" All the fuel in all the reactors and all the storage pools at this site (1760 tons of Uranium per slide #4) would be consumed in such a mega-explosion. In comparison, Fat Man and Little Boy weapons dropped on Hiroshima and Nagasaki contained less than a hundred pounds each of fissile material."[1]
"Accordingly, the worst case scenario at Fukushima will not make the northern hemisphere uninhabitable and the total doom/ death of life on earth predicted by some is impossible. The most likely outcome would be catastrophic contamination of northern Japan forcing total evacuation of the population. The main global consequence will be serious contamination of the Pacific which may catastrophically affect Pacific biota." [2]
If "catastrophically affecting Pacific biota" would not cause the whole ecosystem to collapse,and make life impossible, I'm not sure what would.
If few nuclear plant meltdowns occurred at the same time, and there would be a problem with the cleanup effort (i.e EMP in the States) we could expect spent fuel explosions to release massive amount of radioactive material into the atmosphere.
[1]http://www.dcbureau.org/20110314781/natural-resources-news-s...