Risk assessment of operational events methodology for Indian nuclear power plants

Operational Event is any event which happened at a nuclear power plant leading to an unplanned transient or significantly affecting the plant response characteristics. Such events may be caused by operator errors or equipment failures. The range of events could be from an apparently insignificant event to a transient that leads to reactor trip and subsequent actuation of designed safety systems or operator action. The efficient feedback of operating experience is a valuable source of information for improving the safety of nuclear power plants (NPPs). Among the various efforts to improve operational safety of nuclear installations, the systematic collection, evaluation and feedback of operational events (OE) are considered valuable and effective. This may be achieved by establishing a system for the effective feedback of operating experience. It is important that the assessment of events is carried out to provide confidence that the safety consequences have been fully understood, the causes have been correctly established and appropriate corrective actions identified. The traditional ways of investigating operational events have been predominantly qualitative. A method established upon probabilistic safety assessment (PSA) called probabilistic precursor event analysis is being increasingly used as it allows for a quantitative estimation of the safety significance of operational events. It is crucial that no events that are relevant to plant safety are screened out. The PSA based view helps to ensure that safety related aspects of an event are identified and do not get missed out. The purpose of this paper is to outline this method that makes more effective use of event information by mapping it with the plant specific PSA. A pilot study on PSA based Event Analysis was conducted for few events of an Indian PHWR, to establish a methodology for Event Analysis in Indian Nuclear Power Plants. The risk measure suitable for the purpose of operational events analysis is the Conditional Core Damage Probability (CCDP). This paper presents a case study bringing out the methodology for estimation of exposure times and CCDP.