I don't know whether my correspondent was serious – I doubt it, but I am going to state the obvious anyway just in case there are people out there who don't understand the joke. A nuclear meltdown can go a certain depth from the surface – meters possibly – not kilometers. It may actually be able to go deep enough to allow itself to be sealed up and rendered harmless, but that's not too likely in areas where water tables exist.
In any case, it melts downward because gravity pulls it downward. It cannot go through the earth and come out the other side because gravity doesn't pull UP. It can't even get through the lithosphere without using up all of its potential energy. We suspect that the core of earth is fissioning anyway – or at least a ball of it a few hundred feet in diameter. It isn't bubbling up as radioactive volcanoes. There are boundary layers and…gravity. The volcanism we see is not material from the center of the earth. It is heated by a completely separate process and squeezed out through the action of plate tectonics.
Therefore, the smell of fire and brimstone is sulphur and volcanic magma is really molten basaltic rock and not actually the overflow of burnt souls from Hell. If you read that second definition within the past twenty years or so at Landover Baptist, I am sorry to tell you that I owned that site for many years and made the story up – and a few others – pseudonymonously (and coined the adjective).
The horrible possibility that someone, somewhere, actually believes that The China Syndrome could potentially describe an actual event, and might be working somewhere in a position of authority, terrifies me almost as much as the concept that humans are, at this very second, plotting to kill each other by the millions to satisfy the psychotic bloodlust of an imaginary being who lives in the clouds and whispers orders to nutcakes and pedophiles.