For those who didn't catch the agenda last night, Duncan Hawthorne (pres Bruce Power ) was on to explain the Japanese reactor situation.
As it happened, the earthquake triggered shutdown on all units, either because of the quake itself or because it was disconnected from the electrical grid. For the first hour(until the flood) everything was fine, and all the units shut down properly, the subsequent core cooling operated perfectly.
Then the water came and washed away the diesel fuel supply for the generators that were running the whole plant. That was the first real issue.
Getting that cooling going is imperative. Without it, the water flooding the core boils off into steam and creates a pressure buildup inside the primary containment vessel. Systems exist to vent that steam into a secondary containment vessel(surrounding the primary vessel. One is inside the other)
With the cooling pumps down and water replenishment unavailable, the water boils away dropping the level till some fuel bundles are exposed above the water. This allows hydrogen to be created. It is vented to secondary containment with the excess steam. Hydrogen is lighter than air so it collects at the top of the secondary containment.
Hydrogen buildup here is a foreseen issue, and the systems to deal with it are in place to burn it off in a controlled manner. However, those control systems operate on electricity.
So the hydrogen builds up until a little spark comes along and you get the big explosions you are seeing on tv. Secondary containment is blown sky high. However, the primary containment is still intact, and holding the really nasty elements.
All this venting puts small amounts of radioactive steam into the atmosphere. But its not like the core exploded, nor will it, nor can it. So right now the issue is keeping cooling water moving through the primary vessel. They have decided that since the units are no longer salvageable, sea water is a good solution. Not sure how they got that flowing, maybe temporary pumps or something. But anyway, Unit 1 is pretty much on its way to being safe, unit 3 temp is coming down, with unit 2 being the most critical right now.
Unit 2 will recover in the same way the other have, it just takes time.
Duncan's bigger concern is that since the backup systems have been destroyed, the rescue of the units is dependent on the actions of the people. Its all back to basics with people doing extraordinary things to keep things working. Most of those people have lost all family, all possessions. They are locked inside the reactor site, and have no idea what has happened to everything they care about.
Their ability to maintain the effort is the biggest issue of all.