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As Alex mentioned, all reactors reportedly successfully SCRAM'd when the
earthquake was detected.
A 'SCRAM' involves the rapid insertion of the reactors control rods which
absorb neutrons and moderate the fission reaction.
Immediately after the control rods are inserted, the core is still at high temperatures for some time
due to the natural decay of fission products. It is then vital to cool the core by continually pumping cool
water through it.
Unfortunately the emergency coolant systems failed undoubtedly due to the earthquake.
However, workers have been pumping sea water through the core.
The temperatures though are sufficiently high that the water's hydrogen becomes
disassociated with the oxygen. The explosions have come from the hydrogen
igniting.
When a running reactor has been shutdown due to a SCRAM, the fission
reaction is largely stopped but some heating sttill occurs due to natural
radioactive decay of the fission products. If the coolant system has been kept operational,
then after the first 72 hours the core will have cooled to a point to
become more manageable.
However, if the core is not kept surrounded by fresh coolant during that
first 72 hours for an extended period of time, then the rods will be sufficiently
hot that a meltdown can occur.
When a meltdown occurs, the steel containment vessel is designed to try and
prevent the molten mass from burning its way through the bottom.
NHK report one of the reactors apparently had the rods exposed for half of
their length for 148 minutes, so a partial meltdown might have taken place.
However, 72 hours have passed for the reactors that have been flooded with
seawater cooling, so hopefully the risk of a full meltdown has passed.
NHK just now report that after the most recent explosion that the radiation levels
were 10,000 times higher than background and that the operators have been evacuated.
There is a concern as to whether the containment vessel has been damaged.
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