From Kyodo News (2/14/2012):
As the temperature at the bottom of the Reactor 2 Pressure Vessel shot up extremely high, TEPCO announced on February 14 that the company had conducted the test of 41 thermocouples on the RPV, and found 8 of them showing abnormality.
In addition to the thermocouple that exhibited the temperature above 400 degrees Celsius, 2 other thermocouples had abnormal electrical resistance. 5 had been deemed broken. TEPCO will continue to monitor the temperature trend of the RPV with the remaining 33 thermocouples.
As for the request from the Nuclear and Industrial Safety Agency to submit a plan to monitor the temperatures at the locations where the thermocouples failed, TEPCO says, "It is difficult to replace the thermocouple, but we would like to consider further whether it is possible at all."
8 out of 41 showing abnormality? That's not quite what I read in the TEPCO's report in December 2011. Page 1-64 of the report has a table that shows the numbers of thermocouples in Reactors 1, 2, 3 that were evaluated with equivalent circuit. In the case of Reactor 1 RPV,
No. of thermocouples that can be used with equivalent circuit: 32
No. of thermocouples that cannot be used with equivalent circuit: 1
No. of normal (functioning) thermocouples: 2
So, as far as I can understand (correct me if I'm wrong, as I could be very wrong) one of the 32 thermocouples on the Reactor 2 RPV shows a certain value, and TEPCO compares it with the value of the equivalent circuit, and figure out over time whether the value shown by the thermocouple can be used after equivalent circuit calibration.
TEPCO's report says the standard deviation for Reactor 1 RPV's thermocouples is about 15 degrees Celsius, and 8 degrees Celsius for Reactor 2 RPV's thermocouples. Thus the margin of 20 degrees Celsius mentioned by TEPCO for the maintenance of "cold shutdown state", I think, to be on the safe side of 15 degrees Celsius.
The report says 14 thermocouples on the Reactor 3 RPV can be used with equivalent circuit calibration, while 18 thermocouples cannot. There is no normal thermocouple on the Reactor 3 RPV.
Hmmm. Maybe it is Reactor 3 we should be worrying about.
FYI, the latest temperature information of Reactor 3's RPV is here.