Hydrogen accumulates in pipes at Fukushima's No. 1 reactor

September 24, 2011

By HIDENORI TSUBOYA / Staff Writer

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, the plant's operator said Sept. 23.

But Tokyo Electric Power Co. officials said an explosion was unlikely because nonflammable nitrogen was being pumped 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.

TEPCO said it is investigating the possibility that 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 had been generated immediately after the onset of the crisis on March 11 or in later stages. Nor could TEPCO measure how much hydrogen may have been generated in the containment vessel.

The nitrogen injections are believed to have lowered the hydrogen concentration considerably, but some hydrogen, being lighter than nitrogen, may be accumulating near the top of the containment vessel without being driven out.

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 was found to exceed 1 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 generated by a reaction under high temperatures between water vapor and the surface of nuclear fuel rods that were 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 is also intended to create higher pressure levels than those outside to prevent oxygen in the air from entering the containment vessels.

In the early stages of the disaster, the Fukushima nuclear plant was rocked by hydrogen explosions that spewed radioactive materials into the environment.

By HIDENORI TSUBOYA / Staff Writer
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Work to put a protective cover over the stricken No. 1 reactor of the Fukushima No. 1 nuclear power plant continued on Sept. 15. (Tokyo Electric Power Co.)

Work to put a protective cover over the stricken No. 1 reactor of the Fukushima No. 1 nuclear power plant continued on Sept. 15. (Tokyo Electric Power Co.)

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  • Work to put a protective cover over the stricken No. 1 reactor of the Fukushima No. 1 nuclear power plant continued on Sept. 15. (Tokyo Electric Power Co.)
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