The question asks why are nuclear plants are built on the coast, I don't see any error here... – Nic Mar 21 '11 at 19:05
Because there a no rivers to use in "black country" those plants will operate on air (vaporisation) coolers. As coal plants have a higher upper temperature (into turbines) than BWR plants, one can afford a bit more temperature at the exhaust of turbines. – Georg Mar 21 '11 at 19:44
Planning permission was also easier in the UK if the stations were a long way form anywhere - there are more uninhabited stretches of coast than inland. Coincidentally the French built a lot of their early stations along the north coast where the winds blow toward the UK – Martin Beckett Mar 21 '11 at 22:07
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Also, access for shipping heavy components is easier. This is probably important for minimizing construction costs. Properly sited, a plant could still be near the shore, but be high enough up to be out of reach of any tsunami.

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The tsunami did not reach the power plants. They are up about some 50 m (estimate from pics) above sea level. The cooling water and harbour facilities of course were damaged presumably. – Georg Mar 21 '11 at 19:40
Georg: As far as I can determine they did. The biggest problems were the backup power deisel generators were only protected for a 5.7M tsunami, and were runied. And the control rooms were for the reactor were in the basement and got flooded with salt water. The last claim I heard was the tsunami reached 14M at the site! The reactors themselves were higher up, but without active cooling, which was taken out serious overheating is inevitable within a day or two. Obviously if this stuff had been at 25M, like the California plants no problem. – Omega Centauri Mar 21 '11 at 20:51
Omega, those were build to withstand a 5.7 earthquake, not tsunami! – Georg Mar 21 '11 at 21:05
I think he means 5.7 meters wave height :) The plant was designed to withstand a magnitude 9 earthquake, and it did (including the generators). It was salt water intrusion into the fuel system of the generators that caused them to shut down, depriving the installation of its source of backup power after incoming powerlines had been cut in the earthquake. – jwenting Mar 21 '11 at 21:39
Georg,jwentig: I meant a 5.7M wave height. Which I think was the height of the backup gen set. I think the design earthquake was more like 7.9. But that was probably assumed to have been closer than the Sendai quake. It is not certain what sort of damage these plants may have sustained from the quake alone, although normal shutdown did happen. I'm not sure what tsunami height was required to flood the basement control room. – Omega Centauri Mar 21 '11 at 22:43
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As Deepak Vaid said above:
WATER especially with older BWR reactors having lots of water nearby proves to be very useful for keeping it running cool.

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