Three ways the nuclear disaster could end

Basically,
1) the reactors are basically destroyed, but public radiation exposure is minimal
2) fires and small explosions in the cores and spent fuel ponds spread radiation around the area, leaving an area highly contaminated and uninhabitable, but only within a few miles of the nuclear plant
3) absolutely everything goes wrong, and major explosions or fires cause significant fallout plumes to spread to a wider radius

This comes from a decent summary by USA Today:

http://www.usatoday.com/news/world/2011-03-18-1Ajapannuclear18_CV_N.htm