Tokyo Electric Power Company, operator of the Fukushima Dai-ichi nuclear plant, said on Friday that one of two thermometers used to monitor temperatures in the bottom part of a reactor vessel, which contains nuclear fuel, has failed.

Workers short-circuited the thermometer by delivering 250 volts of electricity instead of 100 volts during checks, Tepco said.

The problem wasn't identified or announced for more than 24 hours after workers first noticed the problem because the short-circuited thermometer showed a reading of 20 degrees Celsius, the same as the working one.

The utility has been pouring water on the melted nuclear fuel to try to keep it cool.

Tepco workers installed the now-defunct thermometer in September 2012, after finding that one of nine thermometers were working at the bottom of the vessel.

Officials say there are no immediate plans to replace the thermometer because high radiation levels surround the reactor building. A new thermometer would need to be installed through a pipe.

On Thursday, Tepco reported a leak of radioactive water. Tepco has been managing the plant’s cleanup of radioactive water for nearly three years after a massive earthquake and tsunami in 2011 caused cores of three reactors to melt at the plant and release substantial amounts of harmful radioactive material.