Temperature readings at one of the crippled Fukushima nuclear reactors have risen above Japan's stringent new safety standard but there was no immediate danger, its operator said Sunday.

Tokyo Electric Power said one of three thermometers on the number-two reactor at the Fukushima Daiichi nuclear plant indicated gradual heating this month and reached 82 degrees Celsius (179.6 degrees Fahrenheit) Sunday.

The temperature was above the 80-degree safety standard newly employed by Japan's nuclear safety authority, prompting the utility to publicise the reading and notify public agencies.

But it remains below the 100 degree level that the government says is needed to maintain the safe state of "cold shutdown".

The utility said it will check the accuracy of the thermometer in question, as two others on the same reactor have been measuring its temperature at around 35 degrees.

Gas samples from the reactor did not indicate any new critical reaction, and other monitors and data do not suggest heating and increased steam, TEPCO said.

"We believe the state of cold shutdown is being maintained," TEPCO spokesman Junichi Matsumoto told a press conference.

"Rather than the actual temperature rising, we believe there is high possibility that the thermometer concerned is experiencing display error," he said.

As a precaution, TEPCO has increased the volume of water and boric acid solution being poured on the reactor to cool it down.

The Fukushima power plant became the site of the worst nuclear accident since Chernobyl after it lost its cooling systems in the March earthquake and tsunami and went through meltdown and explosions.

Severe radiation contamination has rendered surrounding communities uninhabitable and triggered food and water scares in Japan.