Images taken by a robotic probe inside one of three reactors at Japan’s damaged Fukushima nuclear power plant have revealed exposed steel rods and parts of its thick outer concrete wall are missing, raising concerns about its earthquake resistance in the event of another major disaster.

The station’s operator, Tokyo Electric Power Company Holdings, has been sending robotic probes into Unit 1’s main containment chamber since last year. The new data, announced Tuesday, came out in late March.

A TEPCO spokesman told reporters on Tuesday that the steel reinforcement was largely intact, but the company plans to conduct further analysis of the data and images over the next two months to determine if and how the reactor’s seismic resistance can be improved.

Images of exposed steel reinforcement raised concerns about the safety of the reactor.

About 880 tons of highly radioactive molten nuclear fuel remained inside the three reactors. Robotic probes have provided some information, but the condition of the wreckage remains largely unknown. This amount is about 10 times the amount of damaged fuel removed during the cleanup of the Three Mile Island nuclear power plant in the United States after a partial core meltdown in 1979.

A terrible earthquake of magnitude 9 on March 11, 2011 caused a large-scale tsunami that destroyed the power supply and cooling systems of the station. The ensuing nuclear accident at the Fukushima Daiichi nuclear power plant, which flooded and melted the cores of three of its six reactors, forced tens of thousands of people to leave their homes and left entire areas uninhabited for years to come.

Source: Associated Press.