What happens during a nuclear meltdown?

[oexnorth]

With all the events happening in Japan right now, I was wondering, what exactly happens during a nuclear meltdown? I have a decent idea of how the plant produces power, but I have no idea what happens during the meltdown to create such a hazardous situation.

 

[Evo]

Hi oexnorth, there are already several threads on this.

https://www.physicsforums.com/showthread.php?t=480017&page=21

https://www.physicsforums.com/showthread.php?t=480200

 

[Astronuc]

With all the events happening in Japan right now, I was wondering, what exactly happens during a nuclear meltdown? I have a decent idea of how the plant produces power, but I have no idea what happens during the meltdown to create such a hazardous situation.

It's complicated and different people use the term differently.

The biggest concern is that extremely hot corium - mixture of fuel and steel structural material achieves a sustained temperature of about 1400-1450 C - the melting point of steel. That molten mess drops to the bottom of the pressure vessel, which is more or less dry (superheated steam) - and then melts through the pressure vessel - and all that molten mess drops into a pool of water. That event is expected to cause a big steam explosion depending on the mass of water. Then is expected to cause containment failure and distribution of fission products outside of containment - more than there is now.

If the pressure vessel contains water, and particular if the water covers some of the fuel, that aforementioned scenario won't happen.

If fuel above the water line is breaking and dropping into the water - then it is slow enough that the water doesn't cause a big steam explosion, but just boils. It would be like taking hot shot or ball bearing and dropping them gradually into a pot of hot (saturated) water - the boiling would become more violent as extra heat sends the sat liquid into vapor.

Disclaimer: I don't suggest one do that at home - boiling water will burn one's skin.