I am not sure, I appreciated the risks associated with Nuclear reactor power generation until now. I come from the state of Utah which loves to generate its electricity using clean coal technology. However, nuclear seems like a good option for countries like Japan and France that may not have the coal resources. However, Japan is a very seismically active country and each one of its 55 nuclear reactors carries a significant risk in the face of any severe Earthquake.
Yesterday, Japan was hit by the most severe Earthquake that it has faced in the modern era. The Japanese build to the most strict seismic code. Much of their infrastructure and buildings are designed to withstand up to an 8.24 earthquake. The problem with yesterday's quake was that it measured in at a hefty 8.9. Additionally, a huge section of the northern coast of Honshu Island was inundated with water from a terrible tsunami that was almost as bad as the Boxing Day/Indian Ocean Tsunami in 2004.
If the damage from the Earthquake and Tsunami weren't enough, now government officialls and nuclear engineers are scrambling to prevent a catastrophic nuclear disaster. Word has it that several reactors in Fukushima, Japan were severely damaged in the quake and tsunami.
If I am understanding things, it seems that shutting down a nuclear reactor in response to an Earthquake is not as easy as just flipping the power switch to off. Reports are that all reactors in Japan are designed to power down in response to severe seismic activity. However, it seems once you get one of these reactors going, they require 2-4 days of continually circulating water to cool down even after control rod insertion and shut down.
The problem is that all that circulating water requires functional pumps, intact pipes, and electricity from batteries or on-site generators. That is too many systems that have to be maintained in the event of a catastrophe to prevent a reactor from going critical. It seems to me that any damage to any one of these systems and its backups would prevent a reactor from safely shutting down.
As I understand it, the difficult situation with the Fukushima reactors in Japan are that several reactors cooling systems were damaged in the Earthquake and radioactive steam is being released from at least Fukushhima-1 reactor. Second, the tsunami damaged the generators that supply power to the pumps that pump in coolant water to the other reactors on the site. Even though the reactors are off, they are still hot, and will continue to heat up if not cooled by continually circulating water. However, with the pumps damaged, leaking, and generators damaged, water is not circulating. That means that the water that is there in the undamaged reactors 2-4, is increasing in pressure. If they don't let radioactive steam off, then Fukushima will blow up like Chernobyl. If they let too much steam out and the water level drops then they will have a core meltdown worse than 3-Mile Island.
On the other hand, it seems that the cooling system of Fukushima-1 was already damaged and it has already been losing steam. Water levels may be low enough that some believe the nuclear fuel rods are already partially exposed and melted. The hope is that this molten fuel in Fukushima-1 doesn't burn its way through its secondary containment.
I'm not sure why no one has designed a nuclear reactor than can't shut itself off and cool itself down by inserting control rods alone without the need of continually circulating coolant? Pebble-bed and Thorium reactor designs meet this specification.