There are really two problems, both of them extremely serious because of the toxic nature of the radioactive waste materials in nuclear fuel rods, and because of the enormity of the amounts of these wastes in the rods. Each reactor generates hundreds of pounds of super-toxic plutonium-239, strontium-90, cesium-137, and iodine-131 each year. The most toxic and serious of these is plutonium; it has a half-life of 24,000 years, that is only half of its radioactivity is gone after that time. It's "lifetime" or "Half-life" - the time that it takes to reduce its radioactivity by 1000 - is 240,000 years. Plutonium is an "alpha" emitter; a microgram of it in your lungs will almost guarantee that you will get lung, bone, or some other internal cancer. Strontium-90 and cesium-137 are gamma and beta emitters which are absorbed in bones and cause bone cancer. Iodine-131 is a beta and gamma emitter which is absorbed in the thyroid gland and causes thyroid cancer.

1. The nuclear reactors themselves, even though they were shut down at the time of the earthquake, need constant pressurized cooling. The earthquake disrupted local power and there was likely damage as well to various control systems. Without local power, neither the main pumps nor the Emergency Core Cooling Systems (ECCS) of the reactors will not run. TEPCO ran the ECCS pumps on battery power until the batteries died and/or other failures took place, but there were times when they were not running, some water boiled away and the fuel rods overheated, and some hydrogen gas formed, vented, and exploded in several of the reactors. The explosions further damaged the pumping equipment, compounding the problems. Eventually TEPCO started pumping sea water into the reactors. If they are not able to keep the reactor fuel rods cool or cover them over in some way, they will either catch fire or melt through the floor of their containment, hit groundwater, and create a giant and powerful steam explosion, spewing the waste skyward. This is a "meltdown" or "China Syndrome." TEPCO has acknowledged, that some of the fuel rods have already melted.

2. In each of the 6 reactor buildings there are thousands of spent fuel rods which were replaced in regular replacement cycles. The reactors have been operating for about 40 years. The spent fuel rods also need to be continuously cooled, although not to the same extent as the active reactor rods. The earthquake and subsequent hydrogen explosions have disrupted the cooling systems of these "spent fuel pools." (Approx 600,000 'spent' fuel rods were stored near the roof of reactor #3 -- They were blown sky high.) Without continuously pumping of cool water, some of the fuel rods are physically hot enough to boil off the water in the pools and then catch fire. This has already happened in Fukushima unit 4. Burning fuel rods are what caused the vast radiological pollution and harm from the Chernobyl reactors.

At this point, it is very likely that one or the other or both of the scenarios above will take place for a large fraction of the fuel assemblies at the Fukushima facility. TEPCO has lost control of the reactors, the radiation is too high for personnel to be there for any significant length of time, the waste pools are boiling away, radioactive smoke is coming from the facility, probably from a waste pool fire, and some of the fuel rods in the reactors have already partially melted. The only thing that could possibly squelch the disaster at this point would be for TEPCO and the Japanese government to treat the facility like the Russian government did at Chernobyl and encase it in millions of pounds of cement and sand. And even this would be a gamble, whose odds are unknown.

The radioactive pollution from the Fukushima reactors, will be carried by the prevailing winds and jet stream, eastward towards Hawaii and the continental United States, Canada, and Mexico. The problem is made more difficult by the nature of the pollutants: they are very toxic (that is, they cause cancer if ingested or inhaled), long-lived, and are made up of fine dust particles.