NYT: "The essential problem is the definition of “off” in a nuclear reactor. When the nuclear chain reaction is stopped and the reactor shuts down, the fuel is still producing about 6 percent as much heat as it did when it was running, caused by continuing radioactivity, the release of subatomic particles and of gamma rays.
Usually when a reactor is first shut down, an electric pump pulls heated water from the vessel to a heat exchanger, and cool water from a river or ocean is brought in to draw off that heat.
But at the Japanese reactors, after losing electric power, that system could not be used. Instead the operators are dumping seawater into the vessel and letting it cool the fuel by boiling. But as it boils, pressure rises too high to pump in more water, so they have to vent the vessel to the atmosphere, and feed in more water, a procedure known as “feed and bleed.”
When the fuel was intact, the steam they were releasing had only modest amounts of radioactive material http://amplify.com/u/bu8pr