What Happens During a Nuclear Meltdown?

Technicians are scrambling to contain the damage after March 11’s devastating earthquake and tsunami knocked out power at Japan’s Fukushima Daiichi nuclear power plant. Seawater is being flooded into the reactor core to prevent overheating, and radioactive gas is being periodically vented to prevent pressure from building up. But these are merely stopgap measures to prevent a full meltdown of the reactor core. How likely is it that this strategy will fail and Japan will face a total meltdown?

At the moment, not very. It’s an inexact term, but "meltdown" generally refers to the complete melting of a plant’s nuclear fuel rods. These rods are about half an inch in diameter and 12 feet long and are surrounded by a zirconium covering called cladding. To prevent overheating, water is constantly circulated through the reactor. When the cooling system fails, the rods, made of a ceramic material, can melt. The melted nuclear material drips down and accumulates, possibly penetrating the core.

In the case of the Fukushima plant, it is believed that the top 2 to 3 feet of the rods were exposed after the power went out, causing them to overheat. The vessel containing the nuclear core has not been penetrated. Nuclear engineers prefer the term "partial melting" for events of this type.

The good news is that the plant is not currently operating, meaning that the fuel is only producing about 6 percent of the heat normally generated when it’s up and running. During the 1986 Chernobyl disaster, the plant was still running during a power surge that essentially turned the plant’s reactor core into a small nuclear bomb, pushing actual radioactive material — as opposed to gas with trace radioactive elements — out into the air.

The bad news is that without power, the plant’s technicians can’t resume the normal circulation of water through the core to cool down the rods.

The controlled venting of steam from the reactor — while necessary to prevent overheating — is also problematic. Inside the core, the steam reacts with the protective zirconium casing surrounding the rods, creating hydrogen. When this hydrogen is vented out and interacts with oxygen, it can cause explosions like those that occurred at the plant on March 12 and 14. The steam also contains cesium and iodine — radioactive elements that are dangerous to human health. The level of radioactivity around the plant, while relatively modest, is still twice what the Japanese government considers safe. This venting process could potentially continue for several months.

The most severe instance of partial melting in history occurred at the Three Mile Island plant near Harrisburg, Pennsylvania, in 1979. The melting was caused when a pump pushing water into the reactor core failed for unknown reasons. Nuclear specialists say that the melting at Fukushima Daiichi may release more radioactivity than that incident. However, a disaster on the scale of Chernobyl, which left hundreds of square miles uninhabitable for years, is believed to be nearly impossible because of improved containment facilities at modern nuclear plants.

Thanks to Mujid Kazimi, director of the Center for Advanced Nuclear Energy Systems at the Massachusetts Institute of Technology, and John Lee, professor of nuclear engineering and radiological sciences at the University of Michigan.