U.S. sees new and growing Japan nuclear threats

In this March 24, 2011 photo, the inside of the Unit 4 reactor at the Fukushima Dai-ichi nuclear power plant is seen in Okumamachi, northeastern Japan.

 (Credit: AP/TEPCO)

A team of experts from the U.S. Nuclear Regulatory Commission warned in a report about a week and a half ago that the very methods being used to try and cool down Japan's stricken nuclear plant may be increasing the risk of explosions or cracks in the reactor buildings, reports The New York Times.

According to a report dated March 26 and obtained by the Times, experts warn that the millions of gallons of water -- most of it seawater - that have been dumped into the overheating reactors to cool fuel rods adds to pressure on the already-battered surrounding containment structures.

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That pressure makes it more likely, according to the report, that future aftershocks could crack the containment structures, leading to cracks from which highly-radioactive water can escape into the environment.

One such crack, which allowed contaminated water to spill directly into the sea for days, was finally plugged by workers on Wednesday.

But in a sign that workers still face several challenges before the overheating reactors are stabilized, Tokyo Electric Power Co. said it plans to inject nitrogen gas into one of the reactors. Nitrogen can prevent highly combustible hydrogen from exploding -- as it did three times at the compound in the early days of the crisis.

There is no immediate possibility of an explosion, but the "nitrogen injection is being considered as a cautionary measure," said spokesman Hidehiko Nishiyama of Japan's Nuclear and Industrial Safety Agency.

TEPCO said the process could begin as early as Wednesday evening in Unit 1 -- where pressure and temperatures are the highest -- according to spokesman Junichi Matsumoto. The same measures will eventually be taken at the other two troubled reactors.

The NRC report obtained by the Times also cited an increased risk of explosions due to the release of hydrogen and oxygen from the overheating fuel rods. Salt from the seawater was caking around the fuel rods as the water evaporated away, making it more difficult for the fresh water being added to circulate around the cores and cool them down.