In the aftermath of the earthquake and tsunami that struck northeastern Japan on March 11, engineers are flooding three nuclear reactors with seawater in an effort to cool their radioactive cores and to prevent all of their nuclear fuel from melting down. Explosions have been recorded at two of the reactors, but do not seem to have breached the crucial inner containment vessels.
The grimmest situation is at the final reactor, where water stopped flowing temporarily on March 14, exposing the fuel rather than cooling it. Much now depends on the containment vessels that shield the highly radioactive reactor cores. Even a full meltdown does not necessarily mean that the reactors will release large amounts of radioactive material — as long as the vessels remain intact.
Officials are closely monitoring several reactors at the Fukushima facility, on the northeastern coast of Japan near where the magnitude-8.9 earthquake hit. There are two clusters of reactors at Fukushima. The Daiichi cluster includes six boiling-water reactors, all of which came online in the 1970s.
In the boiling-water design, nuclear reactions in the core generate heat and cause water to boil, which makes steam to drive turbines and produce electricity. Together, the six Daiichi reactors produced 4.7 gigawatts of power before the accident; the largest nuclear facility in the United States, the Palo Verde facility in Arizona, has a capacity of 3.7 gigawatts and serves roughly 4 million people. With 54 nuclear facilities operating before the accident, Japan is the third-largest producer of nuclear energy after France and the United States.