Tokyo Electric Power Co. injected boric acid into a reactor at its crippled Fukushima nuclear plant to prevent an accidental chain reaction known as re-criticality after temperatures rose in the past week.
The temperature of the No. 2 reactor was 70.1 degrees Celsius (158 degrees Fahrenheit) as of 6 a.m. today, according to preliminary data, Akitsuka Kobayashi, a spokesman for the utility, said by phone. The reading fell from 72.2 degrees at 5 a.m. this morning, and is below the 93 degrees that’s used to define a cold shutdown, or safe state, of the reactor.
Since Feb. 1, temperatures at the bottom of the No. 2 reactor vessel have risen by more than 20 degrees Celsius, according to the company’s data. Tepco, as the utility is known, and the government announced that the Fukushima plant reached a cold shutdown on Dec. 16, nine months after the Tohoku earthquake and tsunami wrecked the nuclear station, and caused three reactors to meltdown and release radiation.
“It was too early to say the plant is safe in December. They declared cold shutdown even though nobody is sure about the location of melted fuel,” Tetsuo Ito, the head of the Atomic Energy Research Institute at Kinki University in western Japan. “A similar incident will probably occur again.”
Tepco increased the rate of cooling water being injected into the unit to 13.5 cubic meters per hour from 10.5 cubic meters per hour at 4:24 a.m. today, it said. A cold shutdown describes a reactor’s cooling system operating at atmospheric pressure and below 93 degrees Celsius, according to the U.S. Nuclear Regulatory Commission.
“Tepco is in a dilemma,” Ito said. As Tepco maintains water injection at a high rate, more radioactive water will be accumulated in the basements of plant buildings, he said.
About 95,000 cubic meters, which is enough to fill 38 Olympic-sized swimming pools, of highly radioactive water may still be in the basements, even after the company has processed more than 220,000 cubic meters of contaminated water, according to Tepco’s latest estimate on Feb. 1.
Tepco replaced coolant piping on Jan. 26 to improve reliability of equipment following water leaks caused by freezing temperatures, Taichi Okazaki, a spokesman for the utility, said by phone today. This may have led to insufficient cooling water reaching inside the reactor, according to Tepco.
Tepco found a total of 28 water leaks between Jan. 28 and Feb. 3, according to the utility. The average temperature in Namie town near the wrecked plant in January was 0.5 degrees Celsius, compared with the 2.1 degrees Celsius January average between 1981 and 2010, according to the Japan Meteorological Agency’s data.
No traces of xenon 135, which is associated with nuclear fission, were found when Tepco conducted a gas sampling of the reactor yesterday, the company said in an e-mailed statement.
In November, Tepco said it detected xenon, signs of nuclear fission, in gases taken from the No. 2 reactor, raising concerns that radiation emissions may increase. Tepco later announced the xenon was caused by “natural” nuclear fission and the plant isn’t in a critical state.