Human reliability analysis of the Tokai-Mura accident through a THERP–CREAM and expert opinion auditing approach

• Influence of organizational features on human error probability is evaluated.
• Expert opinion elicitation and auditing process are used to fill data gaps.
• The Tokai-Mura accident is taken as an example for comparison purposes.
• The result is much more conservative than the previous one.
• A radar diagram was elaborated to help improve decision making.

This paper presents a human reliability analysis (HRA) model that allows the incorporation of features related to facility conditions to determine human error probabilities (HEP) used in probabilistic safety analyses of process plants. We present an approach to show the predominance of human factors as an accident cause, as well as existing methodologies for HEP determination and their deficiencies in incorporating socio-technical elements that influence them. Such elements are: inappropriate design, training, procedures, communication, safety culture, management in the production process changes, emergency planning, accident investigation, environmental factors, maintenance workload and human–system interface. A mathematical model is proposed to incorporate these elements taking into account their contribution weights as well as measuring their degree of implementation in the plant. This creates a factor that can modify existing HEPs, giving values that better reflect plant reality. The model was applied to the accident that occurred in 1999 in Tokai-Mura, Japan. The modified HEP was 2 times greater than the nominal HEP. This shows that considering organizational factors thoroughly allows for a more realistic plant behavior modeling in face of abnormal events.