- Associated Press - Sunday, March 13, 2011

TOKYO | Inside the troubled nuclear power plant, officials knew the risks were high when they decided to vent radioactive steam from a severely overheated reactor vessel. They knew a hydrogen explosion could occur, and it did.

The decision still trumped the worst-case alternative — total nuclear meltdown, at least for the time being.

The chain of events started Friday, when a magnitude 9.0 earthquake and tsunami severed electricity to the Fukushima Dai-ichi nuclear complex 170 miles northeast of Tokyo, crippling its cooling system. Then, backup power failed to kick in properly at one of its units.

From there, conditions steadily worsened, although government and nuclear officials initially said things were improving. Hours after the explosion, they contended that radiation leaks were reduced and that circumstances had gotten better at the 460-megawatt Unit 1. But crisis after crisis continued to develop or be discovered.

Without power, and without plant pipes and pumps that were destroyed in the explosion of the most-troubled reactor’s containment building, authorities resorted to drawing seawater in an attempt to cool off the overheated uranium fuel rods.

Robert Alvarez, senior scholar at the Institute for Policy Studies and former senior policy adviser to the U.S. secretary of energy, said in a briefing for reporters that the seawater was a desperate measure. “It’s a Hail Mary pass,” he said.

He said that the success of using seawater and boron to cool the reactor will depend on the volume and rate of their distribution. He said the dousing would need to continue nonstop for days.

Officials placed Dai-ichi Unit 1 and four other reactors under states of emergency Friday because operators had lost the ability to cool the reactors using usual procedures.

An additional reactor was added to the list early Sunday, for a total of six.

Local evacuations have been ordered at each location. Japan has a total of 55 reactors spread across 17 complexes nationwide.

Officials began venting radioactive steam at Fukushima Dai-ichi’s Unit 1 to relieve pressure inside the reactor vessel, which houses the overheated uranium fuel.

Concerns escalated Saturday when that unit’s containment building exploded.

It turned out that officials were aware that the steam contained hydrogen, acknowledged Shinji Kinjo, spokesman for the government Nuclear and Industrial Safety Agency. More important, they also knew they were risking an explosion by deciding to vent the steam.

The significance of the hydrogen began to come clear late Saturday. Officials decided to reduce rising pressure inside the reactor vessel, so they vented some of the steam buildup to prevent the entire structure from exploding and starting down the road to a meltdown.

At the same time, operators needed to keep circulating more and more cool water on the fuel rods to keep the reactor fuel cool and also prevent a meltdown,

Temperature in the reactor vessel apparently kept rising, heating the zirconium cladding that makes up the fuel rod casings. Once the zirconium reached 2,200 degrees Fahrenheit, it reacted with the water, becoming zirconium oxide and hydrogen.

When the hydrogen-filled steam was vented from the reactor vessel, the hydrogen reacted with oxygen, either in the air or water outside the vessel, and exploded.

If the temperature inside the Fukushima reactor vessel continued to rise even more — to roughly 4,000 degrees Fahrenheit — then the uranium fuel pellets would start to melt.

According to experts interviewed by AP, any melted fuel would eat through the bottom of the reactor vessel. Next, it would eat through the floor of the already-damaged containment building. At that point, the uranium and dangerous byproducts would start escaping into the environment.

At some point in the process, the six-inch stainless steel walls of the reactor vessel would melt into a lavalike pile, slump into any remaining water on the floor, and potentially cause an explosion much bigger than the one sparked by the hydrogen. Such an explosion would enhance the spread of radioactive contaminants.

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