A thermocouple device in the pressure vessel of reactor 2 at Tokyo Electric Power Co.’s Fukushima No. 1 power plant read over 285 degrees Monday but Tepco dismissed the reading, claiming the device is faulty.
Two similar reactor 2 thermometers at the same height — about 3 meters from the bottom of the pressure vessel — as the apparent faulty one gave readings of about 31 to 33 degrees Monday. The problematic one read over 90 degrees in the morning but hit 285.4 at 3 p.m. after Tepco checked its status by gauging its electrical resistance.
The resistance was about 1.7 times more than the average, so some cables are probably partially disconnected and causing the malfunction, Tepco said, adding the temperature readings tend to show higher numbers when cables are disconnected.
“Considering the result (of the test), we are quite certain that the thermometer is not working properly,” Tepco spokesman Junichi Matsumoto said in the evening.
Matsumoto said Tepco isn’t sure why the reading shot up right after the test, but “it is nearly unthinkable that the temperature of the pressure vessel can go up this high at this point,” he said.
He also said the utility did not check the electric resistance of the other two thermometers, as their readings have been consistent with the level of coolant water being injected into the reactor.
On Feb. 1, all three thermometers read between 44 and 50 degrees, but the one believed faulty began going up and down over the past couple of weeks.
After more water was injected into reactor 2, the two thermometers in accord showed a drop in temperature while the problematic one kept rising. About 18 tons per hour of water currently is being injected, up from 10.5 tons up until Feb. 7.
Earlier Monday, Toshihiro Yamamoto, a specialist in reactor safety management at Kyoto University Research Reactor Institute, told The Japan Times that the one showing high readings is probably malfunctioning because the two others show almost the same temperature and trend.
“If the temperature is really rising, there may be a spot in the pressure vessel that is not getting sprinkled with water. If nothing is done, the temperature will keep going up and the pressure vessel may get damaged. Tepco will have to change the way it injects water or the amount of the water injected,” Yamamoto said.
No xenon, which has a short half-life, was detected in a test of a gas sample in the containment vessel of reactor 2 Sunday evening, suggesting there has been no recent criticality, or a sustained nuclear chain reaction. Nor was there an increase in cesium, said Matsumoto, adding that this is further evidence of a thermometer malfunction.
Reactors 1, 2 and 3, which all suffered meltdowns last March, were declared in December in a state of cold shutdown, meaning the temperature at the bottom of their pressure vessels is under 100 and the leak of radioactive materials is under control.
But because thermometers can be off by as much as 20 degrees, the government has ordered Tepco to keep the temperature under 80 degrees.
The government and Tepco have said the cold shutdown is being maintained based on the readings of other thermometers.