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, around 10,000 becquerels per hour (Bq\/h) of radioactive materials were being released to the atmosphere from the buildings (TEPCO assessment in September 2023, Fig.1).<\/p>\n
Fig. 1. Radioactivity from Units 1 to 4 of Fukushima Daiichi Nuclear Power Station (Bq\/h) released to the air<\/strong><\/p>\n At the same time, 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 September 2022).<\/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.<\/p>\n The removal of fuel debris has been delayed. Experimental removal using a removal device (a folding robot arm of 22m in length) originally scheduled in the Mid-term Roadmap to take place in Unit 2 in FY2021 (now scheduled for some time in FY2023), firstly experienced problems in opening the reactor vessel penetration hatch (X-6 penetration) to be used. Beginning in April, the work was scheduled to be completed in about a month but was finally completed in October. Inside the penetration, cables, etc. which melted during the accident have fused and are lying in a heap, making it impossible to insert the device. TEPCO plans to push the accumulated materials into the reactor vessel using water and machinery, but the prospects appear bleak. In addition, investigations are ongoing due to water level reductions occurring in the Unit 1 pressure containment vessel (PCV) and Unit 3 PCV and suppression chamber.<\/p>\n There has been progress in the removal of piping for the standby gas treatment system (SGTS, the piping that connects the reactor building with the Units 1 and 2 exhaust stack, and which was contaminated inside during venting). Since it interfered with installation work for the large-scale cover on the Unit 1 building, removal work on the SGTS piping had begun in March 2022. That was supposed to be completed in about a month but ran into frequent problems and was temporarily halted. Work was resumed on April 18, 2023, but again ran into a number of problems. The work was due to be completed in May, but the removal work was finally completed on July 14.<\/p>\n Changes in the average number of workers onsite per day is shown in Fig.2. As of September 2023, the number of workers was 4,590.<\/p>\n![\"\"](\"https:\/\/cnic.jp\/english\/wordpress\/wp-content\/uploads\/2024\/02\/F1.jpg\")
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