TOKYO — A nuclear watchdog has asked the operator of Japan’s wrecked Fukushima nuclear power plant to assess potential risks from damage found in a key supporting structure inside the worst-hit of the three melted reactors.
A robotic probe inside the Fukushima Daiichi nuclear power plant’s Unit 1 primary containment chamber found its pedestal — a main supporting structure right underneath the core — was largely damaged. The thick concrete exterior was missing almost all the way around, exposing the internal steel reinforcement.
About 880 tons of highly radioactive melted nuclear fuel remain inside the plant’s three reactors. Robotic probes have provided some information, but the status of the melted debris is still largely unknown.
Based on data collected from earlier probes and simulations, experts have said most of the melted fuel inside Unit 1, believed to be the worst hit, fell to the bottom of the primary containment chamber, but some might have fallen through into the concrete foundation — a situation that makes the already daunting task of decommissioning extremely difficult.
At a meeting Wednesday of the Nuclear Regulation Authority, its commissioners agreed to order operator Tokyo Electric Power Company Holdings to urgently assess the risks from the pedestal damage, including possible leaks of radioactive substances from cracks and holes caused by the meltdown. The authority also requested that TEPCO assess potential risks if, in the event of another disaster, the pedestal fails to support the reactor.
“We need to think about responses in case of an accident,” watchdog commissioner Shinsuke Yamanaka told reporters. “TEPCO has a responsibility to make the risk assessment as soon as possible.”
TEPCO has said that, even though the concrete exterior is largely missing, the steel reinforcement remains intact and there is little safety risk. If the pedestal fails, its surrounding structures can prevent the reactor from collapsing. TEPCO said it planned to further analyze data and images over the next couple of months to find out the extent of the reactor’s earthquake resistance.
The images were the first to be taken from inside the pedestal since the March 11, 2011, disaster. Robots were sent in earlier attempts but were unable to reach the pedestal and take pictures. The images, captured in March by a remote-controlled underwater vehicle, show details of the damage inside the pedestal, where traces of melted fuel can most likely be found and will be key to an investigation by TEPCO and nuclear experts.
The damage is believed to be from the initial earthquake in 2011, but it’s not known if it happened more recently. The images of the exposed steel reinforcement have triggered concerns among local residents about the reactor’s safety.
The Fukushima Daiichi plant’s plan to release treated, but still slightly radioactive, water into the sea has also triggered concerns and protests from the local fishing community and neighboring countries, including South Korea.
A South Korean delegation of government experts visited the plant for two days earlier this week to see the facilities related to the planned water release. The team members were to meet with Japanese officials Thursday in Tokyo, where they said they planned to follow review of the International Atomic Energy Agency, which has been assisting Japan to improve transparency and credibility.
Trial removal of melted debris is expected to begin in Unit 2 later this year after a nearly two-year delay. Spent fuel removal from the Unit 1 reactor’s cooling pool is to start in 2027 after a 10-year delay. Once all the spent fuel is removed from the pools, melted debris will be taken out of the reactors starting in 2031.