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Japan Q&A: What caused the blast at nuclear plant, and what are officials doing to avert a meltdown?

The cause of the explosion isn't yet clear, but the nuclear plant has lost the ability to cool its hot uranium fuel. Seawater is being pumped into the reactor containment vessel and radiation levels outside the reactor are decreasing.

March 13, 2011|By Thomas H. Maugh II, Los Angeles Times

An explosion overnight destroyed the building housing one of the nuclear reactors at Tokyo Electric Power Co.'s Fukushima No. 1 facility about 160 miles north of Tokyo, where authorities have been struggling to bring the reactor under control following the loss of backup power for the cooling system. Officials have feared a meltdown of the reactor core at the plant, also known as Fukushima Daiichi, which could result in the release of large quantities of radioactivity into the environment, but the latest information suggests the situation is coming under control.

What caused the problem?

When the magnitude 8.9 earthquake struck, the shaking caused safety mechanisms to shut down 11 of Japan's 55 nuclear reactors, plunging control rods into the cores where the fuel pellets reside so that electricity could no longer be produced. The cores continued to produce heat, however, and the water that cooled them had to be continually recycled to cooling towers or it would boil away, allowing the cores to overheat and melt. But at five reactors, two at Fukushima Daiichi and three at Fukushima Daini 7 miles up the coast, water from the tsunami that followed the quake damaged the backup diesel generators that supply power to the pumps. Batteries took over, but they had only a limited lifetime.

What happened next?

There was definitely an explosion, but there have been conflicting reports about its origin. Some reports say that the building that houses the reactor blew up. Others suggest that it was a separate building that houses the turbines for the cooling pumps. Similarly, some reports suggest that it was an explosion caused by leaking hydrogen gas. Others suggest that it was the pumping equipment itself that blew up. Japanese officials reported to the International Atomic Energy Agency Saturday that the reactor containment vessel had not been breached and remained intact.

What could have caused a hydrogen leak?

The fuel rods in the reactor are composed of stacks of uranium pellets enclosed in a zirconium sheath. If the fuel rods became overheated, then they could have been reacting with the water, splitting it into hydrogen and oxygen, which could have leaked out of the containment vessel. If there was, indeed, a hydrogen explosion, that suggests there was considerable oxidation of the zirconium, indicating the cooling efforts have not been successful.

Has there already been damage to the fuel rods?

Most likely, yes, experts agree. Radiation levels measured at the boundary of the site are more than 1,000 times background levels, which may not yet be extremely dangerous, but is concerning. Authorities have also detected the presence of cesium outside the containment vessel. Cesium is one of the byproducts of the fission that occurs during the production of electricity. During power generation, it and other byproducts migrate to the gap between the uranium pellets and the zirconium cladding. When the core heats up because cooling fails, the cladding swells and cracks. If the zirconium sheath is breached, the byproducts are expelled, even if the fuel hasn't melted yet.

What happens if the fuel melts?

The General Electric Mark One boiling water reactor, 40 years old, is one of the oldest designs used commercially and one of its biggest liabilities, experts say, is a weakness in the floor of the containment vessel. If the core should melt, there is a significant chance it will melt through the floor of the containment vessel, where it will then spread ash and radioactivity into the environment.

What are authorities doing?

Apparently the explosion has destroyed the ability to use conventional cooling on the reactor. Authorities have begun pumping seawater into the reactor containment vessel in an attempt to cool it using portable pumps brought in from elsewhere. Experts say this has never been attempted before, but it appears to be successful so far. Authorities have told the IAEA that the amount of radiation outside the reactor is falling, which is considered a good sign.

Won't that make the water dangerously radioactive?

No. It might be slightly radioactive, but it will be pumped back into the ocean where it will be massively diluted, eliminating any danger. That is, after all, one reason why nuclear power plants are frequently sited near oceans.

What about civilians?

Authorities have evacuated all civilians within a 12-mile radius around the plant, as well as those within a 6-mile radius of Fukushima No. 2, where the problems do not appear to be as severe — a total of 140,000 people so far. They are also preparing to distribute iodine tablets to those who live farther away in case there is a larger release of radiation. Iodine prevents the thyroid gland, which is particularly susceptible to radiation, from absorbing too large a dose of radioactive isotopes.

How does this compare to other nuclear accidents?

Even if nothing further happens, Fukushima will be considered the third most serious accident in history, trailing only Chernobyl and Three Mile Island. But keep in mind that there is a wide gap between those first two. While at least 50 people died at Chernobyl and more than 4,000 cases of cancer resulted, there were no proved human fatalaties from Three Mile Island.

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