Tokyo Electric Power Co. said it started injecting nitrogen early Thursday into reactor 1 at the Fukushima No. 1 nuclear plant to purge the hydrogen inside and prevent an explosion, and the process went smoothly in the afternoon.

Tepco meanwhile said it believes 25 percent of the fuel rods in reactor No. 3’s core were damaged as of March 15, with their casings ruptured or melted from earlier overheating caused by temporary loss of coolant water. The damage could lead to the release of strong radioactive materials from the reactor’s core.

Tepco said earlier that an estimated 70 percent of the fuel rods in the No. 1 reactor and 30 percent in the No. 2 reactor were damaged as of March 15. This was the first time the utility issued an estimate for reactor 3’s core.

According to Tepco, the operation to inject about 6,000 cu. meters of nitrogen into reactor 1’s containment vessel is expected to last six days. Tepco is also considering injecting nitrogen into reactors 2 and 3.

According to a New York Times report, U.S. experts advising the Japanese government were particularly concerned about a possible hydrogen explosion in reactor No. 1, which could severely damage its integrity and lead to the release of large amounts of radioactive materials. Hydrogen, which is believed being generated inside reactors 1, 2 and 3, could explode if it reacts with oxygen.

According to the Nuclear and Industrial Safety Agency, the pressure inside the No. 1 reactor had risen to 0.168 megapascal by 9:30 a.m. from 0.156 megapascal at 1:31 a.m., when the procedure started, showing the nitrogen was entering smoothly.

Purging the hydrogen from the reactor could also result in release of some radioactive materials. Tepco claimed it was closely monitoring radiation levels in and around the plant compound.

Workers around all four reactors were ordered to temporarily evacuate at the beginning of the operation due to the possible leakage of radioactive materials. But Chief Cabinet Secretary Yukio Edano said they were back at work later in the morning.

“There was a certain amount of risk at the beginning of the infusion of nitrogen and we took measures just in case the unexpected happened,” Edano said. But “the nitrogen is being injected properly and I have been told that the workers have resumed their jobs at the reactors.

“Ever since the March 11 earthquake, preventing an explosion inside the reactors has been one of the main goals of our countermeasures,” Edano said.

“The possibility of a hydrogen explosion at the moment is not necessarily high, but we wanted to bring it down to as close to zero as possible.”

Meanwhile, Tepco has decided not to estimate the percentage of damaged fuel rods beyond March 15 because it can’t obtain reliable data, a company official said.

The utility made the estimate by measuring the rate of change in the amount of gamma rays inside the pressure vessels for reactors 1, 2 and 3, which hold nuclear fuel rods. Reactor 4 has no fuel rods inside its core because it was shut down before the quake.

“Judging from the temperature of the pressure vessels, the damage to the fuel rods may be greater now (than 70 percent) for reactor No. 1, while No. 2 may not have changed much,” said Ken Nakajima of the Kyoto University Research Reactor Institute.

Still, Nakajima said the fact that the radiation level of water in the turbine building of reactor No. 1 isn’t that high means damage to the reactor’s pressure vessel and containment vessel — two key barriers preventing the release of radioactive materials — must have been relatively small.

“It is lucky that the containment function at reactor No. 1 is not so damaged,” he said.

He also said it makes sense that Tepco began pumping nitrogen into reactor 1’s containment vessel first because the level of damage to its fuel rods means it poses the greatest danger of releasing radioactive substances should it explode.

Nakajima said the damage to pressure and containment vessels of the No. 2 reactor is probably greater than that of No. 1 and No. 3, judging from the level of radiation in water in their pipe trenches.

Tepco said March 27 that the radiation level in reactor No. 1’s trench was 0.4 millisievert, while that in reactor No. 2’s trench was more than 1,000 millisieverts. It could not monitor the level at No. 3 because of debris.

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