I had a four hour car ride today with a fellow engineer. His specialty was running nuclear power plants and has years experience with the exact design run in Japan.
Way too much for me to repeat here, but a few keys points really enlightened me.
First, today's plants are using a hotter uranium core than the plants were originally built for. This is for producing more power. A lot of upgrades have occurred to handle this. Not all upgrades have been installed in every plant.
Second, the original design called for 6 months down time for a uranium core change. A lot of this time was to cool the core coming out. Today the core change is being done in one month. that's to get the plant back up and running and not lose the millions per day in down time. This means the core come out of containment and goes to the spent pool far hotter.
Again a number of design improvements were made to handle the hotter material. And at least some improvements certainly haven't been done in Japan. he knew exactly what was missing with the first explosion and had installed design improvements for a spent core overheat in the plants he ran. He also knew the biggest danger is in a spent core overheat.
He was also involved in an emergency cooling system improvement. This would allow the fire department to pull up to a tap on the outside of the plant and hook up high pressure water. The water pressure would trip all the necessary valves and cool the whole plant with no one going inside. In other words the fire department could shut it down. It sounds like this was real expensive and hasn't been done in most plants, certainly not in Japan.
Lot more, but I learned more about power plants today than I thought possible.
Karl