Water temperatures in the pressure vessels of three reactors at the Fukushima nuclear plant were all below 100 degrees for the first time since the crisis started, Tokyo Electric Power Co. said Sept. 28.
Water temperatures in the pressure vessels of three reactors at the Fukushima nuclear plant were all below 100 degrees for the first time since the crisis started, Tokyo Electric Power Co. said Sept. 28.
Achieving water temperatures below 100 degrees at all reactors of the Fukushima No. 1 nuclear power plant is one of the required conditions for a stable state known as "cold shutdown."
However, further action is needed to prevent radioactive substances from spreading from the reactors, eliminate the dangers of hydrogen explosions and mitigate other potential risks.
The buildings housing the No. 1 through No. 3 reactors, which were operating when the Great East Japan Earthquake hit northeast Japan on March 11, were rocked by hydrogen explosions in the days after the tsunami knocked out power to the plant.
"It would be premature to say now we have reached the cold shutdown stage," said Junichi Matsumoto, deputy head of TEPCO's Nuclear Power and Plant Siting Division. "To further stabilize the cooling system for the reactors, we will establish multiple cooling systems to ensure that any potential incident, including aftershocks of the March 11 quake, does not disrupt the cooling process."
The water temperatures in the pressure vessels were 78 degrees at the No. 1 reactor, 99.4 degrees at the No. 2 reactor and 79 degrees at No. 3 reactor as of 5 p.m. on Sept. 28.
Earlier this month, TEPCO changed its water injection methods for the No. 2 and No. 3 reactors, which had continued to record temperatures exceeding 100 degrees.
Since then, the water temperatures at the No. 3 reactor have remained below 100 degrees. The water at the No. 1 reactor has been under 100 degrees since July.
The No. 2 reactor finally dropped below 100 degrees on Sept. 28.
"The temperatures at the No. 2 reactor still fluctuate, although they are declining overall," a TEPCO official said. "We will continue to make our best efforts to cool the reactors."
A cold shutdown stage normally only requires the condition that water temperatures at a reactor remain below 100 degrees.
In the Fukushima case, however, the Nuclear and Industrial Safety Agency also included a significant reduction in radiation leaking from the reactors as a requirement for cold shutdown, given that the fuel rods had melted in the plant's reactors.
TEPCO is studying the spread of radiation from the plant by increasing the number of monitoring sites in the upper parts of the reactor buildings and areas surrounding the plant. The utility is also installing equipment to remove radioactive substances from gas leaking from the containment vessels.
Highly concentrated hydrogen detected in a pipe connected to the No. 1 reactor's containment vessel is another problem for TEPCO. The concentration level of hydrogen was 63 percent on Sept. 28, raising concerns about another explosion that could spew radiation into the atmosphere.
TEPCO said the current situation at the No. 1 reactor does not pose a danger of an explosion because the oxygen level was zero percent. A certain level of oxygen when mixed with hydrogen causes an explosion
However, hydrogen could be gathering at the No. 2 or No. 3 reactors or in other pipes at the No. 1 reactor, TEPCO said.
The company will inject nitrogen into the pipe at the No. 1 reactor on Sept. 29 to pump out the hydrogen. During the process, TEPCO will check other pipes for hydrogen accumulations.