Heat makes all the difference. That's the key in understanding how a healthy nuclear reactor works and how a meltdown occurs in a compromised one.
First, let's look at a basic coal-burning power plant: We burn coal to create heat. That heat boils water into expanding, pressurized steam that heads to a turbine, which spins a generator to produce that valuable spark.
A nuclear power plant operates similarly, only the heat comes from an induced fission reaction that occurs in the reactor. Fission refers to when a material's atoms steadily split in two, releasing a lot of energy and a heat we call decay heat. See, uranium and other radioactive elements already undergo spontaneous fission at very slow rates without any human help. In a nuclear power plant, operators artificially spur on, or induce, that fission reaction by bombarding the uranium-filled fuel rods with neutrons from prior fission reactions. This means more heat to boil water into steam.
Of course, you don't want temperatures inside the nuclear reactor to rise too high, lest they damage the reactor and release harmful radiation. So, the coolant (often water) inside the reactor's core also serves to moderate the temperature of the nuclear fuel rods.
It's like driving an automobile: You don't want to overheat the engine, because that could damage it. The difference, however, is that you can turn off a vehicle and allow its engine to cool off. A car only generates heat while it's running and possibly for a short time after.
The radioactive materials inside a nuclear reactor, however, are a different story. The uranium and even radiated tools and parts will continue to generate decay heat even if plant operators shut down all induced fission reactions. Which they can do in minutes flat.
On the next page, we'll go inside a nuclear meltdown.