TOKYO (Kyodo) -- The operator of the disaster-struck Fukushima Daiichi nuclear power plant said Thursday an attempt to take a close look inside the severely damaged No. 2 reactor using a scorpion-shaped robot ended in failure due to a technical flaw.
The failure of the first full-fledged attempt to examine the highly radioactive residue of melted nuclear fuel inside the reactor is expected to complicate the effort by Tokyo Electric Power Company Holdings Inc. and the government to decommission the plant.
However, TEPCO said at a press conference, "We have received great hints for the next step."
The robot sent data showing that about three meters from the entrance to the pressure vessel, the radiation level in the air was 210 sieverts per hour, a level where a person could die from even brief exposure, and the temperature was 16.5 degrees Celsius, the company said.
In a preparatory survey, an extraordinarily high radiation level -- estimated as high as 650 sieverts per hour -- was detected in the Nov. 2 reactor.
By using the self-propelled robot, TEPCO aimed to obtain precise radiation readings, images of the reactor and other details needed to remove the debris.
But the robot was unable to climb over obstacles. The company gave up on retrieving the robot by cutting its remote control cables.
TEPCO said it is uncertain whether high radiation caused a track glitch of the robot, co-developed by Toshiba Corp. and the International Research Institute for Nuclear Decommissioning.
According to TEPCO, the robot probe was inserted into the reactor containment structure around 7:50 a.m. It became unable to move when it encountered debris on the rail it was meant to travel along to get deeper into the reactor, one of three at the plant that melted down and then exploded in the days after a massive earthquake and tsunami on March 11, 2011, knocked out power needed to run the reactor cooling system.
TEPCO had planned for the robot to get to the end of the rail and continue to a metal grating directly underneath the pressure vessel, where a black mass had been found in previous surveys.
The mass is believed to be nuclear debris. The company had also found on the grating a 1-square-meter hole believed to have been created by melted fuel.
TEPCO personnel attempted to continue by dragging the robot but gave up and cut its cables around 3 p.m.
The latest survey faced many difficulties, with little clear surface for the robot probe to move on and high radiation that was feared would cause problems with the robot's electrical components, as happened in preparatory surveys.
TEPCO plans next month to survey the No. 1 reactor, which also melted down in the 2011 disaster.
The Fukushima catastrophe was the world's worst nuclear accident since the 1986 Chernobyl disaster.