Summary Report for Conduct of Kozloduy NPP Stress Tests

“Kozloduy NPP” PLC
SUMMARY REPORT
for Conduct of Kozloduy NPP
Stress Tests
2.2.2 Methodology for reassessment of seismic design bases
For reassessment of the seismic characteristics of Kozloduy NPP site project IAEA BUL
implemented during the period from 1990 to 1994 guidance of the applicable IAEA documents
[40],[41] have been applied.
Both standard levels of peak ground acceleration with recurrence periods respectively 100
(Seismic Level-1) and 10000 years (Seismic Level-2) are defined based on tectonic, geological,
geomorphologic, seismic and geophysical data using probabilistic and deterministic methods. The
RLE is defined according to the rules of defining of Seismic Level-2.
The methodology of probabilistic analysis of seismic hazard is based on standardized
mathematical model of Cornel and software of McGuire 1976 and Toro and McGuire 1988.
The seismotectonic characteristics of the regional and local area of Kozloduy NPP are
defined based on comprehensive geological, geophysical, geodesic, geomorphologic, seismic,
seismologic and other studies and the results provided are within scope and scales required in [40].
The following conclusions are made based on the summary of the results:
· there are no large fault structures with high energy potential in the area investigated
(there are no data of presence of a capable fault).
· Kozloduy NPP site is located in the relatively most stable part of the Moesian
platform. This conclusion is confirmed also by cumulative database of the local seismologic
network around the site existing already for 14 years.
The used earthquake catalogue covers period from 375 to 1990. The catalogue data are
unified and standardized in accordance with the existing requirements. The intensity of the
earthquake is assessed by МSК-64 scale. The catalogue contains 812 independent seismic events.
Most part of the observed seismic events are associated with well-known eight seismogen
areas: Sofian, Marishka, Gornooryakhovska, Kresnenska, Negotinska-Krayna and Kampuling-
Vrancha (shallow and medium depth) and local. For this areas space, time and energy
characteristics have been studied in detail.
Through analysis of registered accelerograms for medium soil conditions for each of them
attenuation principles have been defined. Also, magnitude-frequency dependencies for specific
zones are obtained.
Seismic input uncertainties are studied and reported through the so called logical tree
(logic diagram). 24 hazard curves have been obtained.
Design seismic level characteristics are also defined – design floor response spectra and
respective 3D accelerogram regarding the site geological conditions. The peak ground
accelerations, defined using deterministic method, are significantly lower than those defined from
seismic hazard (1.35 – 1.7 times). The local earthquakes effects М=4.5 under the site at the depth of
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