Nitrogen Injection to Continue Despite Aftershock

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.

A 7.1 magnitude earthquake triggered a tsunami warning in the vicinity of the stricken Fukushima Dai-1 Nuclear plan, at 12:32 a.m. Friday. According press reports, Tokyo Electric Power Co. (TEPCO) says there was no additional damage, and its efforts at cleaning up and controlling the plant would continue.

One key to those efforts has been the injection of nitrogen gas into the damaged containment vessel of the No. 1 reactor. That got under way around 1:30 a.m. Thursday. In a Thursday morning press briefing, a TEPCO official said that, as expected, the pressure inside the reactor’s containment vessel was “rising slightly,” indicating the operation was “going smoothly.”

The official said that as of 6 a.m. Thursday, the pressure in the reactor had risen 5 kilopascals from the 157 kPa the previous day. According to a paper published by the Citizens’ Nuclear Information Center, an antinuclear organization which has been following the events at the plant closely, the Unit 1 containment vessel is built to withstand a maximum pressure of 429 kPa; pressure inside the vessel soared to 850 kPa after the earthquake, forcing TEPCO to vent steam into the air to relieve the pressure and prevent the vessel from rupturing.

The injection of nitrogen is needed to prevent a repeat of the 12 March hydrogen explosion that partly destroyed the No. 1 reactor building. It is currently believed that hydrogen has been building up in the containment vessel because of melted zirconium in the damaged fuel-rod cladding interacting chemically with the steam and water in the vessel. Hydrogen can also be generated when radiation breaks water molecules into hydrogen and oxygen. Injecting nitrogen will thin the hydrogen and oxygen, decreasing the likelihood of another explosion.

Hidehiko Nishiyama, deputy director-general of the Nuclear and Industrial Safety Agency (NISA), has warned that there is the possibility of gases and radioactive materials escaping while the nitrogen gas is being injected. Fear of such an escape is the reason why the procedure is being undertaken slowly, over six days, and is being carefully monitored.

Meanwhile, TEPCO reported a new development following its successful stoppering yesterday of contaminated water that had been flowing into the ocean from a cracked concrete pit near the Unit 2 seawater intake area. An inspection of the vertical shaft of the trench carrying cables and pipes for the Unit 2 turbine building showed that the highly contaminated water pooling there had risen 5 centimeters. TEPCO assumes that this is the result of its having stemmed the leak from the cracked pit. TEPCO suspects the water is flowing from the turbine building into the trench, then via a series of tunnels and pipes to the cracked pit, and until yesterday on into the ocean. A TEPCO official said they have not found any other sources of water leaking into the sea.

The water in the trench shaft remains just 1 meter from the surface, so the company is closely monitoring the situation.

In a press conference Wednesday night Nishiyama responded to a question about a New York Times article based on a confidential  U.S. Nuclear Regulatory Committee (NRC) document that negatively assessed the problems of the Fukushima plant. Citing the document, the article suggested that among other problems the reactor containment vessels are under mounting stress as they fill up with radioactive water, making them vulnerable to aftershocks, which could cause them to rupture.

Nishiyama said that “members of the NRC are here in Japan, and we are working closely with them every day.” He added that NISA was sharing with them “almost all information regarding the plant.” He agreed that the containment vessels were “undergoing a rare experience …. But even if we were to have an aftershock right now, I don’t believe that would lead to any emergency or dangerous situation.”

His words seem prescient.

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