TOKYO The key to averting a nuclear disaster at the quake-stricken Fukushima power complex in Japan is ensuring the thick protective steel walls around the reactor cores remain intact.
So far that is the case, authorities say, but experts will be watching closely for any of the following developments:
* Some experts insist a Chernobyl-style nuclear disaster can be averted, despite a second hydrogen explosion on Monday at the complex, which sent a plume of smoke into the air. If they are right, authorities would be expected soon to announce that the reactors had been safely cooled down.
* So long as the thick containment walls shielding the plant's three overheating reactor cores remain intact, a major escape of radioactive clouds could be avoided, even if there was a meltdown of the nuclear fuel rods inside them.
"Everything I've seen says that the containment structure is operating as it's designed to operate. It's keeping the radiation in and it's holding everything in, which is the good news," said Murray Jennex, of San Diego State University.
"This is nothing like a Chernobyl... At Chernobyl (in the Ukraine in 1986) you had no containment structure -- when it blew, it blew everything straight out into the atmosphere."
* Low-level radiation has been detected outside the plant but at very low levels. These levels would need to rise by something like a thousand times before real fears are justified, experts say.
* Every day without a major release of radiation increases the odds of a good outcome. Once a reactor is shut down, there is a lot of decayed heat that needs to be gotten rid of, with the first four-to-seven days critical. Jennex says Japan, at three days out from the quake and shutdown, would likely "be OK" if another day goes by without a major radiation release.
* Some of the fuel rods in the Fukushima Daiichi power plant's reactors are exposed, so raising water levels around the rods is critical and will help cool them down. Pumping seawater into the reactors is a last-ditch measure, because the reactor may not be able to be used again, but it can be effective.
Again, the key is keeping the reactor container intact.
* A release of plutonium at the number 3 reactor would be a real sign of trouble because it is the only one using plutonium as a fuel. This would signal that the fuel cladding had melted and ruptured, and that a meltdown was in progress, experts say.
Similar signs from the other two reactors would be a release of uranium-235 or maybe cesium. Cesium was detected on Saturday, but the level remained low. The cesium released could also have been due to contamination from old fuel issues, not a meltdown.
* If the containment wall is breached, the level of radiation would rise sharply.
* At one point on Monday, levels at the stricken plant were around 751 microsieverts, a dosage similar to a stomach x-ray, Japans' nuclear authorities said. A fatal dose of radiation would be more than 7 million microsieverts.
* A sharp rise in radiation would be measured in millisieverts, rather than microsieverts, as it has been up to now, which would be potentially a thousand times higher.
* An explosion within the reactor vessel itself could damage the container, but no signs of that have been detected yet. The containment vessel also is designed to channel any melted fuel to separate areas, which will help keep heat from building up.
* Additional warning signs would be a total evacuation of the plant area or the wind dispersion area.
* Neighboring countries could be the first to sound the alarm about a major radioactive leak, as happened with Chernobyl, according to Jennex of San Diego State University. In Japan's case, that would most likely be Korea, China or Russia.
(Reporting by Elaine Lies; Editing by Mark Bendeich)