Nitrogen Injected at Reactor to Prevent Explosion

Special Report: Fukushima and the Future of Nuclear Power

Editor's Note: John Boyd is an IEEE Spectrum contributor reporting from Kawasaki, Japan. This is part of IEEE Spectrum's ongoing coverage of Japan's earthquake and nuclear emergency. For more details on how Fukushima Dai-1's nuclear reactors work and what has gone wrong so far, see our explainer and our timeline.

Tokyo Electric Power Co (TEPCO), operator of the troubled Fukushima Dai-1 nuclear plant, began work aimed at heading off explosions like the ones that damaged buildings housing reactors in the first few days following the 11 March earthquake and tsunami.

“We assume the Number 1 reactor core is damaged and that hydrogen is accumulating inside the containment vessel,” a TEPCO official told reporters. In a press briefing, a company official said TEPCO would start injecting nitrogen gas into reactor No. 1 as early as Wednesday evening, in an effort to thin the concentration of hydrogen and prevent its reacting with oxygen.

Hidehiko Nishiyama, deputy director-general of the Nuclear and Industrial Safety Agency (NISA), told reporters Wednesday morning that there was no immediate danger of an explosion. “This measure is being taken in advance to prevent that happening,” he said.

NHK, Japan’s national broadcaster, reported that the power company planned to inject some 6000 cubic meters of nitrogen gas into the reactor over the next six days. And a TEPCO official added that it is also considering injecting nitrogen into the No. 2 and No. 3 reactors.

All three reactors underwent a loss of power after the 11 March tsunami took out primary and backup power systems. As a result cooling water levels fell and exposed the fuel rods in the three reactors. It is assumed that the resulting heat partially melted the rods’ zirconium cladding, which then reacted with steam and produced hydrogen. A high concentration of hydrogen carries the risk of exploding in the presence of oxygen. In the first of at least two explosions at the Fukushima plant, a hydrogen blast on 12 March damaged walls and took off the roof of the No. 1 reactor building.

“In normal conditions nitrogen is used to purge the air from the containment vessel during operations,” Kazuaki Matsui, executive director of the Institute of Applied Energy, an independent research organization, told IEEE Spectrum. “If you have an intrusion of air and there is no nitrogen, only air and hydrogen, it is very dangerous.”

In opposition to the unsettling idea of another hydrogen explosion, TEPCO had some welcome news to report Wednesday. That morning, the company announced that it had finally managed to stop water contaminated with radioactive material from pouring into the sea through a cracked concrete pit near the No. 2 reactor's seawater intake area.

“At 5:38 a.m. on April 6, we observed the stoppage of water spilling from the crack on the concrete lateral of the pit,” said a company official. NHK reported that the company had injected 6000 liters of liquid glass (sodium silicate) through a series of holes into a gravel layer between the pit and the reactor. TEPCO suspects the water is leaking from a damaged pipe upstream from the pit and is collecting in the gravel before spilling out into the sea. The liquid glass was intended to solidify the gravel, and it seems to have worked.

But, in a morning update for the press, Yukio Edano, the Chief Cabinet Secretary, was cautious about TEPCO’s success in halting the leak, asking rhetorically: “But has it completely stopped? Are there any other routes where the water is being released?” He added that TEPCO was now surveying and investigating the situation to answer these questions and verify the source of the leak. “What we can say right now,” said Edano, “is that we cannot be optimistic just because we were able to halt this (leak).”

Meanwhile efforts to make room for the contaminated water pooling in turbine basements and in outside trenches carrying cables and pipes continue for reactors No. 1, 2, and 3. The Nuclear and Industrial Safety Agency (NISA) estimates that the combined total of such water is 60 000 metric tons, some of it, especially the water from reactor No. 2, highly radioactive.

In order to create storage space, the seemingly drastic decision was taken to release into the ocean 11 500 tons of low-level contaminated water from two onsite containers. NHK reported that as of Wednesday evening most of this water had been discharged. Government spokesman Edano the measure was regrettable but inevitable, and that the contamination level in the water that was released was “one two-hundred-thousandth” that in some of the radioactive water pooled in the turbine basements and trenches. He also admitted that Japan should have explained to neighboring countries why it was taking this measure ahead of time, and that the foreign ministry would now inform neighboring countries if more such important actions become necessary.

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