What is a nuclear meltdown?

Answer 1

A meltdown is probably about the most catastrophic failure of the core of a nuclear reactor that can be experienced. We need to review some things about the reactor core so we can get where we need to go with this. But you asked, so pull up a chair and let's get to it.

A lot of the reactors we see are pressurized water reactors, and that's what we'll talk about here. Water under pressure is the primary coolant. At the "heart" of the beast is the core, and we know that the core of a nuclear reactor is where the nuclear fuel, the fissionable material, is located. An actual fuel element is usually either a long tube or a long, flat plate. In the tube-type, a special alloy steel tube is filled with fuel pellets (little cylinders that slide down into the tube) and has caps welded on both ends. In the plate-type, a long ribbon of fuel is encased or "sandwiched" between plates of that specially alloyed steel, and it's welded along the sides and across the ends to completely seal in the fuel. In either case, the fuel is sealed up inside a steel casing (which we call cladding) and it is designed to stay there. The fuel bundles are made up by welding together a number of fuel elements, but with a little spacing between them for coolant. By welding together a number of fuel bundles, we now have a reactor core.

We stick the whole thing in a big pressure vessel with the control rods and all the supporting paraphernalia. And run the thing hot; 500oF isn't uncommon. The water stays liquid at these temperatures because we pressurize the primary system. The heat is transferred out of the heart of the fuel elements (where the fissions are happening) into and through the cladding, and into the primary coolant - (usually) water. The heat is transferred out of the water (our primary coolant) into water that is in the steam generators, and this makes steam. The cooler water (still super hot - well over 450oF) goes back into the reactor to suck up more heat. The temperature at the centerline of the fuel elements can run in the 1500oF range. That's hot.

What happens if a gross failure of our primary piping befalls us? Wooosh! Water blasts out and turns immediately to steam. We shut the reactor down immediately, but our troubles have only begun. The pressure that kept the primary coolant in a liquid state is disappearing. The primary coolant begins to boil while the pressure falls through the floor. Look at the surfaces of the fuel elements where the primary coolant comes in contact with them. The now-underpressurized water is turning into steam there. Steam is a lousy conductor of heat compared to water. The fuel elements cannot transfer heat out of themselves into the steam. A little jump here.

When fuel fissions in the core, the fuel atoms "break" into a pair of smaller atomic nuclei which we call fission fragments. A ton of energy is released, and much of it is heat. Those fragments that were created are almost invariably radioactive, and they continue to decay and release heat doing it. This is what we call decay heat, and just because we shut down doesn't mean the core stops generating heat. It will continue to generate huge amounts of heat after shutdown, and for an extended period. There won't be as much of this decay heat as was generated by fission during operation, but it's still lots and lots and lots of heat. And now we have lost the effectiveness of our primary coolant.

At this point, the emergency cooling (XC or EC) system should kick in. It dumps cold ("room temperature") water directly into the reactor vessel to cool the core. It's supposed to work. But it must pump in lots of cool water continuously over a long period. Also, remember that the reactor vessel and the fuel elements are extremely hot. And they have a huge, huge "heat capacity" that the XC system is trying to overcome. The water from the XC system will be turning to steam from almost the moment it enters the vessel and hits the core. The centerline temperatures in the fuel are climbing. XC is not effective (for some reason). Soon, the centerline temperatures of the fuel cause the metals to begin to liquify. Things get worse. Heat is still being generated. More melting occurs. The cladding melts. The fuel in now exposed to the steam in the core. Highly radioactive materials are carried out of the hole in the primary Plumbing and into the area inside the emergency containment structure (the "dome" of concrete built over the reactor building). Back to the core.

Molten fuel and fuel byproducts, the latter being incredibly radioactive, begin running down inside the core, and a puddle of this stuff forms at the bottom of the reactor vessel. This puddle is still generating a tremendous quantity of heat. The core has melted down. We've had a reactor meltdown. Should the magnitude of the failure be over the top (like this isn't bad enough), the puddle will melt its way through the bottom of the pressure vessel, through the bottom of the concrete floor of the reactor facility, and into the earth. Not all the way to China, but that's where it's headed - hence the name "China syndrome" from the book and movie. It's possible. We engineer to avoid it, but it can happen. Not a good thing. Not a good thing at all.

Wikipedia has posted an article on a nuclear meltdown, and a link is provided.