/>Summary

The main objective of the project was to provide expert support to the Bulgarian nuclear regulatory authority (BNRA) in the review of the probabilistic safety assessment (PSA) reports prepared for the Kozloduy NPP units 5&6 for the Periodic Safety Review purposes as follows:

• />The project provided EU expertise and assistance to the BNRA by provision of independent peer review to give assurance of the results and by establishment of the regulatory requirements on the basis of the assessment of the PSA reports.
  

• /> />The project also focused on the transfer of know-how to the BNRA and its TSOs from the member states regulatory authorities for long term perspective in enhancing Bulgarian competence and capabilities in the area of PSA review.
  

• /> /> />The process of transfer of know-how also included development of a regulatory guidance document for the review and assessment of PSA. In the future, this guide will serve as guidance when PSA reviews are made by the BNRA and its TSOs.
  

Concerning the overall framework, the PSA work in
Bulgaria/>/> is in good progress. The first PSA project was carried out in 90'ies and the plant status in that study corresponded to the plant structure in 1992. In the second PSA project that was reviewed in this task, plant status corresponded to the plant structure at the end of 2000. The main aim was to provide reference risk value after the large plant modification programme performed during 90'ies. The objective of this study was also to support the Periodic Safety Review currently going on. The PSA work continues actively in
Bulgaria/>/>. The next Level 1 PSA is already under preparation and the plant status will correspond to the plant structure at the end of 2007.

The Level 1 PSA project has been ordered and financed by the Kozloduy NPP. The full power PSA project was entirely implemented by the consulting company, Risk Engineering Plc. The low power and shutdown states PSA project was implemented by Framatome ANP in co-operation with Risk Engineering Plc. For the implementation of the projects, Risk Engineering Plc. applied thorough QA practices including Quality Management Manual and detailed procedures as well as a specific Quality Plans for the projects that were followed in the project implementation. The final result reports have been signed and approved by the Kozloduy NPP in November 2005.

Sound international practices have been utilized in the PSA project work. In the project implementation, the IAEA guidelines have been used as the main methodological basis. In addition, reference documents refer to the US NRC practices in performance of PSA work. Computer code used for the Full power PSA is of
US/>/> origin - the code SAPHIRE that is distributed by the NRC, was used. In the low power and shutdown states PSA project, the code Risk Spectrum of European origin was utilized by Risk Engineering Plc. These computer codes are well known and widely used in the PSA work. PSA model is needed to describe the plant event sequences and system fault trees including plant specific input data Some methodical improvements were suggested for the future work. The scope and objectives of the Full power PSA and Shutdown PSA are generally according to international practices. The PSA includes typical PSA coverage for full power, low power and shutdown states. Internal initiating events such as loss of coolant accidents and plant transients as well as internal fires and internal floods and external events such as seismic effects (for full power) have been covered in the study.

General impression on the PSA documents provided is that reporting is well done, main reports are easily readable and text is understandable for a technically oriented reader. The PSA study has been divided into two separate parts, one for full power state and another for, low power and shutdown states. The overall level of reporting is good and the structure and forms of presentation in the reports makes it possible to perform a regulatory review. The results can be compared with the international reference values as well as with the Bulgarian regulation. Plant modifications performed during 2001-2007 are not included in the Full power and Shutdown PSA. There is also plant project going on with the objective that the symptom based emergency operating procedures would be available at the end of 2008. These modifications have a considerable impact on the PSA results because of improved plant safety structure and improved operation during emergencies.

Internal and external hazards identification and assessment is a specific area of PSA. During this task all hazards recommended for consideration, are evaluated for applicability on the site. Accepted for the detailed analysis, the hazards, both internal and external origin, are investigated through PSA methodology and final results are included in overall plant risk. In this area, further work is needed to cover all plant states such as shutdown states as well as all important phenomena.

Some internal initiating events were also identified that need further consideration. Initiating events representing support system failures (power supply, cooling systems, MC, etc.) need to be included as separate initiating events in the PSA. The definition of initiating events "triggering of reactor emergency protection" need to be modified in order to meet the international standards. Modelling of failure of the scram (signal or control rod) in case of transient as well as handling of boron dilution needs to be improved.

The system descriptions and simplified schematic system diagrams are available for almost all of the analyzed systems in an appropriate manner. In general, it appears that the development of system fault trees was done according with the state-of-the-art PSA methods. For the handling of computer based systems some improvements were suggested. Handling of dependencies, common cause failures and human interaction are areas where further development is needed. And the most important, to provide the plant specific analysis, plant specific input data is needed for the initiating events and component data. Here the close co­operation between the plant staff and performer of PSA analysis is needed.

The results of the PSA and their interpretation are presented very clearly and they are easily understandable. The general conclusions and recommendations for the application of the analysis results of the full power and shutdown PSA are presented. On the basis of the obtained results and the sensitivity and importance analysis, appropriate measures are presented by the utility to improve safety level of the units 5 & 6 of Kozloduy NPP.

PSA work in
Bulgaria/>/> has started well but further work is needed to reach the full coverage of Level 1 PSA and to extend it to the Level 2 PSA. The review team presents four recommendations and about forty important suggestions how to improve PSA methodology on the basis of international practices including binding Bulgarian regulation. Implementation of corrective actions on the basis of findings and suggestions will improve the quality of the PSA study as a whole and will allow meeting the state of the art PSA practices.