* NRC says No. 4 reactor fuel pool may be empty
* Dry pool could expose rods to air, start fire
* Rods could release radioactive material into atmosphere
* One expert says accident might reach level of Chernobyl (Adds further expert comments)
NEW YORK, March 16 - Japan's nuclear crisis may have taken its most dangerous turn yet after a U.S. official said one of the pools containing highly radioactive spent fuel rods at the stricken plant had run dry.
One nuclear expert said that there was now even a possibility that the disaster may approach the extent of the Chernobyl accident, the worst ever in the industry's history. When the Chernobyl plant in Ukraine exploded in 1986 it spewed a radiation cloud over a large area of Europe.
And a nuclear engineer said that it may be time to consider ways to bury or cover the entire complex in some kind of material that would stop radiation from leaking into the atmosphere.
Triggering the new levels of alarm were comments by U.S. Nuclear Regulatory Commission Chairman Gregory Jaczko in Congress on Wednesday. "There is no water in the spent fuel pool and we believe that radiation levels are extremely high, which could possibly impact the ability to take corrective measures," he said.
Japanese officials have been working desperately for two days to try to get more water into the pool to cover the rods, which remain hot for months after they are removed from the reactors and can quickly release radioactive components if exposed to the air.
"If they don't get water to these spent fuel pools in view of the containment breaches in the other plants the actual radiation releases could approach that category of Chernobyl," said Victor Gilinsky, who was an NRC commissioner at the time of the Three Mile Island accident in 1979, which was the worst nuclear accident in U.S. history.
Earlier Japanese authorities told the International Atomic Energy Agency that radioactivity was being released directly into the air at the pool for the No.4 reactor at the Fukushima Daiichi plant. Experts say the pools could present a bigger threat to public health than the reactors, which appear to be still encased in steel containment systems.
"Up until now they have not been able to get close to the spent rods, as even with protective clothing it only stops workers from breathing in radioactive particles, not from radiation itself," Dr Peter Hosemann PHD of the University of California Berkeley Nuclear Engineering Department said Tuesday.
While the building holding the rods has been rocked by fire and a blast, officials in Japan had not said how much water remained in the 40-foot deep tanks.
"There is either a leak in the spent fuel pool or the rods are hot enough to cause evaporation," Acton said.
When a fuel rod is exposed to the air the zirconium metal cladding on the rods can easily catch on fire, releasing a deadly mix of radiation into the atmosphere. The earlier fires and explosion in the area of the spent fuel storage tank left the pool partly open to the air allowing the radiation to escape into the atmosphere.
Experts say the water should remain at least eight feet over the spent fuel to maintain acceptable radiation levels but the level usually is kept much higher.
"As the water drains there's going to be less stability - and higher levels of radiation released," said Tara Neider, President and CEO of Areva Federal Services, a U.S. arm of the world's biggest nuclear power plant builder, French giant Areva SA.
Tokyo Electric Power Co (TEPCO), the operator of the six-reactor complex, was considering spraying water into the spent fuel pools through the holes in the roof by helicopter but later canceled that mission.
Arnie Gundersen, a 29-year veteran of the nuclear industry who has worked on reactors similar to the Daiichi plant and is now chief engineer at Fairewinds Associates Inc, warned that dropping water on the spent fuel pool could make matters worse.
"It's a bad idea to drop water onto the fuel racks. You could get an inadvertent criticality. That means you could have a nuclear reaction, similar to that in a reactor core, in the fuel pool," Gundersen said.
"There is more radiation in the spent fuel pool - which is about ten stories in the air -- than in the reactor core," Gundersen said, noting used rods contain more dangerous radioactive materials than new rods, including elements cesium, strontium and plutonium.
Plutonium, in particular, is a very nasty isotope and could cause cancer in very small quantities if ingested, he said.
The uranium fuel is burned in the reactor for three to six years before being placed into the pool. About one third of the fuel is removed from the reactor core to the pool every 18 to 24 months during refueling outages.
Used fuel rods must sit in the spent fuel pool for at least five years. Though, much to the consternation of environmental and anti-nuclear groups, the rods usually sit in the pool much longer while waiting for either reprocessing or storage in dry casks.
Gundersen also said he recommended evacuating children and pregnant women to more than 50 miles away from the plant to avoid the radiation risk.
In a sign that other spent fuel rod pools could be in a deteriorating condition, the NRC Chairman Jaczko said he also believed the pool at the No. 3 reactor may also have a leak.
"Every day it seems like things may be stabilizing but you wake up the next morning and it seems like things have not stabilized or maybe gotten worse," said Brian Woods, nuclear engineering associate professor at Oregon State University and a former engineer for the U.S. Department of Energy.
Woods said that it may be time to think of encasing the complex in some kind of protective material.
"More than likely you're probably looking at some kind of external containment," he said.