Fukushima
There has been a lot in the press about the reactor at Fukushima that was damaged by the earthquake in Japan. The type of Reactor is not the same as the type that is in most text books so I did a bit of research so I would be able to answer my students questions. The type of reactor I normally describe to my students is a Pressurised water reactor that has a high pressure container surrounding the fuel, the fuel heats the water that is pumped into a heat exchanger that boils water used to drive the turbines. The Fukushima reactor is a boiling water reactor, the main difference is that the water used to cool the fuel is the same as the water that turns to steam and drives the turbines. This means that the water does not have to be kept at such high pressure.
When there is an earthquake the control rods are pushed into the fuel slowing down the chain reaction. In BWR tthe water also acts as the moderator so the rate of reaction can also controlled by the flow of water. When it flows slowly bubbles are formed in the gas which reduce its moderating effect so the reaction slows. A neat bit of feedback since if the reaction slows there isn't so much heat produced so the rate of flow of coolant does not have to be so great. Contrary to what I told my class yesterday the fuel does not have to be removed to stop the reaction as it should stop producing heat if the control rods are fully inserted, however it does take time for the fuel to cool down so water must be kept circulating until the temperature drops.