Jose is likely wrong. Even if the worst possible contamination happens, all life on earth will not cease next year, much less in the near future. Insects, bacteria, and some other life-forms are incredibly more resistant to the effects of ionizing radiation than are the higher life forms. That being said, studies of survivors of Hiroshima and Nagasaki showed a surprisingly and unexpected low incidence of birth defects and sterilization. The greatest risk seems to be an elevated incidence of cancers, primarily among the survivors (who did not continue to be contaminated after a relatively short period of time) and a lesser increase in cancers than the norm among their offspring. Therefore, total meltdowns with long-term, wide spread, impossible to clean-up contamination will cause a very real threat to the continuation of our life expectancy (and that of higher life-forms) for a very long time into the future, but probably not eradicate any species.
The total amount of extremely radioactive, uncontrolled, very hot, spent reactor fuel in the 4 badly damaged pools in the reactor buildings plus the 3 badly damaged reactor cores is huge. Each spent fuel pool is probably holding the equivalent of at least 4 fully loaded reactor cores and that spent irradiated fuel is much more radioactive and dangerous if released than the fuel inside each of the 3 damaged cores.
The power plant is essentially on a beach. A full fuel melt down through the bottom of the plant of any of these 7 collections of out of control fuel rods will put multi-thousand degree molten, incredibly radioactive material in direct contact with the pacific ocean once the fuel burns through the sand (the temperature is high enough to fully and easily liquefy sand). The resulting steam explosion is likely to disperse far more radioactivity than multiple Chernobyls.
The big problem is not I-131. The half-life of that isotope is only 8 days, and uptake can be blocked by appropriate use of potassium iodide until environmental levels return to a safe level. A much larger problem is Cesium-137, which has a half-life of 30 years, is water soluble and therefore taken up by all the plants and animals in our environment.
Also released will be Strontium-90, which behaves like calcium and is taken up into the bones and teeth and has a half-life of almost 29 years.
Plutonium isotopes are released, which decay into several much more dangerous isotopes of Americium, one of which has a half life of 7,370 years. This stuff is an alpha particle emitter, which is almost harmless when outside the body but is highly carcinogenic from the inside.
If this incredibly radioactive brew comes into direct contact with the Pacific Ocean, it will be easily and relatively rapidly spread throughout the world. Although the dilution factor will be tremendous, the total amount of very long-lived radioactive matter will also be tremendous. There is no way to predict how many of the 7 at-risk collections may go to full melt-down, but if even a few do, the world-wide consequences could be huge for tens of thousands of years.
It will not be Armageddon, but will probably be the worst disaster the planet has endured since the great meteor crash approx. 65 million years ago.