News out of Japan is replete with the technical lingo of nuclear power generation: fuel rod, exposure, feed and bleed, hydrogen explosion and meltdown. While no one intends to become a nuclear physicist overnight, these terms are not as complicated to understand as many think.
When understanding a Nuclear Reactor Meltdown, first consider what is melting. It is not the reactor, but instead, the fuel rods that are melting. Nuclear power reactor fuel rods are not solid rods of radioactive material. The rod is actual a container for a column of radioactive fuel pellets.
In a boiling water reactor (BWR) such as those at the ill-fated Fukushima power plant, water is used to keep the rods cool during power generation. Meltdown can occur when the coolant is no longer covering some or all of the fuel rod assembly – and that is what has occurred in Japan.
Nuclear Fuel Rod Meltdown
Once the fuel rod assembly is partially or fully exposed, or having a portion of its surface not covered by coolant, intense heat goes unchecked. The nuclear fuel will then heat-up to a point where the containment rod will melt. In the most extreme case, the fuel pellets will then fall to the bottom of the reactor chamber, continue heating and melt into a lava-like mass of super-heated radioactive sludge. This pool of nuclear material could then melt through the floor of the containment vessel and expose the environment to massive amounts of radiation – this is commonly known as “China Syndrome”.