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).<\/p>\n
![\"\"](\"https:\/\/cnic.jp\/english\/wordpress\/wp-content\/uploads\/2025\/11\/229_M_F1-e1762763861902.jpg\")
<\/a>Fig. 1. Radioactivity from Units 1 to 4 of Fukushima Daiichi Nuclear Power Station (Bq\/h) released to the air<\/p><\/div>\n
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.<\/p>\n
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<\/sup>\u00a0per hour in May 2011 to 1.4-3.7m3<\/sup>\u00a0per hour as of January 2025). At the same time, the tritium concentration in contaminated water is on a rising trend (Fig.2).<\/p>\n Fig. 2. Changes in Tritium Concentration at Intake of Reverse Osmosis Device<\/p><\/div>\n 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.<\/p>\n 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.\u3000<\/p>\n Table 1.\u3000State of Spent Fuel Pool Cleanup<\/p><\/div>\n Work for the second experimental removal of fuel debris was conducted from April 15 to 23.<\/p>\n Table 2. Worker Exposure due to Removal of Fuel Debris<\/p><\/div>\n 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\u2019s 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.<\/span><\/p>\n 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).<\/p>\n Fig.3. Change in average number of workers (actual values) per day on weekdays<\/p><\/div>\n<\/a><\/a><\/a><\/a>