What I haven't heard is what happens to the molten matter after it releases this radiation ball. How long it goes on for, how they deal with the molten stuff, likelihood of a chain-reaction explosion?
I can give you some facts on Chernobyl (I was in the Red Army when it blew, we were at the field exercises and withstood the edge of radioactive fallout, and they recruited volunteers from all units).
In Chernobyl, the reactor blew open and there was an uncontrolled release of heat and accelerating chain reaction (no nuclear explosion is possible, though, as the fuel has too low an enrichment level). When the first firemen got to the roof of 4th energy block, they could actually look into the reactor and see how uranium burned. It had an extraordinary purple colour - they who saw it died in the next 36 hours and were buried in tin coffins.
The chain reaction in uranium fuel produces plutonium that is as toxic as cyanide in addition to being radioactive. Some other elements are produced as well. The fire in the nuclear fuel created an ascending air current that took all that radioactive and poisonous stuff up and carried far away.
Soviets had basically only one option - to extinguish fires in surrounding buildings (at a huge cost in lives) and fill the reactor with sand, clay and boric acid to slow down the chain reaction, stop the leak of radioactivity and gradually put out the fire in the nuclear fuel. They had to fly helicopters right over the reactor and drop their sand, clay and boric acid payloads onto the reactor. In other words - simply bury it. The assessment is that the nuclear fuel and graphite kept burning for two weeks in Chernobyl. Having done that, they began erecting concrete sarcophagus.
For me, today's developments with helos show that Japanese are getting desperate - they simply
have to fly in there. I interviewed one nuclear expert and his assessment was - they really have a little idea about what is happening there. What they know for sure is that temperatures keep rising and radioactive contamination levels of the plant as well.
Japanese have also another problem - their reactors seem to be intact and if anything is going to burn, then initially within the reactors. If the fuel and gaphite was to ignite, it is possible that the molten mass would break out of reactor and would have to be buried in fashion similar to Chernobyl. The worst option is if the spent fuel storage ignites - it is open, above the ground and has a high potential to contaminate large areas. In both cases Japanese would have to do what Soviets did at Chernobyl to check the disaster.