Japan’s nuclear watchdog has asked the operator of the Fukushima plant to assess the risk of reactor damage

TOKYO — A nuclear watchdog has asked the operator of Japan’s crippled Fukushima nuclear power plant to assess the potential risks of damage found in a key support structure, the worst-hit of the three melted down reactors.

The robotic probe pedestal in the primary containment chamber of Unit 1 of the Fukushima Daiichi nuclear power plant – a main support structure directly below the core – has been extensively damaged. The thick concrete exterior was almost completely missing, and the internal steel bracing was exposed.

About 880 tons of highly radioactive molten nuclear fuel remain in the plant’s three reactors. Robotic probes have provided some information, but the state of the molten debris is still largely unknown.

Based on data collected from previous probes and simulations, experts said most of the molten fuel inside Unit 1, which was the worst hit, fell to the bottom of the primary storage chamber, but some may have fallen into the concrete foundation. a situation that makes the already daunting task of disarmament extremely difficult.

At a meeting of the Nuclear Regulatory Authority on Wednesday, commissioners agreed to task operator Tokyo Electric Power Company Holdings with an urgent assessment of risks from damage to the pedestal, including possible leakage of radioactive materials from cracks and holes caused by the meltdown. The authority also asked TEPCO to assess the possible risks if the pedestal fails to hold the reactor in the event of another disaster.

“We need to think about the response in the event of an accident,” said Shinsuke Yamanaka, the supervisory commissioner. “It is TEPCO’s responsibility to carry out the risk assessment as soon as possible.”

TEPCO said that while the concrete exterior is largely missing, the steel reinforcement remains intact and there is little safety risk. If the pedestal fails, surrounding structures can prevent the reactor from collapsing. TEPCO said it plans to analyze more data and images in the next few months to determine the extent of the reactor’s earthquake resistance.

The images were the first taken from inside the pedestal since the March 11, 2011 disaster. In previous experiments, robots were sent, but they were unable to reach the pedestal and take pictures. Images taken by a remote-controlled underwater vehicle in March show details of the damage inside the pedestal, where traces of melted fuel are likely to be found, and will be key to the investigation by TEPCO and nuclear experts.

The damage was caused by the first earthquake in 2011, but it is not known if it happened recently. Images of the exposed steel bracing have raised concerns among local residents about the safety of the reactor.

The Fukushima Daiichi plant’s plan to release treated but still slightly radioactive water into the sea has also sparked concern and protests from the local fishing community and neighboring countries, including South Korea.

A delegation of experts from the South Korean government visited the plant for two days earlier this week to see facilities related to the planned water release. Members of the team met with Japanese officials in Tokyo on Thursday, where they said they plan to follow up on an International Atomic Energy Agency review that has helped Japan improve transparency and credibility.

The trial removal of the molten debris is expected to begin in block 2 this year after a delay of almost two years. The removal of spent heating elements from the cooling pool of the reactor of Unit 1 must begin in 2027, with a 10-year delay. Once all spent fuel is removed from the pools, molten debris will be removed from the reactors starting in 2031.