2.1.2.3 Accident Analyses

The approach to safety review has been based on internationally approved safety standards and guidelines, accounting for the corresponding national regulations and recommendations. The ÚJD guidelines for safety analyses of WWER reactors are viewed as particularly important since they set the classification of initiating events, the set of acceptance criteria, and more stringent requirements concerning their analyses.

Initiating events (and induced processes) have been classified into two basic categories: anticipated transients and postulated accidents. In accordance with the international practice and the above mentioned references, a specific procedure was applied for selected events (Anticipated Transient Without Scram - ATWS, Pressurized Thermal Shock - PTS, radioactivity releases, internal structure loads, and selected beyond design basis events with realistic approach).

The general approach to the application of the single failure and deterministic conservative approach to the definition of scenarios were used, in accordance with the recommendations of the IAEA-EBP-WWER-01 Guideline.

Additional conservative assumptions were used (exceeding the single failure criterion), such as assumed position of the control rod stuck at the upper position or expected loss of off-site power supply.

In general, the scenarios did not include operator actions. This is based on the principle that automated systems provide sufficient time for the operator to identify emergency conditions, to analyze the conditions of the unit and to take the appropriate mitigating measures. The interval of 30 minutes is considered as sufficient.

The entire set of initiating events for sufficient numbers of variants (cases) was analyzed to cover the evaluation of every criterion and to justify the selection of the adequate conservatism of the analysis. The SAR contains explicit values for all important parameters, with the definition of initial and limit conditions for every case, enabling control as well as repeated analyses if needed.

Summary of accident analyses

Accident analyses were performed over the entire extent of the anticipated initiating events, while applying qualified methods and practices. The analytical work is in accordance with the IAEA recommendations for emergency analyses of WWER type reactors and the selection of the events was confirmed by comparing with power-plant specific PSA. All of them are included in SAR chapter 15. The whole SAR was updated so as to describe the condition of the power plant and its safety following the gradual upgrading. The structure is in accordance with the practices adopted in many developed countries.

The analyses have shown that acceptance criteria are met for transients and accidents, including design bases accident (i.e. LOCA 2 x F 200 mm) and for selected beyond design basis accidents, including LOCA 2 x F 500 mm. The analyses play a key role with respect to the conviction that the project and the implementation of the gradual upgrading of NPP V-1 have been successful.