Photo/IllutrationImages show black lumps on grating for maintenance work below the No. 2 reactor’s pressure vessel at the Fukushima No. 1 nuclear power plant. TEPCO says melted fuel likely caused at least two holes in the metal grating, including an opening measuring 1 meter by 1 meter. (Provided by Tokyo Electric Power Co.)

  • Photo/Illustraion
  • Photo/Illustraion

Radiation levels that can kill a person in a minute and holes created by melted nuclear fuel could further delay decommissioning operations at the No. 2 reactor of the Fukushima No. 1 nuclear plant.

Tokyo Electric Power Co., operator of the crippled plant, said Feb. 2 that the maximum estimated radiation level near what is believed to be melted fuel in the reactor was 530 sieverts per hour, the highest so far since the triple meltdown in 2011.

In its investigation into the interior of the No. 2 reactor, TEPCO also confirmed at least two holes on grating for maintenance work below the bottom of the reactor’s pressure vessel.

“The holes were likely made when the melted nuclear fuel fell from the pressure vessel and melted the grating,” a TEPCO official said.

The findings were made by studying images taken from a video camera attached to a pipe that was inserted into the reactor on Jan. 30.

Radiation levels were estimated at 20 sieverts per hour, 50 sieverts per hour and 530 sieverts per hour at three spots inside the reactor’s containment vessel.

The company estimated the doses from the extent of disturbances in the images caused by radiation.

Although a TEPCO official said “there is a margin of error because radiation levels were not measured directly,” the company believes the scattered melted nuclear fuel inside the containment vessel was emitting high levels of radiation.

After a number of failed attempts, the remote-controlled camera took the first pictures of possible melted fuel at the plant.

However, closer inspection of the images have revealed additional problems for TEPCO, which had believed most of the melted fuel had remained inside the reactor’s pressure vessel.

TEPCO plans to send an investigative robot, called Sasori (scorpion), into the containment vessel this month to more accurately measure radiation doses at various spots and take additional footage of the scattered nuclear fuel.

The utility plans to use the data to determine a fuel-removal method.

But the robot was expected to use the circular grating, measuring 5 meters in diameter, to move around. One of the holes is 1 meter by 1 meter, a potential pitfall for the robot, which is 59 centimeters long and 9 cm high.

TEPCO said it will consider a different route for the robot in its survey.

Fumiya Tanabe, an expert on nuclear safety who analyzed the 1979 Three Mile Island nuclear accident in the United States, said the findings show that both the preparation for and the actual decommissioning process at the plant will likely prove much more difficult than expected.

“We have few clues on the exact locations, the sizes and the shapes of the nuclear fuel debris,” he said. “The planned investigation by the robot needs a rethink. Work to decommission the plant will require even more time.”

TEPCO said it will need 30 to 40 years to complete the decommissioning process. The utility plans to start work to remove the melted nuclear fuel at the No. 2 and two other stricken reactors in 2021 after deciding on a removal method in fiscal 2018.

TEPCO has yet to determine the location and the condition of the melted fuel in the other two reactors.

(This article was written by Takashi Sugimoto, Keisuke Katori and Eisuke Sasaki.)