You gotta love these talking heads on TV.
A few minutes ago, I was listening to a CNN expert (I made a note of his name but it’s not relevant; this is not meant to be a personal attack on anyone but a criticism of television journalism in general, putting talking heads who know little more than the general public in front of live cameras). The expert was discussing the possible fate of the Fukushima nuclear power plant, making comparisons between the events unfolding in Japan and what happened 25 years ago in Chernobyl. This is when he uttered the sentence, “in Chernobyl, the core got uncovered“.
The problem with this utterance is that the core in Chernobyl was never covered (with water) in the first place.
The RBMK reactor used in Chernobyl has a graphite core that is not submerged. Water circulates in channels. What happened in Chernobyl was not that the core was uncovered, but that water boiled away. The resulting voids (containing only steam, far less dense than water) were no longer absorbing neutrons (which were still moderated by the graphite, but now in greater numbers), further accelerating the rise of heat in the reactor, producing more voids in a runaway reaction. Nothing like that can happen in a water-moderated reactor, where boiling the water away reduces the reaction rate, as fewer neutrons are moderated.
That is not to say that such a reactor cannot suffer a catastrophic meltdown. This is what happened at Three Mile Island, when a reactor’s core there was indeed uncovered due to errors in operating procedure and a stuck valve. In Three Mile Island, what saved the day was a reactor containment vessel that prevented a Chernobyl-type release of radioactive material to the environment. I fear that rather soon, we’ll find out just how good the containment vessel is at Fukushima I-1.