What if cooling in one or more of the reactors at the Fukushima nuclear plant were lost? Richard Lester, chair of the department of nuclear science and engineering at the Massachusetts Institute of Technology (MIT) in Cambridge, emphasizes the “very, very” unlikely possibility of that scenario. But if it were to occur, the inherent heat of the radionuclides would cause the fuel in the reactors to melt. Here's what would happen next. In the event of a meltdown, the fuel could melt through and flow out of the primary pressure vessel, falling into the so-called core capture chamber which sits below the reactor for this very purpose. That vessel has water that would hopefully cool the molten fuel down, eventually ending the crisis. If this didn't happen, however, a steam explosion could blow out the primary containment vessel, spewing massive quantities of radioactive aerosols as well as particulates. With towns evacuated at a perimeter of 30 kilometers, the lethality of that release “would depend on the winds,” says Lester.
How would this compare to the disaster at Chernobyl? As noted in the New Scientist:
At Chernobyl the pressure vessel was breached and the reactor had no containment. There, the core itself burned fiercely, largely because it was made of graphite – which was used as the moderator… once the reactor exploded the graphite made the situation worse, because it burned so readily. The fires carried radioactive material from the reactor core high into the atmosphere, where it spread far and wide. This could not happen at Fukushima Daiichi, as it does not use graphite as the moderator.