TOKYO/NEW YORK (Reuters) - Japan is counting on four to eight inches of super strong steel to prevent the crisis at its stricken nuclear plant from becoming a radioactive disaster.
The steel containment vessel, 60 feet high and 16 feet wide, is the most critical line of defence protecting the outside world from the nuclear core. Most experts are confident the unit can hold, even in the event of a full-scale meltdown.
Still, the design has been tested only once: at the Three Mile accident in 1979. Conditions are far worse at the Fukushima plant on Japan's northeastern coast, rocked by a massive earthquake and tsnumani.
"Will the steel liner hold? That is the million dollar question," said Michael Marriott, head of Nuclear Information and Resource Service, an environmental group based in Takoma Park, Maryland.
"If they can't cool the rods at some point relatively soon -- maybe just over 24 hours -- the temperatures will be hot enough to melt steel."
Beyond that steel shell, only a larger concrete-and-steel containment dome stands between a potentially lethal radioactive mix and the public. Up to 185,000 people have already been evacuated to a 20-kilometer radius.
Workers at TEPCO are scrambling to keep the uranium fuel rods covered with seawater at reactor No. 2. The fuel in the two other reactors at Daiichi are already being kept cool by seawater pumped in from the Pacific Ocean.
Operator Tokyo Electric Power Co (TEPCO) said fuel rods at the No. 2 reactor had been fully exposed when water levels keeping them cool suddenly dropped.
U.S. government nuclear officials, who asked not to be named, said in the worst-case scenario a full-scale meltdown could produce temperatures above 2000 degrees F that might breach the steel, though that is not their working premise.
Pumping seawater into the core and over the rods may not be as simple as it sounds. Water is pumped in from the ocean via a pipe into the cooling system, but operators must release an almost equal amount of radioactive gas into the next containment structure to equalize the pressure.
That gas will contain radioactive particles and hydrogen, which can explode when mixed with oxygen. Experts believe that caused the blasts at the outer shells of the reactors at Units 1 and 3 at the Daiichi plant.
Experts do not believe the situation will reach the catastrophic scale of the 1986 explosion at the Soviet Chernobyl plant in Ukraine. Chernobyl, the world's worst civilian nuclear disaster, was blamed for thousands of deaths due to radiation-linked illnesses.
Yet conditions are far from stable. On Tuesday morning, another explosion was heard at Unit 2. Officials said it too appeared to be caused by a build-up of hydrogen.
"The primary containment building, the steel vessel surrounding the reactor, can cope with a full core meltdown," said Professor Mark Prelas, Professor of Nuclear Engineering at the University of Missouri.
"That is what it's designed to do. There is no risk of a nuclear explosion here. This is nothing like a Chernobyl event when they had no containment structure around the core and a fire."
Concern over conditions at the No. 2 reactor were heightened on Monday after Japan's nuclear safety agency failed to give it a classification on the accepted international scale for nuclear events. Units 1 and 3 were rated at a 4 on the 7-step scale; Unit 2 was conspicuous by its absence.
France's nuclear safety authority said the accident could be classified as a level 5 or 6 on the scale, putting it on par with the 1979 U.S. Three Mile Island meltdown.