At the bottom of the reactor vessel is a drain pipe plugged with solid salt, maintained using a powerful electric cooler.
Twenty minutes later, an uncontrolled meltdown had begun, as fuel started to slump to the bottom of the reactor pressure vessel.
Some of the salt might be settling to the bottom of the reactor vessel rather than sticking to the fuel rods, however.
Fuel that had already melted into a heap at the bottom of the reactor could melt through the steel pressure vessel and react with the concrete below, releasing vapors carrying highly radioactive materials such as strontium and technetium.
By 9 p.m. on the 11th, temperatures had reached 2,800 degrees Celsius, the melting point of the uranium nuclear fuel rods, leading most of the fuel to fall to the bottom of the reactor vessel.
If not, the little crust of white powder that technicians found at the bottom of the reactor vessel, a discovery that has brought operations here to a halt for the indefinite future, could be the beginning of a broad problem for the nuclear power industry.
Tokyo Electric Power said Tuesday that after the explosion at the No. 2 reactor, pressure had dropped in the "suppression pool" -- a section at the bottom of the reactor that converts steam to water and is part of the critical function of keeping the nuclear fuel protected.
But in a full meltdown — which could occur within hours if all cooling water was lost and the rods became completely uncovered — melting is all but guaranteed, as thousands of fuel pellets fall to the bottom of the reactor and heat themselves into a molten pool at several thousand degrees Fahrenheit.
Hydrated air was sparged through the bottom of the reactor.
Much of the fuel is now likely at the bottom of the reactor pressure vessels.
The residue left at the bottom of the reactor (approximately 5%) was discarded.
