Tokyo:
A small crew of technicians braved radiation and fire through the day on Tuesday as they fought to prevent three nuclear reactors in northeastern Japan from melting down and stop storage ponds loaded with spent uranium fuel pods from bursting into flames.
Tokyo Electric Power Company officials announced on Tuesday evening that they were preparing to use helicopters in an attempt to douse with cold water a boiling rooftop storage pond for spent uranium fuel rods. The rods are still radioactive and potentially as hot and dangerous as the fuel rods inside the reactors if not kept submerged in water.
(Read: Spent fuel rods a long-term risk) With hydrogen gas bubbling up from chemical reactions set off by the hot fuel rods, the storage pond produced a fire and powerful explosion on Tuesday morning that blew a 26-foot-wide hole in the side of Reactor 4 at the Fukushima Daiichi nuclear power plant.
(Read: Third blast at N-plant) Reactors 4, 5 and 6 at the plant, 140 miles northeast of Tokyo, were not operating on Friday afternoon when an offshore earthquake with a magnitude now estimated at 9.0 suddenly shook the site. A tsunami with waves up to 30 feet high rolled into the northeast Japanese coastline minutes later, swamping the plant.
(In Pics: Japan earthquake triggers tsunami)(Watch: Biggest quake in 140 years) At least 750 workers evacuated on Tuesday morning after a separate explosion ruptured the inner containment building at reactor No. 2 at the Daiichi plant, which was crippled by Friday's earthquake and tsunami. The explosion released a surge of radiation 800 times more intense than the recommended hourly exposure limit in Japan.
(Read: Most plant workers evacuated, radiation levels rise) But 50 workers stayed behind, a crew no larger than would be stationed at the plant on a quiet spring day. Taking shelter when possible in the reactor's control room, which is heavily shielded from radiation, they struggled through the morning and afternoon to keep hundreds of gallons of seawater a minute flowing through temporary fire pumps into the three stricken reactors, where overheated fuel rods continued to boil away the water at a brisk pace.
By early afternoon radiation levels had plunged, according to the International Atomic Energy Agency in Vienna. Workers have released surges of radiation each time they bleed radioactive steam from the troubled reactors in an attempt to manage the pressure inside the reactors, but the reactors are not yet releasing high levels of radiation on a sustained basis, Japanese officials said.
In a brief morning address to the nation, Prime Minister Naoto Kan pleaded for calm but warned that radiation that had leaked earlier had already spread from the crippled reactors and that there was "a very high risk" of further leakage.
The sudden turn of events, after an explosion Monday at one reactor and then an early-morning explosion Tuesday at yet another -- the third in four days at the plant -- already made the crisis at the plant the worst nuclear accident since the Chernobyl reactor disaster a quarter century ago. It had already become impossible for workers to remain at many areas within the plant for extended periods, the nuclear watchdog said.
Shigekatsu Oomukai, a spokesman for the Nuclear and Industrial Safety Agency, said the substantial capacity of the pool at reactor No. 4 meant that the water in it was unlikely to evaporate soon. But he said workers were having difficulty reaching the pool to cool it because of the high temperature of the water.
Worryingly, temperatures appeared to be rising in the spent fuel pools at two other reactors at the plant, No. 5 and No. 6, said Yukio Edano, the chief cabinet secretary.
Earlier Tuesday the Japanese government told people living within about 20 miles of the Daiichi plant to stay indoors, keep their windows closed and stop using air conditioning. More than 100,000 people are believed to be in that area.
Mr. Kan, whose government was extraordinarily weak before the sequence of calamities struck the nation, told the Japanese people that "although this incident is of great concern, I ask you to react very calmly." And in fact there seemed to be little panic but huge apprehension in a country where radioactivity brings up memories of Hiroshima and Nagasaki, the haunting images of post-war Japan.
(Read: For Japan's elderly, echoes of war's horrors)Radiation measurements reported earlier Tuesday showed a spike around the plant that revealed that the leakage was significantly worse than it had been, with radiation levels measured at one point as high as 400 millisieverts an hour. Even seven minutes of exposure at that level will reach the maximum annual dose that a worker at an American nuclear plant is allowed. And exposure for 75 minutes would probably lead to acute radiation sickness.
