A pressure washer-equipped robot clears the path inside the containment vessel of Fukushima No. 1 nuclear power plant's No. 2 reactor on Feb. 9. The black lumps are believed to be melted fuel. (Provided by Tokyo Electric Power Co.)

The road to decommissioning Fukushima No. 1 nuclear power plant’s No. 2 reactor could be rockier than expected, as radiation levels on Feb. 9 were even deadlier than those recorded in late January.

Tokyo Electric Power Co. announced that day that radiation levels inside the reactor were estimated at up to 650 sieverts per hour, much higher than the record 530 sieverts per hour marked by the previous survey.

A camera made its way inside the reactor's containment vessel for the first time on Jan. 30 and spotted fuel rods that had melted into black lumps in the nuclear accident in the aftermath of the 2011 Great East Japan Earthquake and tsunami disaster.

The plant operator made the latest estimate from the amount of camera noise experienced by the robot that ventured into the lion’s den that morning.

Equipped with a pressure washer, the machine was deployed to pave the way for the Sasori (scorpion) robot that is set to survey the reactor’s interior in greater detail.

The robot’s task was to hose down melted fuel and other substances as it traveled along a rail measuring 7 meters long and 0.6 meter wide connecting the outer wall of the containment vessel with the reactor’s core. It started operating from a point located 2 meters from the exit of the tunnel bored into the side of the vessel.

But about two hours into its journey, in which it had progressed about a meter, the camera footage started getting dark, TEPCO said. The amount of radiation emitted by the melted fuel may have taken a toll on the camera’s well-being.

As the robot could be left stranded inside the vessel if the camera broke down completely, the utility called off the operation seven hours earlier than scheduled and retrieved the device.

TEPCO analyzed the footage and concluded that the doses amounted to about 650 sieverts per hour, which is deadly enough to kill a human in less than a minute.

As the robot’s camera was designed to withstand a cumulative dosage of 1,000 sieverts per hour, the utility commented that “it’s consistent with how the camera started to break down after two hours.”

The plant operator plans to deploy the Sasori surveyor robot before the end of February.

“We will be assessing the amount of deposits and debris to decide how far Sasori can advance,” a TEPCO official said.