A scare over temperatures rising near danger level in a reactor at Fukushima Daiichi nuclear plant, where workers are battling to prevent a resurgence of the radiation crisis, could be a false alarm, the plant operator said on Monday.
Instruments showed the temperature inside the plant's No 2 reactor topped 90 Celsius on Monday, double what it was a month ago and close to boiling point, in which water cooling nuclear fuel in the reactor could evaporate and start a new meltdown.
But a faulty thermometer was likely giving false readings, said Tokyo Electric Power Co, operator of the plant 240 km (150 miles) northeast of Tokyo.
The Fukushima plant's cooling system was wrecked by the March 11 earthquake and tsunami, triggering reactor meltdowns and a radiation crisis that has caused widespread contamination and mass evacuations.
Tokyo Electric, or Tepco, said it was able to bring the temperature down at two other places in the reactor to about 33C from over 40C a week ago by pumping more water into it.
"Following our cooling efforts temperatures at the two other locations are declining steadily while that at the location in question keeps rising. This leads us to think that the thermometre at the location in question is not functioning properly, rather than the actual temperature rising," Junichi Matsumoto, Tepco's general manager, told reporters on Monday.
Matsumoto said there was little sign of steam, which would be produced when water is at such a high temperature, and Tepco believes the reactor is still in cold shutdown, meaning temperatures are stable below boiling point.
The government announced on Dec. 16 that the plant's reactors had reached a state of cold shutdown, a milestone in cleanup efforts and a pre-condition for allowing about 80,000 residents evacuated from a 20-km (12-mile) radius of the plant to return home.
Environment Minister Goshi Hosono said he believed the plant was still in cold shutdown but warned against complacency.
"The instruments are showing readings that are difficult to understand but I believe we don't have to change our view that the plant is in cold shutdown," Hosono said in parliament.
"Nevertheless we continue to assess the situation ready for all possibilities."
Glitches continue to dog Tepco nearly a year after the disaster. Heat is not the only problem the utility and its workers have to battle -- sub zero winter temperatures have frozen many parts of the miles of hastily installed plastic pipes at the plant, creating ruptures and causing the radioactive water they carry to leak.
Shattered trust in the safety of nuclear energy has prevented the restart of reactors elsewhere shut for routine maintenance, straining power supply and threatening blackouts.
Only three of Japan's 54 nuclear reactors are now operating. Without approval for restarts, all of them could be shut by the end of April, boosting the use of fossil fuels and adding over $30 billion (Dh110.19bn) a year to the nation's energy costs, a government estimate said.
A visiting team of UN nuclear experts has backed stress tests aimed at showing Japan's nuclear plants can withstand the sort of disasters that devastated Fukushima Daiichi.