Unit 2 temperature rise may be down to a faulty thermometer

14 February 2012

The highest temperature readings of the Fukushima Daiichi unit 2 reactor vessel bottom head have continued to rise, and on 13 February reached 94.9°C. Were the water in the vessel to reach boiling temperature, it might no longer be considered in cold shutdown.

However, a few hours after the high reading, data from the sensor in question jumped from 91.3°C to 285.4°C in the interval of an hour.

The jump casts doubt on the reliability of the temperature gauge, and also suggests the possibility that the entire temperature rise may have been an artefact of a sensor subject to drift. Although TEPCO has not publicly concluded that the sensor's readings may not reflect an actual temperature rise, it has noted that the sensor's readings since 13 February were being reviewed. A DC resistance test was carried out on 13 February; results are being evaluated.

The temperature gauge is one of three in the vessel at the same height, and one of six in the RPV (the other three are higher, at the support skirt junction). All of the other five have maintained more or less constant readings of approximately 30-40°C since 10 February. In addition, sensors monitoring the unit 2 primary containment vessel exhaust have not detected the presence of short half-life (9-hour) xenon radionuclides that would suggest the start of a fission reaction.

As a precaution, TEPCO has again increased the amount of cooling water injected into the system from the top, via the core spray system, from 7 to 7.5 m3/hr. The amount of water injected into the bottom via the feedwater injection system is 9.9 m3/hr. The amount of water injection is now more than double that of unit 1 (6.1 m3/hr), and a third again as much as unit 3 (9 m3/hr).


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