The operator of a crippled nuclear power station in north-eastern Japan started Thursday to inject nitrogen into a reactor to prevent a possible hydrogen explosion.

Tokyo Electric Power Co (TEPCO), which runs the Fukushima Daiichii Nuclear Power Plant, said the company has been pumping nitrogen gas into the containment vessel of reactor number 1. It could take about six days to put in a total of 6,000 cubic metres of nitrogen gas. The plant was crippled by the March 11 earthquake and ensuing tsunami.

Hidehiko Nishiyama, a spokesman for the government’s Nuclear and Industrial Safety Agency, called the injection a “preventive measure” and flatly denied there was an “immediate danger” of explosion.

The agency instructed the operator to monitor radioactivity in surrounding areas and to fully disclose information. Mr. Nishiyama said nitrogen gas would be also injected into reactors number 2 and 3 later.

TEPCO also estimated that 70 per cent of the nuclear fuel rods have been damaged at reactor 1, 30 per cent and 25 per cent of the fuel rods have been damaged in reactor 2 and reactor 3 respectively.

The U.S. government is making final arrangements for Secretary of State Hillary Rodham Clinton to visit Tokyo late next week to enhance bilateral cooperation in addressing the disaster in Japan, Kyodo News reported, citing unnamed U.S. diplomatic sources.

If realized, Ms. Clinton is expected to discuss with Japanese Prime Minister Naoto Kan and Foreign Minister Takeaki Matsumoto how to deal with the ongoing nuclear crisis and reconstruction efforts.

The U.S. has pledged its support for Japan, mobilising about 20,000 personnel, 140 aircraft and more than 20 ships from the military for humanitarian assistance and disaster relief efforts in Japan, Kyodo reported, citing the Pentagon.

Meanwhile, United Nations experts said Wednesday that the consequences of the Fukushima nuclear accident rate between those of past disasters at Chernobyl and Three Mile Island, while taking Tokyo to task for a lack of adequate data.

The Vienna-based U.N. Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) announced it would produce a report on the health effects of the Fukushima nuclear crisis within two years.

“It is not as dramatic as Chernobyl, but it is certainly much more dramatic than Three Mile Island,” UNSCEAR chairman Wolfgang Weiss told reporters.

While not much radioactivity was released in the 1979 reactor meltdown at the U.S. nuclear plant at Three Mile Island in Pennsylvania, Mr. Weiss noted that “we have seen traces of iodine in the air all over the world now.” The earthquake, the largest ever recorded in Japan, and the resultant tsunami killed a confirmed 12,596 people, but as of 10 a.m. (0100 GMT) on Thursday, 14,747 people were still listed as missing, Japan’s National Police Agency said.

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