By late Tuesday the measurements at the main gate of the plant were down to 0.6 millisieverts per hour, a drop from the 11.9 millisieverts per hour observed six hours earlier, Japanese officials told the International Atomic Energy Agency.
The extent of the public health risk depends on how long people are exposed to such elevated levels as well as how far and fast the radioactive materials spread, and whether the limited evacuation plan announced by the government proves sufficient.
(Read: Japan's multiple crises)In Tokyo, 170 miles south of the plant, the metropolitan government said Tuesday it had detected radiation levels 20 times above normal over the city, though it stressed that such a level posed no immediate health threat and that readings had dropped since then.
Japan has requested assistance from the United States Nuclear Regulatory Commission, as well as the International Atomic Energy Agency. The succession of problems at Daiichi was initially difficult to interpret, with confusion compounded by incomplete and inconsistent information provided by government officials and executives of the plant's operator, Tokyo Electric Power Company.
But industry executives in close contact with officials in Japan expressed extreme concern that the authorities were close to losing control over the fuel melting in three reactors at Daiichi, especially at the crippled No. 2 reactor where the containment vessel was damaged.
Tokyo Electric Power said Tuesday that after the explosion at the No. 2 reactor pressure had dropped in the "suppression pool" -- a section at the bottom of the reactor that converts steam to water and is part of the critical function of keeping the nuclear fuel protected. After that occurred radiation levels outside No. 2 were reported to have risen sharply.
"We are on the brink," said Hiroaki Koide, a senior reactor engineering specialist at the Research Reactor Institute of Kyoto University. "We are now facing the worst-case scenario. We can assume that the containment vessel at Reactor No. 2 is already breached. If there is heavy melting inside the reactor, large amounts of radiation will most definitely be released."
Another executive said the chain of events at Daiichi suggested that it would be difficult to maintain emergency seawater cooling operations for an extended period if the containment vessel at one reactor had been compromised because radiation levels could threaten the health of workers nearby.
If all workers do in fact leave the plant, the nuclear fuel in all three reactors is likely to melt down, which would lead to wholesale releases of radioactive material -- by far the largest accident of its kind since Chernobyl.
Even if a full meltdown is averted, Japanese officials have been facing unpalatable options. One was to continue flooding the reactors and venting the resulting steam, while hoping that the prevailing winds did not turn south toward Tokyo or west, across northern Japan to the Korean Peninsula. The other was to hope that the worst of the overheating was over, and that with the passage of a few more days the nuclear cores would cool enough to essentially entomb the radioactivity inside the plants, which clearly will never be used again. Both approaches carried huge risks.
While Japanese officials made no comparisons to past accidents, the release of an unknown quantity of radioactive gases and particles -- all signs that the reactor cores were damaged from at least partial melting of fuel -- added considerable tension to the effort to cool the reactors.
"It's way past Three Mile Island already," said Frank von Hippel, a physicist and professor at Princeton. "The biggest risk now is that the core really melts down and you have a steam explosion."
The sharp deterioration came after a frantic day and night of rescue efforts focused largely on the No. 2 reactor. There, a malfunctioning valve prevented workers from manually venting the containment vessel to release pressure and allow fresh seawater to be injected into it. That meant that the extraordinary remedy emergency workers had jury-rigged to keep the nuclear fuel from overheating no longer worked.
As a result, the nuclear fuel in that reactor was exposed for many hours, increasing the risk of a breach of the container vessel and more dangerous emissions of radioactive particles.
By Tuesday morning, Tokyo Electric Power said that it had fixed the valve and resumed seawater injections, but that it had detected possible leaks in the containment vessel that prevented water from fully covering the fuel rods.
Then an explosion hit that reactor. After a series of conflicting reports about what level of damage was inflicted on the reactor after that blast, Mr. Edano, the chief cabinet secretary, said, "there is a very high probability that a portion of the containment vessel was damaged."