Off-the-chart temperature readings at the Fukushima No. 1 nuclear power plant's damaged No. 2 reactor are due to a faulty thermometer and no cause for alarm, according to plant operator Tokyo Electric Power Co.
TEPCO's assessment is backed by the government’s Nuclear and Industrial Safety Agency and the Atomic Energy Society of Japan, which comprises academics and nuclear specialists.
The utility also said there was no evidence of the release of nuclear material that would suggest a heat-producing chain reaction known as nuclear fission, which could be occurring if the alarmingly high readings were accurate.
It said other thermometer readings remained stable.
The temperature at the base of the No. 2 reactor’s pressure vessel went over 400 degrees and became unmeasurable on the afternoon of Feb. 13 after steady spikes for several days.
Aside from the one thermometer, readings on 34 other devices remained flat or showed slight drops in temperature.
The temperature surge initially triggered alarm, giving rise to speculation that the government could be forced to repudiate its announcement in December that a state akin to cold shutdown had been achieved. The term refers to the state in which temperatures at the base of the crippled reactor's pressure vessels are below 100 degrees, government officials said.
In its announcement, the government also said it had reduced the volume of radioactive materials spewing from the plant.
Given the fact the thermometers were exposed to extremely high temperatures following a meltdown at the reactor last March, readings can be off by as much as 20 degrees.
The loss of all power sources at the nuclear complex following the Great East Japan Earthquake and tsunami led to meltdowns at three reactors.
According to TEPCO, the thermometer in question had a reading of 272.8 degrees at 6 p.m. on Feb. 13. It later soared to beyond 400 degrees, the maximum figure that can be measured.
TEPCO officials attributed the glitch to a broken cable connection.
Two other thermometers set at the same height as the one showing unusually high temperatures gave readings of 31.5 degrees and 32.3 degrees.
One installed right below the faulty thermometer gave a reading of 38.4 degrees.
TEPCO pointed out that the other devices would have had much higher readings if the temperature levels had really reached 400 degrees.
Some 35 thermometers are set around the outer wall of the reactor’s pressure vessel, which is about 22 meters tall and has an inner diameter of 5.6 meters.
Figures recorded near the base of the reactor, where melted nuclear fuel ended up, are slowly dropping as more cooling water is pumped in, TEPCO said.
Readings in the upper part of the reactor pressure vessel remained flat despite the increased flow of cooling water.
TEPCO cited additional grounds for its conclusion.
It noted that readings have been all over the place, fluctuating significantly in a range of 70 degrees but under 100 degrees since the afternoon of Feb. 12.
It said the readings were inexplicably high, given that 10-degree water is flowing inside the steel wall of the pressure vessel.
TEPCO also said that cesium levels around the nuclear complex had not changed despite the high readings. This would have been expected, along with more steam, if the temperature had risen significantly.
“It is not plausible that the temperature at one location is soaring when the overall trend of the temperatures is downward,” said Takashi Sawada, vice chairman of the Atomic Energy Society of Japan. “It (the specific thermometer) must be broken, since other devices nearby show low readings.”
The device in question is known as a thermocouple, which is a widely used type of temperature sensor for measurement and control.
(This article was written by Hidenori Tsuboya and Tatsuyuki Kobori.)
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