A faulty thermometer ?

TEPCO:broken thermometer may show high temperature

http://www3.nhk.or.jp/daily/english/20120213_28.html

Tokyo Electric Power Company says a malfunctioning thermometer at the disabled Fukushima Daiichi nuclear power plant is likely to blame for the high temperature reading in one of the reactors.

The reading of one of the thermometers at the bottom of the No. 2 reactor began to rise late last month. On Sunday the temperature exceeded the critical safety threshold of 80 degrees. On Monday the reading rose to 94.9 degree Celsius at noon.

The utility firm says it thinks the thermometer is broken since the readings of 2 other thermometers set at the same height dropped to about 33 degrees.

The company says an inspection showed that a cable inside the thermometer is probably cut, resulting in a false reading.

Possible sensor failure throws Fukushima reactor temp. data into doubt

http://mdn.mainichi.jp/mdnnews/news/20120213p2a00m0na015000c.html

Doubt has been cast on the accuracy of temperature readings from the No. 2 reactor at the Fukushima No. 1 nuclear plant, as the government and plant operator Tokyo Electric Power Co. (TEPCO) increasingly suspect a temperature sensor inside the reactor has malfunctioned.

According to TEPCO, readings from the suspect sensor -- which registered 94.9 degrees Celsius at the bottom of the reactor's pressure vessel on Feb. 13 -- have been rising since the end of January this year. At the time, unable to determine if the elevated readings were real or not, the utility injected extra water into the core to prevent overheating. However, TEPCO began to suspect a malfunction on Feb. 12, when readings from the sensor -- one of three in the reactor -- fluctuated wildly between 75 and 90 degrees. The "safe" temperature for maintaining cold shutdown has been set at 80 degrees.

The sensor in question uses metal components whose electrical resistance changes with temperature, and calculates the ambient temperature based on changes in the electrical current. TEPCO plans to confirm the sensor's condition by rechecking its electrical resistance, among other measures.

However, the very fact that the sensor may have malfunctioned casts doubt on the cold shutdowns of reactors No. 1-3, declared by Prime Minister Yoshihiko Noda in December last year. The declaration was based on readings showing pressure vessel temperatures had come below 100 degrees. Since then, TEPCO has assumed a maximum margin of error of 20 degrees Celsius. Even so, the admission of a possible sensor failure calls all the temperature data collected thus far into question, and with it the condition of the reactor cores.

"Because we haven't been able to grasp how the nuclear fuel in the cores has been distributed, it's impossible to rule out localized high temperature spots," says Kazuhiko Kudo, a special professor of nuclear engineering at Kyushu University. "As the high radiation rules out installing new temperature sensors, if the last two sensors (in the No. 2 reactor) fail, the situation will be much more serious indeed."

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