In a nuclear reactor meltdown what melts?

Answer 1

The short answer is that at least the fuel melts, but if it can melt anything else, it does, including concrete.

There are different types of meltdowns. At Three Mile Island, the meltdown happened when hot steam reacted with the zirconium cladding of reactor rods. This allowed uranium fuel pellets to come into contact and melt together. Upwards of half of the fuel in the reactor melted.

A worse case is if the reactor itself melts, so the nuclear fuel can go through it. In Chernobyl, the reactor was opened by a series of explosions caused by steam and chemical reactions. Molten fuel escaped the reactor, and there was a threat that when it melted through the concrete floor the reactor stood on, it would fall into a water containment below. If this had happened it almost certainly would have caused another explosion and a much worse disaster.

Fortunately for all of us who are alive, a number of people went into the area of the disaster, knowing they would be killed as a result, to get the water out of the containment under the reactor. The molten fuel combined with other materials, and was diluted in the process, finally solidifying in the area under the reactor.

As a matter of interest, other people also worked to contain the radioactive material, knowing the work would kill them. Some of these people lived as long as six weeks in the hospital, and some hospital workers came down with radiation poisoning as a result of this exposure. The disaster workers who died were buried in graves over 300 feet deep to isolate the radiation in their bodies.