In addition, when the zirconium cladding surrounding the cores was exposed to air, it may have oxidized and become so brittle that radioactive fuel particles could have escaped through cracks. If enough of the escaped fuel has collected at the bottom of the reactor vessel, it could become hot enough to melt through the steel container and escape into the environment, Lyman said. Even if the steel was not breached, the collection of fuel at the bottom of the container would also make it more difficult to cool.
"These cores may not be as easily cooled as if they were undamaged," Lyman said.
Workmen have been spraying all three with seawater for several days in an attempt to keep temperatures down, but the water has combined with the steam and radioactivity to make it difficult for workmen who are attempting to reconnect power.
Had there been no intervention at the stricken power plant, the nuclear fuel would have completely melted within six hours, Lyman said. That would have formed a "hot pool" of fuel that would have melted through the bottom of its stainless steel shell within two hours, he said. But neither of those scenarios has come to pass.
"If the seawater pumping had not been effective, this would have ended days ago," Lyman said. But as long as workers can continue to feed water into the plant, the situation could be stabilized indefinitely, he said.
"I actually think it's an amazing thing that they have been able to maintain the cores," he added. "It is truly heroic."
However, Lyman criticized the Japanese government for failing to expand its evacuation order to all people within 50 miles of the Fukushima plant, as recommended last week by the U.S. Nuclear Regulatory Commission. Japanese officials have maintained that only those within about 12 miles of the reactor should evacuate, and that people within 18 miles should stay put but remain indoors.
"The Japanese are squandering the opportunity to be able to initiate an orderly evacuation," Lyman said. "Our concern is they are wasting valuable and precious moments."
Contamination of foodstuffs in the area surrounding the Fukushima plant is a growing concern, particularly in light of the shortages of food that are occurring in the wake of the magnitude 9 Tohoku quake that rocked the area 10 days ago. The government had already said that it had detected contaminated milk at 37 farms in the area.
Photos: Unrelenting crisis grips Japan
Now, authorities said they have also found contaminated spinach, canola and chrysanthemum greens. Monitors detected low levels of iodine-131 and cesium-137 on the leaves of the plants.
The biggest concern is not with food that is clearly too unsafe to eat, but rather with items that contain a small amount of radioactivity but still meet government safety guidelines, Lyman said.
"It certainly is going to pose a dilemma for people, to be able to trust the food they're eating," he said.