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.
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.