Tokyo Electric Power Co. said Sunday the temperature at the bottom of the No. 2 reactor at its crippled Fukushima Daiichi nuclear plant rose further to 82 C, but the reactor has not gone critical.
While the thermometer reading at shortly after 2 p.m. marked a new high since the reactor attained a cold shutdown in December, the utility said it has confirmed that sustained nuclear reactions are not taking place in the reactor as no radioactive xenon has been detected inside its containment vessel.
Tepco reported the latest development immediately to the Nuclear and Industrial Safety Agency of the Economy, Trade and Industry Ministry as the temperature exceeded the limit of 80 C designated by the company’s safety regulation for maintaining a cold shutdown, it said.
It is considered desirable to keep the temperature below 80 C, while the bottom of a reactor pressure vessel must be kept below 100 C in a stable cold shutdown, in view of the margin of error of thermometers, according to Tepco officials.
Tecpo plans to increase the amount of water injected as a coolant by 3 tons per hour and pour 1 ton of boric acid later Sunday to prevent any event of criticality.
As a reason for what is causing the temperature rise, Tepco said it is possible the water flow is unstable and thus failing to cool the reactor stably, while also saying it will check the thermometer for any irregularities. The temperature was measured at 78.3 C at 10 a.m. and fell to 75.4 C at 11 a.m.
The decline occurred after Tepco on Saturday night increased the amount of water being injected into the reactor to 14.6 tons per hour from 13.6 tons, after seeing the temperature rise to 73.3 C at 9 p.m. It reached 74.9 C at 11 p.m. Saturday. The temperature readings began rising on Feb. 1.
One of the three thermometers at the bottom of the reactor’s pressure vessel stayed between 67 C and 71 C from Friday evening to Saturday evening after hitting 73.3 C on Monday.
Readings from two other thermometers that check the temperature at the bottom of the No. 2 reactor vessel were around 35 C, Tepco said.
The Nos. 1 to 3 reactors at the Fukushima Daiichi plant in northeastern Japan experienced meltdowns as a result of the loss of key cooling functions in the wake of the devastating earthquake and tsunami on March 11 last year.