Tokyo Electric Power Co. slowed the pace of water injection into one of the six reactors at its crippled Fukushima Dai-Ichi nuclear station after a drop in pressure and temperature increased the risk of an explosion.
“We wouldn’t want the pressure of the containment vessel to fall below that of the atmosphere,” Junichi Matsumoto, a general manager at the company known as Tepco, said at a media briefing in Tokyo today. The water flow was reduced to 6 cubic meters an hour today from 10 cubic meters, and nitrogen is also being injected to help avoid a hydrogen explosion, he said.
Tepco will take longer to cool the reactor by flooding it as it struggles to contain the worst nuclear accident since Chernobyl in 1986. Four reactors have been damaged by hydrogen blasts after the March 11 earthquake and ensuing tsunami knocked out backup power and cooling systems, and radiation leaks have forced the evacuation of tens of thousands of people.
“When the pressure inside the reactor containment vessel falls below that of the atmosphere, air could enter through gaps and mix with hydrogen and cause an explosion,” Kazuhiko Kudo, a research professor of nuclear engineering at Kyushu University, said by telephone today.
Tepco’s Matsumoto said the concentration of hydrogen inside the reactor is less than 1 percent, and with the injection of nitrogen the risk of an explosion “is not high.” The containment vessel can still be flooded to the brim at the current rate of inflow, he said.
The U.S. Nuclear Regulatory Commission, which has sent engineers to help Japan contain the country’s biggest civilian atomic disaster, said yesterday the crisis “has definitely improved” even though it isn’t yet “quite stable.”
“The Japanese are making progress,” Bill Borchardt, executive director for operations at the U.S. atomic regulator, said at a meeting at NRC’s headquarters in Rockville, Maryland.
Tepco has used fire engines and external pumps to pour water to prevent reactors and spent-fuel pools from overheating. Contaminated water has overflowed and accumulated in basements and trenches near the buildings, preventing workers from repairing damaged cooling equipment.
The utility was forced to dump some of the tainted water into the sea and is trying to pump some of it to a waste storage facility. The company is adding storage capacity and is building a water decontamination unit using technology from Areva SA and Kurion Inc. that’s expected to be operational in June.
Tepco today halted the transfer of radioactive water from trenches near the No. 2 reactor to check for leaks at the waste treatment building, spokesman Daisuke Senda told reporters in Tokyo today. Pumping will resume in one to two days, he said.