China syndrome is a misleading term in several ways and part of the Fear Uncertainty and Doubt surrounding nuclear power.

If you assume a runway reaction where the molten reactor fuel actually makes it down to the mantle (not really all that deep compared to the center of the planet) the actual environmental effect would be pretty low. The radioactive material would disperse by convection throughout the mantle, which is in any case kept liquid by natural nuclear decay.

It is more likely that the melted core would not get much past 30-50 feet of depth before stopping. The ground beneath and around the reactor would absorb the heat and transfer it conductivity to the surrounding area so the ground under it wouldn't get hot enough for it to burn through. Wikipedia has a decent writeup on this.

- Eric

PS - I do not work for the nuclear power industry or have any vested interest in nuclear power.