In this recent article from IEEE Spectrum on the Fukushima disaster, they mention something called CORIUM. I’d not heard of it before so I went to the Wikipedia page on Corium and became a little less ignorant about what it is and what this article really means. What the authors say in Spectrum isn’t really complete without understanding what physically happens during a meltdown.
You can imagine what is involved when a term like meltdown is used as it is quite discriptive. But, to know that concrete burns under that condition, and that reactor elements fuse to become corium removes safety from the conversation. Never mind that they are also talking about radiation levels so high that robots can’t survive to return good data let alone pictures, it is a completely contained physics crucible inside Fukushima, as it was in Three Mile Island and Chernobyl.
The Wikipedia entry is good for understanding the role hydrogen plays in the process of a reactor meltdown. It also shows that there is nothing we can use to contain this reaction once it begins. All of the materials discussed, serpentinite, concrete, graphite, melt or burn under these conditions.
While they are still piecing things together at Fukushima, it isn’t heartening to know that approaching 7 years after the disaster, no one knows what occurred. They have some good guesses and have worked out some highly likely scenarios, but the work of containment is still years off.
I have worked with the Japanese and have a very high regard for their engineering and problem solving. That they are still discovering the extents of the disaster is a bad omen. It makes me more convinced that nuclear fission is not safe.