as far as the nuclear power plants in Japan are concerned there is a real good article
i found that explains things better then the "OHNOES PANIC!" you might hear
The basics are that the Reactors are shut down, and they are attempting to cool down the fuel rods with seawater. Yes the Rods have been exposed causing a partial meltdown. But it is unlikely there is going to be a full meltdown. The recent explosions are caused by a reaction akin to electrolysis of water as it is used for cooling. Hydrogen gas pools on the roof and the venting fans creates a spark, which causes an explosion.
It is bad, 3 mile island bad, but not Chernobyl bad
Any radiation right now that gets into the atmosphere is going to be dispersed to the point of being negligible by the time it hits the US west coast