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 approximately 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 15,000 becquerels per hour (Bq\/h) of radioactive materials were being released to the atmosphere from the buildings (TEPCO assessment in August 2025, Fig.1<\/strong>).<\/p>\n Fig. 1. Radioactivity from Units 1 to 4 of Fukushima Daiichi Nuclear Power Station (Bq\/h) released to the air<\/p><\/div>\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 1.3~2.5m3<\/sup>\u00a0per hour as of January 2026). At the same time, however, the tritium concentration in contaminated water is on a rising trend (Fig.2<\/strong>).<\/p>\n Fig. 2. Changes in Contaminated Water Tritium Concentration<\/p><\/div>\n It is thought that the causes of this, 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, include 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. Treated water with a tritium concentration exceeding 1 million Bq\/L before dilution cannot be released under the implementation plan, but the concentration is now approaching this level and therefore requires careful observation.<\/p>\n The state of removal of spent nuclear fuel from the SFPs is summarized in Table 1<\/strong>. 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.<\/p>\n Table 1. State of Spent Fuel Pool Cleanup<\/p><\/div>\n Changes in the average number of workers onsite per day is shown in Fig. 3<\/strong>.<\/p>\n Fig.3. Change in average number of workers (actual values) per day on weekdays<\/p><\/div>\n![\"\"](\"https:\/\/cnic.jp\/english\/wordpress\/wp-content\/uploads\/2026\/03\/e9c920f0f67bb393a914b353b24b7518-e1774599692384.jpg\")
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