The internal wall of a cylindrical foundation supporting the reactor pressure vessel of the Fukushima Daiichi nuclear power plant's Unit 1 may be badly damaged, its operator said Tuesday, triggering concerns about its earthquake resistance in case of another major disaster.

The operator Tokyo Electric Power Company (TEPCO) has been sending robotic probes inside the Unit 1 primary containment chamber since last year, and the newly released videos taken during a survey from March 28 through March 31 found the concrete wall missing in over half of the pedestal measuring 5 meters in internal diameter.

The underwater, remotely operated vehicle named ROV-A2 was sent inside the Unit 1 pedestal, coming back with images seen for the first time since an earthquake and tsunami crippled the plant 12 years ago. The area inside the pedestal is where traces of the melted fuel can most likely be found.

An approximately five-minute video, part of 39-hour-long images captured by the robot, showed that the 120-centimeter-thick concrete exterior of the pedestal was significantly damaged near its bottom, exposing the steel reinforcement inside.

TEPCO spokesperson Keisuke Matsuo told reporters Tuesday that the steel reinforcement is largely intact but the company plans to further analyze data and images over the next couple of months to find out if and how the reactor's earthquake resistance can be improved.

About 880 tonnes of highly radioactive melted nuclear fuel remain inside the three reactors. Robotic probes have provided some information, but the status of the melted debris is still largely unknown. The amount is about 10 times the damaged fuel that was removed in the cleanup of the Three Mile Island nuclear plant in the U.S. after its 1979 partial core meltdown.

Fukushima Governor Masao Uchibori urged TEPCO to "swiftly evaluate levels of earthquake resistance and provide information in a way prefectural residents can easily understand and relieve concern of the residents and people around the country."

The video taken by the robot also showed equipment that slipped down as well as other types of debris, possibly nuclear fuel that fell from the core and hardened, piling up as high as 40 to 50 centimeters from the bottom of the primary containment chamber, Matsuo said.

Struck by a magnitude-9.0 earthquake and ensuing tsunami that hit Japan's northeast on March 11, 2011, the No. 1, 2 and 3 nuclear reactors at the Fukushima plant suffered core meltdowns, resulting in a level-7 nuclear accident, the highest on the International Nuclear and Radiological Event Scale.

(With input from agencies)