Summary

The main objectives of the project were:

• The identification, classification and review of significant documents prepared during the last decade relating to KNPP site seismic subjects and Unit 1-4 installations
• Updating the essential lists of equipment, structures and distribution systems required for safe shutdown of the plant
• />The preparation of a Final Licensing Report (FLR) structured following TOR recommendations, following commonly accepted guidelines such as USNRC RG1.70 (R), Safety Analysis Report (SAR) and adapted to operating plant conditions which is the case for Units 1-4 of KNPP

The Final Licensing Report comprises a description of the first two tasks and the activities carried out during the period 1991 to 1998 with their main results relating to the seismic issues which affect the safety of Units 1-4 of KNPP.

The assessment of the results obtained can be calibrated according to their degree of compliance with the main objectives which a nuclear power plant must satisfy according to officially recognised general criteria. These criteria basically demand thefollowing:

That the seismic excitation which could potentially affect the site is characterised,

• based on recognised, sound, state-of-the-art studies and procedures
• That the NPP structures, components and systems necessary for the safe shutdown of the plant can be shown to be capable of withstanding and operating during a seismic event as defined above and in current facility conditions

For the KNPP site, seismic characterisation is clearly defined based on broad deterministic and probabilistic hazard studies using normal procedures in NPP site studies and state-of-the-art practices. A Review Level Earthquake (RLE) equivalent to S2 of the IAEA or the SSE of American regulations has been characterised and a Local Earthquake has also been defined.

Both are defined basically by their design Ground Response Spectra.

It can be concluded that the basic subject of defining a seismic input for facilities at the Kozloduy site is adequately covered according to current knowledge.

Structures

All building structures containing equipment and systems necessary for the safe shutdown of Units 1-4 in the event of an RLE, (reactor building, turbine hall, pumphouse buildings, etc), have been evaluated using well known analytical procedures. Some local potential vulnerabilities have been detected in some structural systems and the corresponding upgrading requirements have been designed in order to increase their performance at currently acceptable levels in NPP seismic engineering practice. The implementation and optimisation of these upgradings could be begun based on existing information in accordance with the NPP plans for development.

Distribution Systems

An analytical evaluation of primary circuit piping systems has been done and supported by in-situ walkdowns, covering also the secondary piping systems of the various units, a formal processing of activities is required but basically, it can be concluded that the seismic capacity of the main systems has been evaluated and verified.

New systems such as the complementary emergency feedwater systems and other main steam piping improvements have been seismically evaluated using recognised procedures.

Minor formal documentation and inspection coordinated with unit outages will be completed.

Equipment

For equipment evaluation, techniques that have been developed for assessment of components in operating plants have been effectively used. Systematically carrying out walkdowns was an efficient tool for evaluating the real plant conditions. The systems necessary for the basic functions required to bring the plant to a safe shut down have been studied, from these the complete SSEL lists of critical equipment to be evaluated has been drawn up and the qualification procedures required have been developed and systematically applied. Seismic upgradings have been designed, these are mainly related to anchorage, deficiencies and interactions through reinforcement of adjacent brick walls.

It can be concluded that the evaluation of equipment covers all essential equipment of the 4 units and the upgrading carried out have meant a great increase in the seismic performance and safety of equipment and only requires an additional effort mainly in evaluation of microcomponents which is considered to be an issue currently being developed in the NPP.

It can be considered that the studies, projects, designs and seismic upgrading carried out on all four units of KNPP represent a quantitative increase in the seismic safety of the facilities in real terms. Minor effort is required to enhance its scope in relation to distribution systems and equipment. The implementation of the building structures which is pending should close the integrated circle of seismic performance in its entirety.