Instead of burning coal or natural gas like in some power stations in a nuclear one you create the heat with uranium instead. There are two kinds of Uranium, U-235 and U-238. The difference between U-235 and U-238 is very simple. A U-238 atom has three more neutrons in its nucleus than a U-235 atom does. Everything else is the same. The funny thing is, though, that little difference has a big effect on the way the two atoms behave. U-235 will do something called spontaneous fission, and when they do, they give off a lot of heat.
The word “fission” means “splitting.” If you hit a U-235 atom with a flying neutron, it splits into two smaller atoms, throws out several new neutrons, and creates heat. And it creates a lot of heat, a lot more than you get, say, by burning a carbon atom. Then the new neutrons hit other uranium-235 atoms and make them split. You can call this a chain reaction, because one splitting atom makes others split, and those splitting atoms make others split, and so on. You mine the uranium, purify it, and then put it in a nuclear power plant. The uranium contains a huge amount of heat, just a few pounds contain the same amount of heat as a million gallons of gasoline — and a little uranium is cheap by comparison.
The problem is that the uranium left over after a power plant uses it is still radioactive. And it will be radioactive for thousands of years. Just a little bit of uranium can kill or sicken a lot of people. So you have to store it safely for thousands of years. The problem is, we don’t have a safe storage place yet. Then you have to transport the uranium to the storage area, and there is always a chance for an accident. The second problem is that, if something goes wrong at a nuclear power plant, (like in Japan) all of the radioactivity can create big problems. A nuclear accident releases radiation from the plant, and the radiation can make lots of people sick.
The other advantage of nuclear power is that the uranium fuel is long lasting. A nuclear sub can go for a decade or more without refueling. The only reason it needs ever come to port is to replace the crew and load up a new supply of food for them. For this same reason, modern aircraft carriers use nuclear power. They can stay at sea without having to worry about refueling.
The image and description here (http://notalemming.files.wordpress.com/2011/03/melbibson_spent_rods.jpg?w=594&h=624) shows you what went wrong at Fukushima in Japan.