The level 1 probabilistic safety assessment (PSA) of the VVER/440-213 type Paks Nuclear Power Plant (NPP), Hungary is updated annually in the framework of a living PSA program. The purpose of the update is to (1) ensure an up-to-date and credible risk assessment of the plant, and (2) provide an appropriate technical basis for PSA applications in support of risk-informed decision-making.
The PSA for the Paks NPP includes dedicated analyses for each of the four units of the plant. The reactors and the adjacent spent fuel pools are considered as potential sources of large releases of radioactivity. Design and site specific internal events, internal hazards and external hazards have been analyzed as causes to initiating events. Full power as well as low power and shutdown modes of the NPP units have been subject to risk assessment.
The regular update of such a complex PSA is a challenging task. Modeling of plant modifications, feedback from operating experience, extensions to PSA scope and level of detail, and improvements in analysis methods, models and data are the major areas of PSA update. The update is performed by the developers of the basic PSA models, as external analysts, in cooperation with the technical support staff of the plant. Detailed information on plant modifications and relevant operating experience are the main types of input provided by the technical support staff to enable PSA update.
The PSA models and the associated data bases are reviewed and modified as necessary, risk is re-quantified, and the PSA documentation is revised as part of the update. The summary report of the PSA is a dedicated chapter of the Final Safety Analysis Report (FSAR) of the plant; therefore, PSA update infers regular updating the FSAR too. After performing PSA update every year, the complete electronic PSA documentation and the PSA model are submitted by the plant to the nuclear safety authority for information.