Confirmation of severe accident code modeling in light of the findings at Fukushima Daiichi NPPs

On March 11th 2011 the Great East Japan Earthquake and Tsunami hit the Fukushima Daiichi Nuclear Power Station. The Fukushima Daiichi Units 1 to 3 lost all DC and AC power supplies, which set in motion a chain of events that led to releases of radioactivity to the environment. Since then, TEPCO has made many efforts to investigate the accident progressions and the status of the reactors and containment vessels, mainly by using severe accident analysis code, such as MAAP, MELCOR and SAMPSON. However, there still exist several tens of unsolved issues to be investigated for the fully understanding of the accident. In recent years, massive amounts of information of the accident plants were obtained through the investigation activities by using robots or other devices. The investigation results suggested us the possibilities that some phenomena identifications established before the accident were incorrect. And also, there are many difficulties to reproduce the accident progressions of the Fukushima Daiichi unit 1 to 3, due to lack of knowledge real BWR plant accident phenomena. In this paper, we will introduce the special characteristics, which made difficult to reproduce the accident. And we will discuss how to use the investigation results obtained in Fukushima Daiichi NPP for the SA code validation by comparing with investigation results and calculation results.