Operational events have been evaluated qualitatively since the first unit of the Paks Nuclear Power Plant (Paks NPP) was put into operation. Criteria for the events being evaluated or even reported to the authority underwent several modifications. Some events evaluated in the past are found to be not significant nowadays, thus they are not reported or even not evaluated at all. Since the completion of the first Level-1 PSA study an issue has been raised on the possibility of evaluating the operational events quantitatively, determining the risk significance of the real occurrence and drawing conclusions based on its impact on safety. The initial activities on the development of the methodology and tools were started in 1995 by the VEIKI Institute for Electric Power Research Co. (VEIKI) on the request of the Hungarian Atomic Energy Authority (HAEA). In the process of development basically the objectives of the US NRC Accident Sequence Precursor (ASP) Program were followed, but additional country specific objectives have been taken into consideration as well. With these in mind a precursor event analysis methodology for core damage risk and, in general, the framework of a full scope quantitative event analysis program has been established at the HAEA. The paper introduces the key steps of the development process, the general framework of the full scope event analysis program and the precursor event analysis methodology at the HAEA. A summary of the results of the precursor analyses performed in 2000 is presented also in the paper. The relationship of the precursor event analysis to the nuclear safety authority's event investigation activities is described in the paper. The introduction of the experiences gained so far during the past trial application of the precursor analysis program and the future enhancement of event investigation at the HAEA conclude the paper.
