If you're still trying to get your head wrapped around how nuclear reactors like the ones in Fukushima operate, and what went wrong after the Tohoku earthquake, Mother Jones editorial fellow Joe Kloc might be able to help. He's put together a nifty diagram of the cooling system from a GE Mark 1 nuclear reactor—the kind found in 5 of the 6 reactors at Fukushima Daiichi. By coloring in the pipelines he demonstrates how coolant is supposed to flow through the reactor normally, and how it's supposed to flow during various emergency shutdown situations.
This version of the diagram shows normal operation.
Mark I Reactor Running Normally: Recirculation loops (RED) keep pressurized water circulating through the uranium core of the reactor. When water is heated by the uranium core it turns to steam. It passes through the steam separator and dryer assemblies positioned above the core (ORANGE) and then moves through the steam pipe. The steam is used to turn a turbine connected (PURPLE) to an electrical generator. It is then turned back into liquid by a condenser and cooled by a pipe (GREY) of circulating cold water. The water is then pumped back into the reactor, where the process begins again.
I have attempted to create a diagram of the reactors based on the US Nuclear Regulatory Commission's Boiling Water Reactor Systems Manual, which contains maps of the various Mark I emergency systems. In places where the manual was unclear, I used Japanese news broadcasts. The drawing is not to scale and the layout of the pipes entirely my own (though their location in relation to the various containment walls is based on the USNRC manual). To my knowledge, the diagram is accurate to the extent that a New York City subway map is accurate. It shows the various components, connections, and relationships between the emergency water systems.