Hydrogen has accumulated to a level higher than previously thought in pipes connected to the No. 1 reactor containment vessel at the Fukushima No. 1 nuclear power plant, Tokyo Electric Power Co. (TEPCO) officials said.

But TEPCO officials said an explosion was unlikely because they were pumping nonflammable nitrogen in to prevent oxygen from entering the containment vessel and triggering a blast. TEPCO also said the discovery of the hydrogen buildup will not affect the road map toward achieving a cold shutdown and other schedules.

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TEPCO is looking at the possibility hydrogen has also accumulated in a similar manner at the plant’s No. 2 and No. 3 reactors.

However, the company acknowledged there was no way to tell whether the hydrogen in the pipes came right after the onset of the crisis on March 11 or in later stages. Nor could TEPCO measure how much hydrogen generated in the containment vessel.

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Nitrogen injections have lowered the hydrogen concentration considerably, but some hydrogen, being lighter than nitrogen, may be accumulating near the top of the containment vessel.

The discovery of the hydrogen accumulation came when TEPCO was analyzing gas in the pipes connected to the interior of the No. 1 reactor containment vessel while installing a device to reduce the amount of radioactive substances leaking from the vessel.

The hydrogen concentration exceeded one percent, the threshold of the measurement device. TEPCO said it was conducting a more detailed analysis on the concentration level.

TEPCO said most of the accumulated hydrogen was the result of a reaction under high temperatures between water vapor and the surface of nuclear fuel rods exposed after water was lost following the March 11 earthquake and tsunami.

Even now, the damaged reactors may be generating small amounts of hydrogen as water decomposes through irradiation from the melted fuel rods.

An explosion can occur in a gas containing more than 4 percent hydrogen and more than 5 percent oxygen.

TEPCO has been pumping nitrogen into the No. 1, No. 2 and No. 3 reactor containment vessels to drive out hydrogen from their interiors. The injection of nitrogen also should create higher pressure levels than those outside to prevent oxygen in the air from entering the containment vessels.

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