Experience feedback from Fukushima towards Human Reliability Analysis for level 2 Probabilistic Safety Assessments

In the years 2000, the IRSN developed its first level 2 Probabilistic Safety Assessment (PSA) for the 900 MWe French PWRs. It was an ambitious project and one of the important tasks was to build a Human Reliability Analysis (HRA) model able to model the human actions to be implemented after the core melted. These actions are performed by operators in the main control room or by field operators outside but most of the decisions are taken, on the basis of the Severe Accident Management Guide (SAMG), by the crisis organization. A Human and Organizational Reliability Analysis in Accident Management (HORAAM) model is born from this enterprise. It is based on the "Decision Tree method". HORAAM has been developed from the observation of the nuclear crisis exercises that are regularly practiced in France. Several influence factors which particularly affect human and organizational reliability in such a situation were identified. Currently HORAAM is used at IRSN but it has never been compared to the experience feedback of a real accident. After the Fukushima accident, IRSN conducted a study to confront HORAAM with the difficulties encountered to implement actions after the core meltdown. The purpose of this article is to present the main conclusions drawn from this study.