20%? That's a lot closer to Chernobyl, radiation-wise, than I expected. Why haven't the effects been anywhere near as bad, given that the released radioactivities are of the same order of magnitude?
You can get this from wikipedia, but the short answer is that the graphite moderator Chernobyl used ignited. The burning graphite released smoke fallout, which spread very far due to wind. Fukushima released about an order of magnitude less nastiness, and it hasn't spread as far in quantity (though some of it is in the ocean).
The Chernobyl reactor blew wide open, sent core bits flying, then burned for a couple of weeks. Fukushima remains mostly contained. I think release has been mostly through water. But it's not over yet and we don't know what the total effects will be.
