4.5.3 Basic principles of ÚJD issued decisions on safety improvements of nuclear power plants in operation

In Slovakia, similarly as in other countries, there are no officially encoded rules or requirements concerning safety improvements of nuclear reactors. The requirements of the regulator therefore are set specifically for the individual reactor types. Safety improvement programs are developed by operator of nuclear power plants that has primer responsibility for nuclear safety.

The safety concept of nuclear power plants in Slovakia is based on so-called „defense in in-depth concept“, a concept generally used world-wide upon designing and operating nuclear power plants. In reviewing safety of NPGI, ÚJD reviews the ability of the facility to fulfill safety functions in terms of the design to provide for the required standard of defense in-depth.

The process of safety improvements follows the current international safety standards. Some specific measures were taken based on comparisons of some national standards with those applicable in developed countries. As a rule with respect to WWER-440 reactors, measures to improve safety are generally oriented towards improving reliability, redundancy (in particular with respect to V-230 reactors), and physical separation of safety systems.

The list of safety related shortcomings the solution ,of which is contained in the safety improvement programs for specific reactor types, has been the result of the recent developments in the field of primary circuit integrity, evaluation of events at nuclear installations, results of BDBA analyses, etc.

ÚJD is using deterministic approach to efficient management of the process of safety improvement, in particular to improve the safety of safety systems (independence, redundancy). PSA for specific reactor is used to set priorities for individual measures to improve safety, above all those that make the most significant contribution to core damage.

Requirements on safety improvement are partly set with respect to accident probability. Acceptance criteria for accident analyses set by ÚJD are generally expressed as acceptable radiological consequences that differ according to the probability of the initiating event. Moreover, conservative or so-called best-estimate procedures for accident analyses have been prescribed. Best-estimate procedures are only accepted for accidents with a very low probability of occurrence (less than 10^-6).

Another principle used by ÚJD in the process of safety improvement is limitation of the duration of operation of nuclear power plant units through granting approvals for a limited period of time, which enables a management of the safety measures implementation process. This approach has so far been applied to units with type V-230 reactors at NPP Bohunice.

Based on previous experience, ÚJD set probability targets of acceptability at systemic level for safety systems, for the reactor protection system, for core damage, for so-called early release of radioactive substances, as well as exclusion criterion for external initiating events of emergency sequences.