Photo/Illutration An image captured underneath the No. 1 reactor on March 29 shows chunks on left that are believed to be fuel debris. (Provided by International Research Institute for Nuclear Decommissioning)

A robotic study provided the first visual confirmation that melted nuclear fuel broke through a pressure vessel at the Fukushima No. 1 nuclear power plant, the government and Tokyo Electric Power Co. said March 30.

Images taken by the robot under the No. 1 reactor at the plant also confirmed heavy damage to a concrete “pedestal” under the pressure vessel.

The inspection by the robot started on March 29. It was the first such study at the No. 1 reactor, one of the three reactors that melted down at the plant following the Great East Japan Earthquake and tsunami in 2011.

More than 90 percent of the nuclear fuel at the No. 1 reactor is believed to have fallen from the pressure vessel.

The robot found a large amount of melted fuel debris under the pressure vessel.

The cylindrical pedestal, which supports the 440-ton pressure vessel, is about 6 meters in diameter, and its walls are about 1.2 meters thick.

The high-temperature fuel debris apparently melted the concrete of the pedestal, leaving its reinforcing bars exposed.

The robot’s recorded images from the inner wall of the pedestal showed bare bars in the lower part of the pedestal.

“It was big progress that we could clearly see inside,” Akira Ono, who heads the cleanup project as chief of TEPCO’s decommissioning unit, said at a news conference on March 30. “We hope to thoroughly analyze the collected information.”

TEPCO still faces the difficult challenge of how to remove the fuel debris and how to protect the damaged pedestal from future earthquakes.

The meltdown at the No. 1 reactor is believed to be worse than those at the No. 2 and No. 3 reactors at the plant.

The International Research Institute for Nuclear Decommissioning estimates the No. 1 reactor building contains 279 tons of melted fuel debris.

Naoyuki Takaki, a professor of nuclear safety engineering at Tokyo City University, said the fuel debris “cannot be taken out unless it is broken down into small pieces.”

Takaki said the method for cutting up such chunks will depend on the ratio and hardness of metal mixed in with the melted fuel.

But the information on objects within the fuel debris is limited so far.

“To put it briefly, it is unknown,” Takaki said.

The No. 1, No. 2 and No. 3 reactors at the Fukushima plant contain an estimated total of 880 tons of melted fuel debris.

TEPCO officials aim to start removal work of the fuel debris at the No. 2 reactor in the latter half of fiscal 2023. The initial plan is to take out a few grams, analyze their elements and hardness, and then increase the amount to be removed.

No timetable is set for such work at the No. 1 and No. 3 reactors.

The damaged pedestal has raised concerns that an earthquake could knock down the structure.

“I am worried about the pedestal collapsing in an earthquake and allowing the reactor pressure vessel to fall,” said Chihiro Kamisawa, a researcher at the Citizens’ Nuclear Information Center, a nonprofit organization.

“The shock caused by the fall would crack the pressure vessel, leading to the release of radioactive materials,” he said.

TEPCO has cited an analysis that the quake-resistance of the pedestal would be sufficient even if about a quarter of it is damaged. It said the reinforcing bars in a longitudinal direction have not changed much despite a series of strong quakes.

“There would not be a major problem caused by an earthquake,” Ono said on March 30.

The pressure vessel is supported not only by the pedestal but also by edge-on metals at the upper part.

TEPCO is expected to review the quake-resistance based on the robot inspection results.

(This article was written by Keitaro Fukuchi, Ryo Sasaki and Takuro Yamano.）

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