To start I want to say I feel bad for the Japanese and am praying for them but on a lighter note I guess we now know one of the side effects from getting nuked, nuking themselves in the future but hey they should be used to it right?
It's not going to reach the jet stream. Chernobyl is the benchmark for how bad it can get because it was as bad as it possibly could get. Why?
Chernobyl didn't have a containment vessel (like every reactor outside the com bloc does).
Chernobyl was graphite moderated. What's graphite again? Oh, yeah... carbon. Which burns. So when the core melted down and blew the walls of the building out, guess what happened with that carbon exposed to oxygen and 1800 degree F temperatures? It burned. Hot. Hot enough to lift stuff into the jet stream. Nuclear materials are usually pretty heavy, they don't wind up in the jet stream without a lot of help.
The explosions in Japan? They're happening outside the containment vessel. Are they good? Not really. But they're doing damage to the outside of the plant, not destroying the containment. Yes, there is some radiation getting out because of this since the explosions are from hydrogen and oxygen that's a byproduct of a meltdown recombining when "burped" out of the vessel to make room for more cooling water.
In short, the radiation in the immediate area is going to be a little high, but they should get it cleaned up. The Japanese are going to have a couple of severely damaged reactors. Between the meltdowns - yes, they've had at least partial meltdowns - and the sea water they're pumping into the reactors to cool them, things are going to be screwed up. But those reactors were due for replacement anyway. IIRC they were already building a couple of new reactors.
If this stuff had hit the jet stream, it should start showing up on the Left Coast today. So far, no spike in radiation sensors out there.
Stop worrying. The sky is not falling. We're going to be fine. You can breathe again.