A third hydrogen explosion at the Fukushima Dai-ichi nuclear plant is reported early March 15, destroying the outer building of reactor Number 2. This means that all three of the reactors that technicians have lost control of at the six-reactor site have now experienced hydrogen explosions that have destroyed or seriously damaged the outer turbine buildings. Engineers had been struggling overnight to cool the nuclear core at Number 2, and stave off a catastrophic meltdown. As with the explosions at reactors 1 and 3, authorities say there was no damage to the containment vessel.

The Los Angeles Times offers this explanation of what happened:

Officials had feared the possibility of such an explosion because the fuel core had been exposed to air for more than two hours, allowing it to overheat. When the zirconium cladding on the fuel rods was subsequently exposed to seawater used for cooling, it released hydrogen gas, which built up to dangerous levels in the plant and was most likely ignited by a spark.

Official optimism is of course mandatory. AlJazeera reports:

Koichiro Genba, the national strategy minister, said there was "absolutely no possibility of a Chernobyl"—a reference to the 1986 explosion at a Soviet reactor which spread radiation over swathes of Ukraine, Belarus, Russia and northern Europe and is estimated by UN agencies to have caused the deaths of thousands of people.

But Bloomberg informs us that the markets are not assured:

Exelon, Entergy Decline on Japan Reactor Meltdown Threat
Exelon Corp., owner of the largest group of US nuclear power plants, declined as engineers in Japan struggled to stabilize a reactor similar in design to four of its units.

Exelon, based in Chicago, fell 27 cents to $42.89 at 4:15 PM in New York Stock Exchange composite trading. The shares declined as much as 5.5 percent in intraday trading, the most since May 6. Exelon’s 17 reactors at 10 stations in Illinois, Pennsylvania and New Jersey account for 20 percent of U.S. nuclear power capacity, according to the company’s website.

See our last post on the crisis at Fukushima.

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