In light of the emergencies at the Fukushima I and II power plants in Japan, we’re going to talk about nuclear power plants for a while. Nuclear power is not super-complicated: there is nuclear fuel, for example either UOX or MOX pellets (uranium oxide or mixed uranium and plutonium oxides in this case), which are packed into zircaloy ceramic rods. The job of the rods is to get hot. This is just like in a fossil fuel power plant, when it’s the coal or oil’s job to get hot.
The fuel rods are surrounded by water, which gets hot, boils, and carries the heat away. In the picture above, the red line is the heat generation profile in the rods: they generate the most heat at their center. Think of the heat as a thing that is generated there, diffuses through the solid rod to the outside, transfers to the cooling water, and exits with the water as it boils and becomes steam.
If there is an interruption in this heat transfer path, then heat will begin to build up. Think of it as a heat traffic jam. If the heat begins to collect in one spot, the temperature there will rise. The zircaloy and nuclear fuel pellets are ceramics and have extremely high melting points. However, if you get a big enough heat traffic jam, they will eventually melt. That’s called a meltdown.