Japan's Nuclear Regulation Authority (NRA) has ordered plant operator Tokyo Electric Power Company (TEPCO) to urgently assess the risks of a possible collapse at unit 1 of the crippled Fukushima Daiichi NPP, and to formulate emergency measures as soon as possible to avoid any radioactive leakage. Videos taken during a robotic probe in March showed that the pedestal of unit 1’s containment vessel, the main supporting structure directly under its core, was severely damaged. The underwater robot found the concrete wall was missing in over half of the pedestal, measuring five metres in internal diameter, leaving the reinforcing bar exposed.
Based on TEPCO's survey results, experts have said that severe damage to the base will inevitably affect the load-bearing performance. If the base tilts and the pressure vessel falls in the event of another major earthquake, the consequences would be serious. Sankei Shimbun reported that if the reactor pressure vessel falls, it could also damage various pipes connected to the vessel, resulting in the leakage of radioactive materials from the pipes. In a worst-case scenario, some falling materials could combine with the melted nuclear fuel accumulated at the bottom, leading to "re-criticality." Based on this possibility of significant risks, the NRA has urged TEPCO to assess the risks from the pedestal damage and formulate measures as soon as possible.
Unit 1 suffered a hydrogen explosion during the 2011 Fukushima nuclear accident, which blew off the roof and walls of the building housing the reactor. TEPCO originally planned to cover the entire unit with a containment in 2023 to prevent the leakage of radioactive materials during the construction process, such as removing nuclear fuel from the fuel pool. However, due to the high radiation level, waste disposal, such as pipelines around the plant, progress was slow. TEPCO has said that, although the concrete exterior is mostly missing, the steel reinforcement remains intact, and there is little safety risk. If the pedestal fails, its surrounding structures could prevent the reactor from collapsing. However, the NRA is concerned that, if the pressure vessel falls, it could tear the pipeline.
It is a matter of the utmost urgency to clean up the waste, such as high-radiation pipelines, that affect the construction of the containment but TEPCO says its contractor is responsible for the slow construction Tokyo Shimbun newspaper reported. TEPCO “has no time to waste in confronting the issue – it must swiftly assess the damage and take effective action to prevent an accident or leak of radioactive materials,” said Asahi Shimbun.
TEPCO argues that the pedestal has managed to support the reactor vessel even though the plant “has experienced strong earthquakes”, citing one last year that registered lower 6 on the Japanese seismic intensity scale of 7. Although the possibility of the pressure vessel tilting or sinking cannot be ruled out, the company asserts the impact will be limited with no risk of radioactive material leaking to the outside.
However, NRA said it could not decide whether the assumptions underlying TEPCO’s risk assessment are accurate as the extent of damage and condition of the structural materials are not yet fully understood. NRA also said it would be difficult to reinforce the pedestal because of high radiation levels inside the containment vessel.
For this reason, the NRA called on the utility to evaluate the impact of a possible release of radioactive material into the environment and consider steps to deal with such an emergency. NRA Chairman Shinsuke Yamanaka told a news conference: “It is TEPCO’s responsibility to swiftly assess what risks could impact the surrounding environment and its residents.”
Conditions surrounding the reactors that suffered core meltdowns in the nuclear disaster 12 years ago are only now finally being clarified. This has led to the discovery of additional problems that are already difficult to deal with, making the outlook of progress toward decommissioning the reactors even more uncertain, said Asahi Shimbun. “There is absolutely no room for complacency when it comes to the consequences of the Fukushima nuclear disaster. TEPCO must always remain vigilant to a worst-case scenario in tackling related challenges.”
Image: The Fukushima Daiichi nuclear power plant (courtesy of TEPCO)