Tokyo Electric Power Corp. admitted that more than one reactor at the Fukushima Daiichi nuclear power plant suffered a meltdown in the wake of the earthquake and tsunami that hit the country in March.

At a press conference on Tuesday night, TEPCO spokesman Junich Matsumoto said that reactor units 2 and 3 had lost enough water that the fuel rods inside the reactor vessels were exposed. That caused them to heat up enough to melt. Originally the company had only said that meltdowns occurred at reactor unit 1.

After a magnitude 9.0 quake struck Japan on March 11, a tsunami followed, flooding the northeastern part of the country. The wave drowned the electrical generators that power the pumps in the Fukushima plant, disabling them and preventing the flow of cooling water into the reactors. Within two to three days the reactors in units 1, 2 and 3 heated up enough to melt the fuel rods.

Of the six reactor units at the plant, four were damaged by hydrogen explosions that resulted when coolant levels fell. The explosions were the result of the high temperatures inside the reactor vessel, which separates the steam into hydrogen and oxygen. At that point a small spark can cause the hydrogen to react with the oxygen-explosively.  

The admission of another meltdown comes as the International Atomic Energy Agency sent a team of experts to Japan to investigate the nuclear accident there. The IAEA says it hopes to apply lessons learned to improving nuclear safety elsewhere.

The team, consisting of 18 experts from a dozen nations, will conduct on-site inspections at the site of the stricken power plant. The IAEA group will also visit the Tokai and the Fukushima Daini plants.

Radiation levels inside the plant remain high, though TEPCO has taken steps to limit the amount of radiation that is getting released into the environment.

Besides the Fukushima plant, others suffered power losses and damage as well. At the Higashidori power station in Aomori prefecture, workers switched to emergency power, preventing the kind of disaster that occurred at Fukushima. But it raises the specter of similar emergencies in the future, and a hot topic of discussion in the nuclear industry is how to deal with accidents in which there is a loss of coolant.