So if you have nuclear power plants, and a third of your countries power comes from nuclear power plants, and you know you have frequent earthquakes, volcanic eruptions, tsunami's, typhoons and other natural disasters... and you need water to cool the reactor.. and you know that the pumps that are needed to cool the reactor are powered by electricity.. and you could lose electricity if there's a disaster..... ah, nevermind, a realistic plan for this to
not fail would probably cost tons of money.
All Western designed nuclear plants (i.e., not Russian) have redundant safety systems and employ a "defense in depth" strategy.
They have standby diesel generators at that plant, but the news reports I saw, said that the tsunami damaged them.
As Beagle Bay said, they were designed for a lower magnitude earthquake.
As far as a comparison to TMI, that would be a
good thing.
There was little or no impact outside the plant boundary from that accident. Yes the reactor suffered major damage and could not be put back in service, but again there is an example of how the defense in depth approach prevented any significant impact outside the plant.
Based on the news reports I've seen and filtering them through the 30 years experience in the nuclear industry, here's what I think has happened there.
When the Earthquake struck, the reactor scramed and shut down. That stopped all the nuclear reactions, but they still have to deal with the residual decay heat in the core. Think of it this way, when you take a cast iron pan off a hot stove, it doesn't immediately cool off. It takes some time.
The core of a nuclear reactor is big enough and has enough heat in it, that even after the nuclear reaction stops, it can melt the fuel assemblies if you don't maintain cooling water.
So that was problem #1. They lost electricity which is used to run the pumps.
These plants are of the Boiling water reactor (BWR) design, so the coolant that flows through the core is allowed to turn to steam to drive the turbines. Without the pumps, they have a difficult time putting the water back into the reactor.
The earthquake also cause another problem. If they can't send the electricity out into the grid, then they can't run the turbine, which means they don't remove heat as well from the core (i.e. the water doesn't turn to steam).
This is probably why they vented steam from the plant. Because of this venting, they were worried about iodine-131 being released. I-131 is one of the most common volatile radioactive isotopes created as part of the fission process. This is why iodine was given to people in the area. The Thyroid will absorb iodine, and doesnt care which isotope it is. Once it's full it won't absorb any more. So if you take iodine pills, you won't absorb any if the I-131, should you be exposed to it.
The reports of an explosion are almost certainly a hydrogen explosion. When you have that amount of hot water, you get some separation of the oxygen from the hydrogen.
This accumulated someplace inside the reactor building and exploded.
The reactor building is a concrete structure. Inside that is the containment vessel that is a steel pressure container that has pressure doors and air locks to get in and out of it. That has not been damaged, according to reports.
Inside the containment vessels is the reactor vessel that holds the nuclear fuel.
As long as the containment vessel holds, nothing released from the reactor vessel should get out.
