If you have read How Nuclear Reactors Work, you are familiar with the basic idea behind a nuclear power plant. At a high level, these plants are quite simple. Nuclear fuel, which in modern commercial nuclear power plants comes in the form of enriched uranium, naturally produces heat as uranium atoms split (see the Nuclear Fission section of How Nuclear Bombs Work for details). The heat is used to boil water and produce steam. The steam drives a steam turbine, which spins a generator to create electricity. These plants are large and generally able to produce something on the order of a gigawatt of electricity at full power.
In order for the output of a nuclear power plant to be adjustable, the uranium fuel is formed into pellets approximately the size of a Tootsie Roll. These pellets are stacked end-on-end in long metal tubes called fuel rods. The rods are arranged into bundles, and bundles are arranged in the core of the reactor. Control rods fit between the fuel rods and are able to absorb neutrons. If the control rods are fully inserted into the core, the reactor is said to be shut down. The uranium will produce the lowest amount of heat possible (but will still produce heat). If the control rods are pulled out of the core as far as possible, the core produces its maximum heat. Think about the heat produced by a 100-watt incandescent light bulb. These bulbs get quite hot -- hot enough to bake a cupcake in an Easy Bake oven. Now imagine a 1,000,000,000-watt light bulb. That is the kind of heat coming out of a reactor core at full power.
The reactors that failed in Japan are Mark 1 boiling water reactors designed by General Electric in the 1960s. This is one of the earlier reactor designs, in which the uranium fuel boils water that directly drives the steam turbine. This design was later replaced by pressurized water reactors because of safety concerns surrounding the Mark 1 design. As we have seen, those safety concerns turned into safety failures in Japan. Let's take a look at the fatal flaw that led to disaster.