As we discuss what a nuclear meltdown is, it's also important to talk about what a nuclear meltdown isn't. It's not a nuclear explosion. Nor will a meltdown burn a hole through the center of the Earth, as popularized in the 1979 disaster film "The China Syndrome."
In a nuclear meltdown, we're faced with a reactor burning out of control, to the point where it sustains damage from its own heat. Typically, this stems from a loss of coolant accident (LOCA). If coolant circulation through the reactor core slows or stops altogether, the temperature goes up.
The first things to melt are the fuel rods themselves. If plant personnel can restore coolant circulation at this point, the accident qualifies as a partial nuclear meltdown. The 1979 Three Mile Island incident falls under this categorization: The Unit 1 reactor's core melted, but the protective casing around the core remained intact. In fact, the Three Mile Island nuclear power plant's Unit 2 reactor continues to produce power in the shadow of its deactivated counterpart.
If left unchecked, however, a partial nuclear meltdown can worsen into a total nuclear meltdown. Such situations become a race against time as emergency teams attempt to cool off the core remnants before they melt through the layers of protective casing and even the containment building itself. In 1986, Russian teams chased the melted remnants of the Chernobyl Nuclear Power Plant's reactor core into the facility's basement, flooding it with water to cool off the materials before they could burn through the containment building and pollute the groundwater.
In March of 2011, Japan's Fukushima Daiichi nuclear facility experienced a loss of coolant accident when a powerful earthquake damaged the backup generators that supplied power to the plant's water coolant pumps. The events that followed illustrate some of the additional complications that can occur during a nuclear meltdown.
Radiation in some of Fukushima Daiichi's overheated reactors (the facility had six) began to split the water into oxygen and hydrogen. The resulting hydrogen explosions breached the secondary containment structures (or second level of protection) of at least three reactors, allowing even more radiation to escape. A subsequent explosion rocked a unit so hard that it damaged a reactor's primary containment structure.
So how do you stop a nuclear meltdown from occurring or growing worse? Find out on the next page.