Read this New York Times article or watch this short video from the AP to what is at stake in the nuclear crisis in Japan.
This nugget from the Times is particularly frightening:
“We are on the brink,” said Hiroaki Koide, a senior reactor engineering specialist at the Research Reactor Institute of Kyoto University. “We are now facing the worst-case scenario. We can assume that the containment vessel at Reactor No. 2 is already breached. If there is heavy melting inside the reactor, large amounts of radiation will most definitely be released.”
[snip]
Even if a full meltdown is averted, Japanese officials have been facing unpalatable options. One was to continue flooding the reactors and venting the resulting steam, while hoping that the prevailing winds did not turn south toward Tokyo or west, across northern Japan to the Korean Peninsula. The other was to hope that the worst of the overheating was over, and that with the passage of a few more days the nuclear cores would cool enough to essentially entomb the radioactivity inside the plants, which clearly will never be used again. Both approaches carried huge risks. [emphasis mine]
It would seem that the next several hours are critical to determining whether or not a nuclear meltdown at the plant will occur. Even if that is averted, a simple shift of wind can turn this into an international nuclear crisis.