National Report of the Slovak Republic - Radiation and Nuclear Safety , September 1998

4.5.3 Basic Principles of ÚJD-Issued Decisions on Safety Upgrading of Operated Reactors

Similarly as in many other countries, neither Slovakia has officially codified rules or requirements with respect to safety upgrading of nuclear reactors. Consequently, ÚJD's requirements are specified for individual reactor types. Safety upgrading programs are developed by nuclear power plant operator, who bears overall responsibility for nuclear safety.

In Slovakia, the nuclear power plants safety concept has been based on so-called "defence in-depth", a strategy used generally world-wide in designing and operating nuclear power plants. In assessing NPI safety, ÚJD assesses the ability of the installation to fulfill safety-related functions along the lines of the design, to ensure the required in-depth defence level.

The safety upgrading process in Slovakia follows the current international safety standards. Certain specific measures were set based on a comparison of selected national standards with those applied in developed countries. As a rule, safety upgrading measures for WWER 440 reactors have generally been oriented towards improving reliability, redundancy (in particular with respect to V-230 reactors), and physical separation of safety systems.

Lists of safety-related imperfections the treatment of which is included in the safety upgrading program for specific reactor types have been a result of recent developments in the field of primary circuit integrity, assessment of events at nuclear installations, results of beyond-design basis accident analyses etc.

ÚJD uses deterministic approach to effectively manage the safety upgrading process, in particular to improve the safety of emergency systems (independence, redundancy). PSA for specific reactor is used to prioritize the individual safety improvement measures, in particular those which may contribute most to the core melt-down.

Safety upgrade related requirements are partially linked to the probability of accidents. Acceptance criteria set forth by ÚJD for emergency analyses are generally expressed in terms of acceptable radiological consequences which differ according to the probability of the initiation event. In addition, conservative or so-called best estimate procedures were described for emergency analyses. Best estimate procedures are only accepted for accidents with the least probability of a specific initiation event (below 10-6).

An additional principle applied by ÚJD in the safety upgrading process is the limitation of the operation duration of nuclear power plant units by issuing only approvals for operation for specified period of time; this enables the management of the safety measure implementation process. This procedure has so far only be applied with respect to NPP Bohunice V-230 reactor units.

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