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 8.9 earthquake and tsunami severed electricity to the Fukushima Daiichi nuclear complex 170 miles northeast of Tokyo, crippling its cooling system. Then, backup power did not 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 revealed.

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 sea water 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 sea water was a desperate measure.

"It’s a Hail Mary pass," he said.

He said that the success of using sea water 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.

Another key, he said, was the restoration of electrical power, so that normal cooling systems can be restored.

Officials placed Daiichi 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 today, for a total of six — three at the Daiichi complex and three at another nearby complex. Japan has a total of 55 reactors spread across 17 complexes nationwide.

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

A similar "hydrogen bubble" had concerned officials in 1979 during the Three Mile Island nuclear disaster in Pennsylvania until it dissipated.

If the reactor core became exposed to the external environment, officials would likely begin pouring concrete and sand over the entire facility, as was done at the 1986 Chernobyl nuclear accident in the Ukraine, Peter Bradford, a former commissioner of the U.S. Nuclear Regulatory Commission, said in a briefing for reporters.

Another expert on the call, Ken Bergeron, a physicist and former Sandia scientist, added that as a result of such a meltdown the surrounding land would be off-limits for a considerable period of time, and "a lot of first responders would die."