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Update of Level 1 Probabilistic Safety Analysis with expansion of the scope of analysis for Units 5 and 6 of Kozlooduy NPP


  • Employer: Kozloduy NPP EAD
  • duration:
    January 2020 – july 2022


Units 5 and 6 of Kozlouy NPP are the first VVER-type reactors for which a full Level 1 Probabilistic Safety Analysis (PSA) was conducted back in the early 1990s. During the period 2001-2007, a significant modernization program of Units 5 and 6 was implemented. In 2011, the PSA was updated to reflect the state of Structures, Systems and Components (SSC) at the end of 2007.

Since then, a number of modifications of SSC of Units 5 and 6 have been carried out, most notably as part of the findings of post-Fukushima stress tests, as part of the uprating to 104% of nominal power rating and as part of the lifetime extension program (some of which are still ongoing). Therefore, the need arises for the update of the Level 1 PSA for Units 5 and 6 of KNPP to reflect these changes in SSC as well as to implement the state-of-the art analysis methodologies.

Scope of the project

The project includes the update of Level 1 PSA of Kozloduy Units 5 and 6 for three power modes: full power, low power and reactor shutdown. The scope of analysis is to be extended to cover all internal and external hazards, characteristic of the KNPP site, as well as the dependency between the two units.


The internal events model has been updated and is subject to review and acceptance by the Employer before work continues on the other elements of the model.