1. Although some details of the accidents of Fukushima Daiichi Nuclear Power Plant are still not well known, the sequences, causes, and consequences of the accidents have been basically clarified by the efforts of several investigation committees established in Japan.
2. The fission products released to the environment were estimated by the severe accident analysis code from inside the reactor core, and also by the atmospheric dispersion simulations code by coupling with environmental monitoring data in the reverse estimation method from outside the plant. The estimated release amount of 131I is of the order of 120–160 PBq, and that of 137Cs is of the order of 8–15 PBq for both estimations.
3. Lessons learned from the accident identified by investigation committees cover a wide spectrum of insufficient measures, such as for earthquake and tsunami, station blackout, severe accident management, common cause accident at multiple unit sites, education and training, chain of command at the accident, disaster prevention, and safety regulation system. These lessons should be shared all over the world for the higher level of safety assurance of current reactors, and advanced reactors without the need of evacuation in principle should be developed for the future.
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