How a Nuclear Meltdown Works

Heat makes all the difference. That's the key in understanding how a healthy nuclear reactor works and how a meltdown occurs in a compromised one.

First, let's look at a basic coal-burning power plant: We burn coal to create heat. That heat boils water into expanding, pressurized steam that heads to a turbine, which spins a generator to produce that valuable spark.

A nuclear power plant operates similarly, only the heat comes from an induced fission reaction that occurs in the reactor. Fission refers to when a material's atoms steadily split in two, releasing a lot of energy and a heat we call decay heat. See, uranium and other radioactive elements already undergo spontaneous fission at very slow rates without any human help. In a nuclear power plant, operators artificially spur on, or induce, that fission reaction by bombarding the uranium-filled fuel rods with neutrons from prior fission reactions. This means more heat to boil water into steam.

Of course, you don't want temperatures inside the nuclear reactor to rise too high, lest they damage the reactor and release harmful radiation. So, the coolant (often water) inside the reactor's core also serves to moderate the temperature of the nuclear fuel rods.

It's like driving an automobile: You don't want to overheat the engine, because that could damage it. The difference, however, is that you can turn off a vehicle and allow its engine to cool off. A car only generates heat while it's running and possibly for a short time after.

The radioactive materials inside a nuclear reactor, however, are a different story. The uranium and even radiated tools and parts will continue to generate decay heat even if plant operators shut down all induced fission reactions. Which they can do in minutes flat.

On the next page, we'll go inside a nuclear meltdown.

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.

Again, nuclear meltdowns come down to heat and the vital need for an operating coolant system to keep conditions in check. The Fukushima Daiichi disaster reminds us that this system is critical even if all fission activity has been shut down. The Japanese plant automatically submerged the fuel rods when increased seismic activity occurred, effectively stopping all fission reactions within 10 minutes. But those rods still generated decay heat that required a functional coolant system.

This is also why high-level radioactive waste, such as irradiated or used nuclear reactor fuel, poses such a concern. It takes tens of thousands of years for these materials to decay to safe radioactive levels. During much of this time, they'll require a coolant system or sufficient containment measures. Otherwise, they'll burn through anything you put them in.

Past nuclear power plant designs have proven even more prone to meltdowns, however. At the time of the respective accidents, the Fukushima Daiichi and Three Mile Island power plants used water not only as a coolant but also as a moderator. A moderator decreases the speed of fast neutrons, making them more likely to collide with fissionable fuel components and less likely to collide with nonfissionable fuel components. In other words, a moderator increases the likelihood that fission will occur in the reactor. When the water drains from the core of such a reactor, therefore, fission automatically stops.

Chernobyl, on the other hand, used solid graphite as a moderator. If the coolant drains away, the moderator remains behind. As such, loss of water in a Chernobyl-type reactor can actually increase the rate of fission.

In order to prevent a loss of coolant accident from turning into a meltdown, plant operators have to cool down the reactor's core. This means flushing more coolant through the overheating fuel rods. The newer the fuel rods are, the faster this cooldown will occur.

If a partial meltdown begins to occur, the rods will slump. If unchecked, the slumping rods will then melt and pool at the bottom of the reactor core in a large molten sludge. That radioactive sludge would pose an even greater cooling challenge. Not only is it a single mass (as opposed to several independent rods), one side of it is pressed against the bottom of the reactor core, steadily burning through it via the heat it produces.

In Chernobyl's case, emergency teams pumped in hundreds of tons of water to cool the reactor core. Next, they dumped boron, clay, dolomite, lead and sand on to the burning core by helicopter to put out the fires and limit the radioactive particles rising into the atmosphere. In the months that followed, they encased the ruined plant in a concrete shielding often referred to as a sarcophagus.

Again, nuclear power plants ultimately boil down to heat generation, and their maintenance depends on proper regulation of that heat. If coolant systems fail, conditions can steadily burn out of control.

Explore the links on the next page to learn even more about nuclear power.