Important Severe Accident Research Issues After Accident at Fukushima Daiichi Nuclear Power Station

Abstract

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.

Development of the source term PIRT based on findings during Fukushima Daiichi NPPs accident

“…The engineering safety system without electricity such as IC (Isolation Condenser), turbine driven RCIC (Reactor Core Isolation Cooling system) and/or HPCI (High Pressure Coolant Injection) were effective for delaying the core melt however, depletion of DC batteries resulted finally in of Japan) in order to investigate severe accident related issues for future LWR development and to propose action plans for future severe accident research (Sugimoto, 2013). The committee recently developed Thermal-Hydraulic Phenomena Identification and Ranking Table (TH PIRT) (Sakai et al, 2013) to explore the debris distribution and the current condition of molten core in detail and Source-Term PIRT (ST PIRT) to extract the high-priority issues for severe accident code development on the basis of findings during the Fukushima accident.…”

Accident of Fukushima Daiichi Nuclear Power Plant: Sequences, Fission Products Released, Lessons Learned

“…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 identifi ed, for example, by the efforts of the Atomic Energy Society of Japan [ 11 ].…”