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.
Most nuclear reactors use uranium as their primary fuel, although unit 3 at Daiichi uses a mix that includes plutonium. Pellets of enriched fuel are encased inside long, narrow tubes made of an alloy containing the metal zirconium. These tubes, known as fuel rods, are spaced in an array with water flowing between them. Several hundred of these packages are then put together to create the core of the nuclear reactor.
The uranium-235 isotope, which contains 92 protons and 143 neutrons, is inherently unstable, tending to split (or fission) into lighter elements. Such spontaneous fission releases stray neutrons; when one of those neutrons hits a uranium atom, it also initiates fission into lighter elements, releasing more neutrons. Those neutrons can then go on to hit other uranium atoms in the fuel pellets, causing a chain reaction. A reactor is said to have “gone critical” when it has this self-sustaining reaction underway in its core. As long as operators keep variables such as temperature and the flux of neutrons in hand, the fission will continue at a controlled pace.
But the reactor core requires water to cool things down and moderate the flux of neutrons coming from the fissioning uranium. Without water things can heat up quickly — both the temperature and the rate of fission within the reactor core.
According to Japan’s Nuclear and Industrial Safety Agency, the earthquake knocked out power to the Daiichi facility. “Control rods” dropped automatically in between the fuel rods, to absorb neutrons and prevent the reaction with uranium that causes fission. But even with the control rods in place, the reactor still produces heat at a small fraction of its full power, because of the decay products of the uranium fission.
As planned, backup diesel generators kicked in after the monster earthquake and continued to pump water in to cool the reactor cores. But when a tsunami swept across the Japanese coast about an hour later, the wave disabled the backup generators. The next backup system then kicked in: battery-powered pumps.
But the battery pumps could not keep up with the residual heat still coming from the cores of several Daiichi reactors. Excess heat caused steam to build up in the system, which operators eventually vented into the environment along with low levels of radioactive elements like cesium and iodine.
At the same time, though, hydrogen gas had apparently built up within the core, likely created by chemical reactions of the hot zirconium rods with water. The explosions at Daiichi units 1 and 3 were likely caused by that hydrogen igniting.
Potentially far more serious is unit 2, where pumps failed for a time on March 14, causing the water level to expose the fuel rods almost completely. If the rods melt entirely, they could drop their fuel pellets to the bottom of the reactor core. The pellets could then generate enough heat to melt through the bottom of the steel containment vessel. “Once that happens the ability to contain the accident is greatly reduced, because the core is liquefied and spreads across the floor,” says Edwin Lyman, a physicist with the Union of Concerned Scientists in Washington, D.C., a group that has long voiced concerns about the risks of nuclear power.
In the 1986 nuclear accident at Chernobyl in Ukraine, the melting core did not have the heavy shielding of a containment vessel, like the reactors in Japan do. The Chernobyl core exploded, blowing radioactive materials over large parts of western Asia and Europe and causing an ecological and public health castastrophe. In the 1979 Three Mile Island accident in Pennsylvania, the reactor’s core suffered a partial meltdown but its pressure vessel was not breached, and only low levels of radioactive material made it into the environment. The Daiichi incidents, at least so far, may be far more like Three Mile Island than like Chernobyl.
On the international scale used by experts to rank nuclear incidents, Chernobyl ranked as a “major accident” or 7, the highest on the scale. Three Mile Island was a 5, an “accident with wider consequences.” Japanese officials have said they regard the Fukushima incident as a 4, an “accident with local consequences.”
Operators at Daiichi have flooded all three reactors with seawater mixed with boric acid. The boron in the boric acid absorbs neutrons and helps keep them from bouncing around and triggering further fission in the fuel rods. Salts in the seawater will, however, permanently corrode the reactor cores and render them unusable in the future.
It may be years before operators can carefully extract the cores and take them to a containment facility to assess damage, take them apart and dispose of them.
