Release of Fission Products
The Nuclear and Industrial Safety Agency (NISA) and the Nuclear Safety Commission (NSC) estimated the total amount of radioactive materials released to the environment. NISA estimated the total discharged amount from reactors on the basis of the analytical results with severe accident analysis code, MELCOR, by Japan Nuclear Energy Safety Organization (JNES) [5, 6], as typically shown in Fig. 3.7. The NSC estimated the amount of nuclides discharged into the atmosphere with the assistance of the Japan Atomic Energy Agency (JAEA) [7, 8] through inverse calculations, based on the data of environmental monitoring and atmospheric diffusion calculation code, SPEEDI, as shown in Fig. 3.8. The estimated values summarized in Table 3.3 range between 1.2 and 1.6 × 1017 Bq for iodine-131 and 8 and 15 × 1016 Bq for cesium-137. Values estimated by TEPCO are also shown in Table 3.3. Estimated release of iodine-131 by TEPCO is about three times larger than values by NISAor NSC. It is also noted that the estimated releases of iodine-131
Fig. 3.7 Estimated fission products (FP) release ratio to the environment with MELCOR code (Unit 2). Solid lines and dotted lines represent cumulative release fraction and release rate, respectively 
Fig. 3.8 Estimated FP release to the environment with SPEEDI code
Table 3.3 Estimated fission products released to the environment
and cesium-137 of the Chernobyl accident  are about one order of magnitude larger and about 6 to 10 times larger than those estimated by NISA and NSC as shown in Table 3.3.
It may be noted that the containment S/C venting is conducted through several meters of water depth in a suppression chamber, and the effectiveness of this pool scrubbing for FP aerosols is usually very high, of the order of 103 as a decontamination factor. The estimation of FP release during S/C venting by TEPCO for Units 1 and 3 is not necessarily high compared with other periods .
After the accident at Fukushima Nuclear Power Plant, several investigation committees have been established, such as by the Independent Investigation Commission, TEPCO's Investigation Committee, National Diet's Investigation Committee, and Government's Investigation Committee. They have issued investigation reports on the causes of the disaster, major lessons learned from the accident, and recommendations for the future [1–4]. Most of those reports judged that although the accident was triggered by a massive force of nature, it showed existing weaknesses regarding defense against natural hazards, regulatory oversight, and insufficient accident management, emergency response, and emergency training that allowed the occurrence and escalation of the accident .
For example, the Independent Investigation Commission  mentioned that the Fukushima accident is a “man-made disaster–unprepared nuclear severe accident” because of ambiguous private corporate management by TEPCO under the national nuclear policy. It states that the main cause of the accident is complete lack of crisis management and leadership in both the Government and TEPCO. It also emphasized the utmost importance for resilience to be greatly enhanced for the future. TEPCO's Investigation Committee  mentions that enhanced accident measures both in hardware and in software are to be prepared, and recommends that the Government clearly establishes the standards of the emergency offsite center and guidelines of external events that have extremely low probabilities and high consequences. It emphasized the importance of the company-wide enhancement of risk management systems. National Diet's Investigation Committee  raises several recommendations, such as monitoring of the nuclear regulatory body by the National Diet, reform of the crisis management system, Government's responsibility for public health and welfare, and development of a system of independent investigation commissions in the National Diet. The Government's Investigation Committee  points out several important recommendations, such as establishment of a basic stance for safety measures and emergency preparedness, safety measures regarding nuclear power generation, nuclear emergency response systems, harmonization with international practices in nuclear safety, and continued investigation of the accident causes and damage of the Fukushima accident.
Several measures, such as enhanced power supply capabilities, improved severe accident management policies, and strengthened emergency preparedness capabilities, have already been put in place based on the identified causes and lessons learned from the accident, and some mid-/long-term measures, such as a filtered containment venting system and increased seawall, are being implemented at nuclear power plant sites. Also, it is pointed out that professional leadership in nuclear organizations that manage potentially hazardous activities to maintain the risk to people and the environment as low as reasonably achievable without compromise is of utmost importance, thereby assuring stakeholder trust .
The underlying essential lesson will be that a sense of crisis and tension toward a possible severe accident were completely lacking, and groundless overconfidence against nuclear safety covered all nuclear sectors in Japan. Also it is evident that fully flexible resilience by the maximum use of existing hardware and software with enhanced knowledge, experience, monitors, predictive capability, exercises, and management is the only possible way to effectively cope with “unexpected” events that are largely beyond design base. We should learn these important lessons with humility, share them among all throughout the world, and reflect on them to a future even higher level of safety for current nuclear reactors. Because long-term relocation completely destroys local communities, advanced reactors without the need of evacuation, in principle, should be developed and deployed for the future. Associated important severe accident research items are being systematically identified, for example, by the efforts of the Atomic Energy Society of Japan .
Lastly, it may be noted that the Nuclear Regulation Authority (NRA) has been newly established in September 2012, as an independent commission body that solely exercises regulatory authority in the field of nuclear safety and security in Japan. As of July 2013, only 2 units of about 50 units are in operation, although the Government is expecting the restart of the operation of idling nuclear power plants, after satisfying new safety regulation rules  in force by the NRA in July 2013, as an important power source.