Tokyo:
Japan's nuclear crisis verged toward catastrophe on Tuesday after an explosion damaged the vessel containing the nuclear core at one reactor and a fire at another spewed large amounts of radioactive material into the air, according to statements from Japanese government and industry officials.
In a brief morning address to the nation Tokyo time, Prime Minister Naoto Kan pleaded for calm, but warned that radiation had already spread from the crippled reactors and 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 Fukushima Daiichi Nuclear Power Station the worst nuclear accident since the Chernobyl reactor disaster a quarter century ago.
Engineers at the plant, working at tremendous personal risk, on Tuesday continued efforts to cool down the most heavily damaged unit, reactor No. 2, by pumping in seawater. According to government statements, most of the 800 workers at the plant had been withdrawn, leaving 50 or so workers in a desperate effort to keep the cores of three stricken reactors cooled with seawater pumped by firefighting equipment, while crews battled to put out the fire at the No. 4 reactor, which they claimed to have done just after noon on Tuesday.
But late Tuesday, Japan's nuclear watchdog said a pool storing spent fuel rods at that fourth reactor had overheated and reached boiling point and had become unapproachable by workers at the plant. The fire earlier Tuesday morning was sparked by a hydrogen explosion generated by rising temperatures at the fuel pool, which released radioactivity directly into the atmosphere.
The fourth reactor had been turned off and was under refurbishment for months before the earthquake and tsunami hit the plant on Friday. But the plant contains spent fuel rods that were removed from the reactor. If these rods had run dry, theycould overheat and catch fire. That is almost as dangerous as the fuel in working reactors melting down, because the spent fuel can also spew radioactivity into the atmosphere.
Shigekatsu Oomukai, a spokesperson for the Nuclear and Industrial Safety Agency, said the substantial capacity of the pool meant that the water in the pool 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.
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.
Radiation measurements reported on Tuesday showed a spike of radioactivity around the plant that made the leakage significantly worse than it had been, with levels measured at one point as high as 400 millisieverts an hour. Even 7 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 likely lead to acute radiation sickness.
The extent of the public health risk depends on how long such elevated levels persist, as well as how far and fast the radioactive materials spread, and whether the limited evacuation plan announced by the government proves sufficient.
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 levels had dropped since then.
The government said later Tuesday that radiation levels at the Fukushima plant also appeared to be falling sharply.
Japan has officially requested assistance from the United States Nuclear Regulatory Commission, as well as the International Atomic Energy Agency. But on Tuesday, the United States Forces Japan said the Fukushima plant had turned away two fire trucks that had made their way to the plant to offer assistance.
"They said they didn't need them," said Sgt. Maj. Steve Valley of the military public affairs office. "So they came right back."
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 that has been ongoing 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. We are now facing the worst-case scenario," said Hiroaki Koide, a senior reactor engineering specialist at the Research Reactor Institute of Kyoto University. "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."
The steel containment vessels that protect nuclear fuel in reactors are considered crucial to maintain the integrity of the reactor and the safety of the fuel.
Mr. Edano, however, said that the level of leaking at the No. 2 reactor remained small, raising the prospect that the container was sufficiently intact to protect the nuclear fuel inside.