A paradigm shift in the assessment of Natech scenarios in chemical and process facilities

Complex cascading events might arise from the interaction between natural hazards and technological installations handling hazardous substances, leading to the so called Natech scenarios. Recent severe Natech events evidenced that the accident progression was initiated by the lack of operability of auxiliary systems and utilities available in the plant, and that the unavailability of safety systems in place was a key factor in accident development. These features of Natech scenarios are not captured by the current risk assessment procedures, which mainly focus on the release of hazardous substances caused by the structural damage of equipment. Starting from the lessons learnt from two complex Natech events, the Arkema accident and the Fukushima nuclear disaster, a paradigm shift towards a comprehensive assessment of Natech risk is proposed with the support of an innovative holistic framework addressing the identification and characterization of both “direct” Natech scenarios due to equipment damage, and of “indirect” Natech scenarios generated by the failure of auxiliary systems and utilities. The specific role of the modification of safety system performance during Natech events in the escalation of accident consequences is also addressed. Categories of hazardous substances having critical properties which may lead to major accidents in case of utility system failure were identified, in order to support the identification of “indirect” Natech scenarios. The specific methods and tools available to accomplish each step of the proposed framework are discussed, along with the main open issues and future research needs.