LOS ANGELES – An explosion 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 struggled to bring the reactor under control after the loss of backup power for the cooling system.

Q: What caused the problem?

A: 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.

Q: What happened next? 

A: There was definitely an explosion, but there have been conflicting reports about its origin. Some reports say 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.

Q: What could have caused a hydrogen leak?

A: 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.

Q: Has there already been damage to the fuel rods?

A: 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.

Q: What happens if the fuel melts?

A: 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 melts, 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.

Q: What are authorities doing?

A: 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.