Fukushima Now (29) – Part 2: Current State of Post-Accident Operations at Fukushima Daiichi Nuclear Power Station (February to August 2025)
By Matsukubo Hajime
State of the Plant
The water temperature in the containment vessels and the spent fuel pools (SFPs) shows no great variation despite seasonal temperature changes. The state of releases of Xenon-135 (half-life roughly nine hours), released when uranium fuel undergoes fission, is also unchanged and it can therefore be estimated that the state of the reactors is stable. Further, a maximum of around 10,000 becquerels per hour (Bq/h) of radioactive materials were being released to the atmosphere from the buildings (TEPCO assessment in August 2025, Fig.1).
Fig. 1. Radioactivity from Units 1 to 4 of Fukushima Daiichi Nuclear Power Station (Bq/h) released to the air
Further, when the results of measurements fall below the detection limit, the concentration of radioactive materials in the air from released gasses is taken as the detection limit to calculate the amount released. From April, the measured values at Unit 2 fell from three figures to two figures, but this was due to a lowering of the detection limit when ventilation was implemented for one week to collect dust samples.
Decay heat has fallen greatly with the passage of time, and thus the volume of cooling water injected into the reactors has been reduced (falling from 7-10m3 per hour in May 2011 to 1.4-3.7m3 per hour as of January 2025). At the same time, the tritium concentration in contaminated water is on a rising trend (Fig.2).
Fig. 2. Changes in Tritium Concentration at Intake of Reverse Osmosis Device
It is thought that the causes of this are a) the impact of relatively highly concentrated contaminated water leaking into the reactor building side and b) the reduction of rainwater and groundwater flowing into the reactor building due to countermeasures against contaminated water, thus reducing the dilution of contaminated water, which have occurred in association with the work to lower the water level in the suppression chamber (S/C) at the base of the Unit 1 reactor containment vessel. Since treated water exceeding 1 million Bq/L cannot be released under the implementation plan, this requires careful observation.
The state of removal of spent nuclear fuel from the SFPs is summarized in Table 1. Spent nuclear fuel removal from Units 3 and 4 has been completed. Preparations are now being made to remove the spent nuclear fuel from Units 1 and 2. Further, the transfer of spent fuel from Unit 6 to the common pool was completed on April 16 and the transfer of spent fuel from Unit 5 to the common pool was begun on July 23.
Table 1. State of Spent Fuel Pool Cleanup
Work for the second experimental removal of fuel debris was conducted from April 15 to 23.
Table 2. Worker Exposure due to Removal of Fuel Debris
The weight of the collected sample was 0.2g. Worker exposure during the first and second experiments is shown in Table 2. After transporting the debris to the Japan Atomic Energy Agency’s Oharai Nuclear Engineering Institute, it has been distributed among various research institutes. According to the original plan, the analysis was to have been carried out at Building 2 of the Radioactive Materials Analysis and Research Facility planned to be constructed on the nuclear power plant site. The construction work for the facility, originally planned to start operations in 2024, finally began in March 2025 and is due for completion in April 2028.
Changes in the average number of workers onsite per day is shown in Fig. 3. As of July 2025, the number of workers was 4,690. The number of reports of non-compliant work cases is trending downwards (Fig. 4).
Fig.3. Change in average number of workers (actual values) per day on weekdays
Fig.4. Change in number of non-compliant work cases as reviewed in the Performance Improvement Meeting
State of Contaminated Water
Contaminated water countermeasures at Fukushima Daiichi Nuclear Power Station (FDNPS) can be broadly divided into three areas: 1) Reduction of groundwater flowing into buildings, 2) Reduction of contaminated water flowing into the sea, and 3) Reduction of the toxicity of contaminated water. Countermeasures implemented to reduce water inflow into the buildings are, from higher elevations downward, (A) Pumping up groundwater at the groundwater bypass and releasing it into the sea (994,794m3 up to September 12, 2025), B) Installation of a frozen earth barrier (on-land water barrier, total length roughly 1,500m) surrounding FDNPS Units 1-4, C) Pumping up water at the subdrains and releasing it into the sea (1,906,539m3 up to September 11), and D) Paving of the site with asphalt to suppress permeation of rainwater into the soil. Measures to prevent the discharge of contaminated water into the ocean include A) Groundwater leakage prevention by a steel water barrier on the sea-side and B) Pumping up of groundwater dammed up behind the sea-side water barrier from the well points and groundwater drains. Due to these measures, the volume of contaminated water discharged, 540m3/day in May 2014, fell to 70m3/day in FY2024 (80m3/day in times of average rainfall).
In the reduction of the toxicity of contaminated water, cesium and strontium are removed, and after the removal of impurities using a reverse osmosis (RO) membrane, the multi-radionuclide removal equipment (ALPS – Advanced Liquid Processing System) then removes 62 radionuclides. The water is then stored in tanks (containing 1,273,834m3 as of September 4, 2025. However, due to past equipment malfunctions and operational policies, around 64% of the stored water exceeds the notification concentration).
Fig.5. Releases of ALPS-treated water
Oceanic release of the ALPS-treated contaminated water began on August 24, 2023, a total of 78,285m3 being released in ten batches by September 2025. The total released volume of released tritium for each release and the peak value of the concentration of tritium in seawater less than 3 km from FDNPS are shown in Fig.5. In addition, as of September 4, roughly 13,340m3 of water remain in buildings, and a further 10,285m3 of strontium-treated water, etc., 7,679m3 of water treated by RO, and 9,383m3 of concentrated wastewater, etc. also exist onsite.
Further, the number of Unit 1-4 storage tanks containing ALPS-treated water, strontium-treated water, and other water was 1,071 as of August 14, 2025.
