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TOKYO — Amid widening alarm in the United States and elsewhere about Japan’s nuclear crisis, military fire trucks began spraying cooling water on spent fuel rods at the country’s stricken nuclear power station late Thursday after earlier efforts to cool the rods failed, Japanese officials said.The development came as the authorities reached for ever more desperate and unconventional methods to cool damaged reactors, deploying helicopters and water cannons in a race to prevent perilous overheating in the spent rods of the No. 3 reactor.
Moments before the military began spraying, police officers in water cannon trucks were forced back by high levels of radiation in the same area. The police had been trying to get within 50 yards of the reactor, one of six at the plant.
The five specially fitted military trucks sprayed water for an hour, but the full impact of the tactic was not immediately clear.
The Japanese efforts focused on a different part of the Fukushima Daiichi Nuclear Power Station, 140 miles northeast of here, from the reactor — No. 4 — depicted in Washington on Wednesday as presenting a far bleaker threat than the Japanese government had offered.
The decision to focus on the No. 3 reactor appeared to suggest that Japanese officials believe it is a greater threat, since it is the only one at the site loaded with a mixed fuel known as mox, for mixed oxide, which includes reclaimed plutonium.
Western nuclear engineers have said that the release of mox into the atmosphere would produce a more dangerous radioactive plume than the dispersal of uranium fuel rods at the site. The Japanese authorities also expressed concern on Wednesday that the pressure in the No. 3 reactor had plunged and that either gauges were malfunctioning or a rupture had already occurred.
After the military’s effort to cool the spent fuel atop the reactor with fire trucks, Hidehiko Nishiyama, deputy director-general of the Nuclear and Industrial Safety Agency, said it was too early to assess the success of the attempt.
Mr. Nishiyama also said that radiation of about 250 millisievert an hour had been detected 100 feet above the plant. In the United States the limit for police officers, firefighters and other emergency workers engaged in life-saving activity as a once-in-a-lifetime exposure is equal to being exposed to 250 millisieverts for a full hour. The radiation figures provided by the Japanese Self-Defense Force may provide an indication of why a helicopter turned back on Wednesday from an attempt to dump cold water on a storage pool at the plant.
A White House statement late Wednesday said that President Obama had “briefed Prime Minister Kan on the additional support being provided by the U.S., including specialized military assets with expertise in nuclear response and consequence management.”
On Thursday a Pentagon spokesman, Col. David Lapan, said the military expertise made available to the Japanese included a nine-person assessment team that has or will shortly arrive there to work with the Japanese military and government.
The team members, Colonel Lapan said, will then recommend whether additional American military forces are needed to assist in the effort.
The American military is also gathering information on the damaged nuclear power plant. Officials said that a Global Hawk drone was flying missions over the reactor. In addition, U-2 spy planes were providing images to help the Japanese government map out its response to the quake and tsunami.
Earlier Thursday Japanese military forces dumped seawater from a helicopter on Reactor No. 3, making four passes and dropping a total of about 8,000 gallons over it as a plume of white smoke billowed. The Japanese government said that the reactor typically needs 50 tons of water, or about 12,000 gallons, a day to keep from overheating.
The Japanese military later said the measure had little effect on reducing the temperature in the pool where the spent rods are stored. A photograph from the air showed a light that seemed to suggest the presence of water, according to Tokyo Electric, but analysts said it was unclear what the image meant.
The military also announced that it had postponed plans to drop water on Reactor No. 4, which Gregory Jaczko, the chairman of the United States Nuclear Regulatory Commission, on Wednesday pinpointed as a cause for serious alarm.
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