Q&A: What caused the blast at nuclear plant in Japan, and why do officials fear a meltdown? (LAT)

[jpak]

http://www.latimes.com/news/science/la-sci-japan-quake-qa-20110313,0,2957196.story

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. In either case, the reactor containment vessel has not been breached, at least so far.

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.

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What are authorities doing?

Apparently the explosion has destroyed the ability to use conventional cooling on the reactor. Authorities are now planning to pump seawater into the reactor in an attempt to cool it using portable pumps brought in from elsewhere. It is not clear if they are planning to pump water into the containment vessel or are preparing to simply bathe the vessel in water. Experts say neither approach has ever been attempted before.

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