Images captured by a robotic probe inside one of the three melted reactors at Japan’s wrecked Fukushima nuclear power plant showed exposed steel bars in the main supporting structure and parts of its thick external concrete wall missing, triggering concerns about its earthquake resistance in case of another major disaster.

The plant’s operator, Tokyo Electric Power Company Holdings, has been sending robotic probes inside the Unit 1 primary containment chamber since last year. The new findings released Tuesday were from the latest probe conducted at the end of March.