Workers at the severely damaged Fukushima Daiichi nuclear power plant in Japan are trying to prevent two potentially catastrophic outcomes: a complete meltdown, or steam explosion, at the plants’ nuclear reactors; and a massive release of radiation from stored spent fuel. Workers’ efforts over the next few days—combined with events outside their control, such as the weather—will determine how much of the surrounding area is contaminated with radiation, and for how long.
The main priority is keeping the fuel rods inside both the reactor cores and the spent fuel storage pools immersed in specially treated water to keep them from overheating. The extent to which workers have been able to do this so far isn’t clear, in part because high radiation levels have restricted worker movement. The reliability of the water-level sensors in these structures is also questionable. If workers can restore power to the plant and restart the water pumps, cooling will be easier. (This could happen as soon as Sunday.)
At this point, the spent fuel cooling pools are the most worrying. The fuel found in these pools has either been used up or removed from a reactor for routine inspection, but the rods still contain large amounts of radioactive materials. These fuel rods have to be covered with water constantly to keep them from overheating, and there have been unconfirmed reports that at least one of the storage pools has been damaged and is leaking, which would make it difficult to keep enough water in it. This problem is exacerbated by high radiation levels that keep workers from getting close to it. The most recent reports suggest that workers have been successful getting water into the pools.
How fast the fuel rods in these tanks heat up depends on how long they’ve been stored. Freshly removed spent fuel will heat up faster, although it can still take a few days or more of exposure for them to overheat. If the fuel rods do overheat, their zirconium cladding can undergo chemical reactions that generate yet more heat and hydrogen, causing the cladding to break up, and the radioactive material can then be released into the air.
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While the fuel rods inside each reactor are encased inside a strong containment structure, there’s little to keep radioactive materials from escaping from the pools, and explosions ripped the roofs off several of the pools. This means that volatile materials that vaporize could be carried by the wind and distributed widely from the plant.
There are also more fuel rods in these pools than there are in the reactors, since they’ve been accumulating over the course of many years. Some researchers say that the release of radioactive materials from spent fuel pools could be greater than at Chernobyl, the worst nuclear disaster ever, because only fuel inside the reactor core was affected during that accident, and there are typically a larger number of fuel rods in these pools. The amount of spent fuel in these pools is not clear. In three of the pools (at reactors 1 through 3) the number of rods equals that in the reactor.
Other experts caution against a comparison to Chernobyl, saying that the amount of dangerous material in the spent fuel rods varies greatly depending on how long they’ve been stored. They also note that other factors contributed to the dispersion of radioactive materials at Chernobyl.
“There is no comparison between the instantaneous reactive material release from the fuel of the Chernobyl reactor, which was never shut down but became supercritical and exploded, and the spent fuel in the Japanese plant,” says Michael Podowski, a professor of nuclear science and engineering at MIT. At the spent fuel pools, he says, the release of material has been gradual. “Providing sufficient cooling to stop any further releases is only a matter of time,” he says.
Several sources suggest that the situation inside the nuclear reactor cores is stabilizing as workers manage to get water into it. Much of the radioactive material within them should also have decayed by now, meaning they are generating less and should be easier to manage. But workers have yet to establish automated water circulation system needed to keep the fuel rods cool over the long term, and over the last several days they have faced several setbacks that have made it difficult to cool the fuel inside each of the reactors.
There are two hypothetical dangers to be avoided in the reactors. The first is that the fuel will melt, then drop to the bottom of the steel reactor vessel and burn its way through the concrete containment area to reach the outside environment (a complete meltdown). As long as workers can keep injecting cooling water, this shouldn’t happen.
Furthermore, research carried out since Three Mile Island suggests that if the molten material does get out of the steel vessel, it is unlikely to eat through the concrete, as long as there is water present inside the containment. The workers at the Fukushima plant have reportedly flooded the area with water.
The other potential problem is that if enough damaged fuel accumulates at the bottom of the steel reactor vessel, it could reach critical mass, allowing chain reactions to start inside the material—the same ones that produce intense heat inside a reactor during normal operations. This could create the conditions for a violent steam explosion that would eject radioactive materials out of the reactor. Workers are taking measures to prevent this from happening, including trying to keep the fuel rods from completely melting (it’s likely some melting has already occurred), and cooling them with seawater treated with boron, which absorbs the neutrons needed for the chain reaction so it stops the chain reaction. In theory, something similar could happen in the storage pools, but the fuel rods there have less material to sustain a chain reaction.
These issues could be settled in the next couple of days, as workers restore power and get pumps working at the station to help with cooling. Only then will experts be able to predict the likely long-term impact of the crisis.