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After the accident at Fukushima Daiichi Nuclear Power Station several investigation committees issued reports with lessons learned from the accident in Japan. Among those lessons, some recommendations have been made on severe accident research. Similar to the EURSAFE efforts under EU Program, review of specific severe accident research items was started before Fukushima accident in working group of Atomic Energy Society of Japan (AESJ) in terms of significance of consequences, uncertainties of phenomena and maturity of assessment methodology. Re-investigation has been started after the Fukushima accident in this working group. Additional effects of Fukushima accident, such as core degradation behaviors, sea water injection, containment failure/leakage and re-criticality have been covered. The review results are categorized in ten major fields; core degradation behavior, core melt coolability/retention in containment vessel, function of containment vessel, source term, hydrogen behavior, fuel-coolant interaction, molten core concrete interaction, direct containment heating, recriticality and instrumentation in severe accident conditions. In January 2012, Research Expert Committee on Evaluation of Severe Accident was established in AESJ in order to investigate severe accident related issues for future LWR development and to propose action plans for future severe accident research, in collaboration with this working group. Based on these activities and also author’s personal view, the present paper describes the perspective of important severe accident research issues after Fukushima accident. Specifically those are investigation of damaged core and components, advanced severe accident analysis capabilities and associated experimental investigations, development of reliable passive cooling system for core/containment, analysis of hydrogen behavior and investigation of hydrogen measures, enhancement of removal function of radioactive materials of containment venting, advanced instrumentation for the diagnosis of severe accident and assessment of advanced containment design which excludes long-term evacuation in any severe accident situations.

The nuclear accident that occurred at the Fukushima Daiichi Nuclear Power Plant on March 11, 2011 was caused by the extremely massive earthquake and gigantic tsunami, which resulted in a severe accident that extended over multiple reactors simultaneously. In the present chapter the current status of the accident is described in terms of basic information, sequences of the accident, fi ssion products (FP) released, and lessons learned. Although some details of the accident are still not well known, the sequences, causes, and consequences of the accidents have been basically clarifi ed by the efforts of several investigation committees in Japan. The fi ssion products released to the environment were estimated by the severe accident analysis code, MELCOR, from inside the reactor core, and also by the atmospheric dispersion simulations code, SPEEDI, by coupling with environmental monitoring data in the reverse estimation method from outside the plant. The estimated release amount of 131 I is of the order of 120-160 PBq and that of 137 Cs is of the order of 8-15 PBq for both estimations. Lessons learned from the accident identifi ed by the investigation committees cover a wide spectrum of insuffi cient measures, such as for earthquake and tsunami, station blackout, severe accident management, common cause accident at multiple unit site, education and training, chain of command at the accident, disaster prevention, and safety regulation systems. 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 future.

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