Fukushima is going to dwarf Chenobyl . The Japanese government has had a level 7 nuclear disaster going for almost a week but won’t admit it.
The disaster is occurring the opposite way than Chernobyl, which exploded and stopped the reaction. At Fukushima, the reactions are getting worse. I suspect three nuclear piles are in meltdown  and we will probably get some of it.
If reactor 3 is in meltdown, the concrete under the containment looks like lava. But Fukushima is not far off the water table. When that molten mass of self-sustaining nuclear material gets to the water table it won’t simply cool down. It will explode  –not a nuclear explosion, but probably enough to involve the rest of the reactors and fuel rods at the facility.
Pouring concrete  on a critical reactor makes no sense — it will simply explode and release more radioactive particulate matter. The concrete will melt and the problem will get worse. Chernobyl was different — a critical reactor exploded and stopped the reaction. At Fukushima, the reactor cores are still melting down. The ONLY way to stop that is to detonate a ~10 kiloton fission device inside each reactor containment vessel and hope to vaporize the cores. That’s probably a bad solution.
A nuclear meltdown is a self-sustaining reaction. Nothing can stop it except stopping the reaction. And that would require a nuclear weapon. In fact, it would require one in each containment vessel to merely stop what is going on now. But it will be messy.
Fukushima was waiting to happen because of the placement of the emergency generators. If they had not all failed at once by being inundated by a tsunami, Fukushima would not have happened as it did – although it WOULD still have been a nuclear disaster. Every containment in the world is built to withstand a Magnitude 6.9 earthquake; the Japanese chose to ignore the fact  that a similar earthquake  had hit that same general area in 1896.
Making matters worse is the MOX in reactor 3. MOX is the street name for ‘mixed oxide fuel ‘ which uses ~9% plutonium along with a uranium compound to fuel reactors. This is why it can be used .
The problem is that you don’t want to play with this stuff. A nuclear reactor means bring fissile material to a point at which it is hot enough to boil water (in a light-water reactor) and not enough to melt and go supercritical (China syndrome or a Chernobyl incident). You simply cannot let it get away from you because if it does, you can’t stop it.
The Japanese are still talking about days or weeks to clean this up. That’s not true. They cannot clean it up. And no one will live in that area again for dozens or maybe hundreds of years.