Cut through the mis-information here. The article I link at the bottom of this post is not too bad, considering all the wretched reporting on the topic.
The reactor problems in Japan are bad versions of Three Mile Island. The only similarity they have to Chernobyl is that uranium is being used to make steam. The Japanese reactors pretty much cannot catch fire. Chernobyl was a pile of radioactive charcoal briquettes.
1. The reactors "scrammed" as soon as the quake hit a particular threshold. The control rods slid in and stopped the chain reaction.
2. The reactors continued to produce heat due to decay of the fission products. The emergency cooling systems were damaged in this large quake, and they have had trouble removing this heat.
3. Like TMI, they built up hydrogen gas in the reactors, a result of the high temperature and the fuel rod cladding reacting. Either hydrogen or steam can cause the water coolant to be displaced, exposing the fuel rods. The explosions have been the result of venting the hydrogen into the outer containment buildings.
4. Some commentators have seemed to imply that the Japanese reactors have concrete containment buildings comparable to the airplane-resistant domes US reactors use. In fact, they are pretty much like tin Butler Buildings. Which may actually not be too bad when hydrogen explodes in them.
5. Back in the day, we worried about "China Syndrome", in which melted fuel puddles in the bottom of the reactor vessel and the reaction restarts, burning thru the bottom and heading for China (unlikely in this case as the material would have to move sideways). China Syndrome was never especially likely, but material escaping the vessel would be Bad News, especially as one of them is fueled with some plutonium (really toxic stuff in addition to the radioactivity). The basic idea of this is that a meltdown potentially moves the fuel to where the control rods cannot control the reaction.
6. But the article shows these clever nuclear engineers are mixing boric acid (boron 10 is a potent neutron poison) into the seawater they are pumping into the reactors. They are not just pumping in coolant, they are trying to make sure the reaction cannot re-start in the event of a meltdown.
There will be some radioactive material released in these accidents. This is unfortunate, but a look around the country ought to show it is a quibble compared to the other damage. Much will depend on the amount released and where it goes. Consider, though, that there was some contaminated gas leaked from TMI. Estimates of the total deaths caused by leaked radiation from TMI are exactly zero. This event may be somewhat higher, but I doubt it will be much.
I think any environmental concerns about nuclear energy must be balanced against the alternatives. Notice all those fires burning. Many are due to petrochemicals, including refinery fires. There must be fuel spills all over the place. Certainly all those vehicles are contaminating the environment.
I'm hoping a rational view of this does not discourage nuclear industry. It would be wise to learn from it, though. Design the backups so the systems differ enough to not all fail by the same mechanism. Allowing external generators and pumps to be attached from a safe distance, and providing hydrogen vent lines out of the containment building, thru lines that should also resist damage (I'm honestly thinking rubber hose here), may be worth considering.
The reactors will probably wind up being entombed, like the damaged reactor at TMI was.
Factbox: What is happening inside Japan's nuclear reactors?
Reuters, Mon Mar 14, 8:33 am ET
http://news.yahoo.com/s/nm/us_japan_quake_core_factbox