Joe Kloc in Mother Jones:
Workers in Japan are still pouring seawater on overheating nuclear reactor rods at the Fukushima Daiichi Nuclear Power Station in an effort to decrease the risk of further meltdowns. (Read Mother Jones' detailed and regularly updated explainer on the current situation.) Here's what they're up against, as Kate Sheppard and Josh Harkinson explained shortly after the emergency began:
There are six boiling-water reactors on the site, though only three were in operation at the time of the earthquake. These systems, designed by General Electric, rely on an influx of water to cool the reactor core. But the water systems require electricity that was cut off by the earthquake. It also appears that something—the initial quake, the tsunami, or aftershocks—knocked the site's back-up generators offline. Without the cooling system bringing in water, the core of a reactor will start to overheat—which in turn heats up the water already in the system and causes more of it to turn to steam. Emergency responders have been forced to vent some of the steam, releasing radiation, in order to prevent the containment domes from exploding. They are in a race against the clock to bring in new water supplies before the reacting nuclear fuel heats up beyond control.
When I couldn't find a schematic of that showed the Fukushima reactors' failed cooling systems in relation to their various other workings, I set out to remedy the problem in a visually accessible way. Think of the schematic diagram below like a New York City subway map.