For Unit 1, it was confirmed from the investigation conducted in March 2017 inside the PCV (first basement floor) that there was no significant deformation or damage to the existing structure (including valves, piping and steel) visible around the drain sump (X-100B side). In April 2015, an investigation was conducted on the grating on the first floor to confirm that there would be no interference with the B2 investigation. The investigation results showed no significant damage to the existing facilities on the access route (including HVH, PLR piping, and pedestal wall), except that leakage from the sand cushion drain pipe at the bottom of the PCV is presumed to have caused damage to the PCV.

The PCV internal investigation using an underwater ROV started in February 2022. So far, visual inspection, sediment thickness measurement, etc. have been conducted sequentially. In the visual inspection, lumpy sediments were confirmed on the outside and inside of the pedestal around the worker access port, although they have not been identified whether it is the existing structure or fuel debris. In addition, rebar was found near the workers’ access port, and part of the concrete in the pedestal was found to be missing. In March 2023, an underwater ROV survey was conducted to penetrate the pedestal interior, and it was confirmed that the concrete at the bottom of the pedestal had disappeared near the worker access entrance and almost the entire interior wall of the pedestal. IRID conducted a seismic evaluation of the pedestal in a partially damaged condition in FY2016 under “Government-led R&D Program on Decommissioning and Contaminated Water Management” regarding this matter, and confirmed that the pedestal’s support function is not to be significantly damaged. Also, TEPCO is examining the impact of pedestal damage on the plant based on the current information, assuming that TEPCO expand the knowledge and conduct the evaluation through internal investigations. According to the examination report, considering the confirmation status of the external surface of the pedestal, TEPCO assumes that there is little possibility of large-scale damage by movement , colliding and fall of supporting structures assumed by damage of the pedestal, and also presumes that it would not pose a significant risk of radiation exposure to the public in the vicinity as a result of consideration about the impact on cooling the fuel debris, dust dispersion and criticality among the safety impacts in case that the pedestal’s support function were to deteriorate and the RPVs and other equipment were to tilt or sink.

Following the loss of concrete at the bottom of the pedestal, a strength evaluation due to seismic motion was conducted. The evaluation determined that the inner skirt of the RPV pedestal foundation, the stabilizer and bulkhead of the pedestal superstructure were capable of supporting seismic loads, and therefore, it was concluded that no major damage would occur.

The investigation in-pedestal conducted in January 2018 did not identify any major damage in the in-pedestal walls, the existing structure (CRD switchboard) in the pedestal and the CRD housing support, except that a part of the fuel assembly (upper tie plate) had fallen at the bottom of the pedestal. In the investigation in-pedestal conducted in January and February 2017, it was confirmed that the grating on the CRD rail side had fallen off and been warped. In addition, based on the measurement results using muon, the separator and dryer were thought to be remaining in the pedestal though inside the RPV must have been high temperatures caused by the fuel melting (it is unclear whether they maintain their original shape).

An in-pedestal investigation conducted in July 2017 confirmed the existence of CRD housing and platforms. In the CRD housing, the support fittings were damaged/lost in several places, and the level and spacing of adjacent CRD flange faces were observed to be different. Near the CRD housing, there were structures presumed to be CR guide tubes and CRD index tubes. Near the platform, a part of the steel components of the platform was confirmed, indicating that the platform collapsed, while no grating was confirmed. Based on the measurement results using muon, the separator and the dryer were existing there (it is unclear whether they maintain their original shape).