“There is no comparison between the instantaneous reactive material release from the fuel of the Chernobyl reactor, which was never shut down but became supercritical and exploded, and the spent fuel in the Japanese plant,” says Michael Podowski, a professor of nuclear science and engineering at MIT. At the spent fuel pools, he says, the release of material has been gradual. “Providing sufficient cooling to stop any further releases is only a matter of time,” he says.
Several sources suggest that the situation inside the nuclear reactor cores is stabilizing as workers manage to get water into it. Much of the radioactive material within them should also have decayed by now, meaning they are generating less and should be easier to manage. But workers have yet to establish automated water circulation system needed to keep the fuel rods cool over the long term, and over the last several days they have faced several setbacks that have made it difficult to cool the fuel inside each of the reactors.
There are two hypothetical dangers to be avoided in the reactors. The first is that the fuel will melt, then drop to the bottom of the steel reactor vessel and burn its way through the concrete containment area to reach the outside environment (a complete meltdown). As long as workers can keep injecting cooling water, this shouldn’t happen.
Furthermore, research carried out since Three Mile Island suggests that if the molten material does get out of the steel vessel, it is unlikely to eat through the concrete, as long as there is water present inside the containment. The workers at the Fukushima plant have reportedly flooded the area with water.
The other potential problem is that if enough damaged fuel accumulates at the bottom of the steel reactor vessel, it could reach critical mass, allowing chain reactions to start inside the material—the same ones that produce intense heat inside a reactor during normal operations. This could create the conditions for a violent steam explosion that would eject radioactive materials out of the reactor. Workers are taking measures to prevent this from happening, including trying to keep the fuel rods from completely melting (it’s likely some melting has already occurred), and cooling them with seawater treated with boron, which absorbs the neutrons needed for the chain reaction so it stops the chain reaction. In theory, something similar could happen in the storage pools, but the fuel rods there have less material to sustain a chain reaction.
These issues could be settled in the next couple of days, as workers restore power and get pumps working at the station to help with cooling. Only then will experts be able to predict the likely long-term impact of the crisis.