Summary Report for Conduct of Kozloduy NPP Stress Tests

“Kozloduy NPP” PLC
SUMMARY REPORT
for Conduct of Kozloduy NPP
Stress Tests
2.10.5 Evaluation of safety margins against earthquake for DSFS
According to analysis performed [44], it follows that the relative units of DSFS margin are
0,11g or 55% of the design DBE that for the nuclear installation is 0,2g, i.e. DSFS can withstand
without damage to the fuel an earthquake 1,55 greater than the design DBE.
2.10.5.1 Assessment of the seismic impact leading to severe fuel damage
Based on engineering evaluation given in [44] it can be summarized that DSFS can perform
its functions for spent fuel safe storage to seismic accelerations of about 0,31 g, then collapse of the
main support structure can be anticipated. Extension of safe storage of the fuel in this situation
depends on the possibility of cleaning of the constriction debris and restoring, if possible, the
natural ventilation.
2.10.5.2 Scenario with beyond design basis earthquake followed by beyond design basis
flooding
According to the results of [89], no SSCs will be affected by MWL. Therefore, for this
nuclear installation scenario with combination of beyond design basis earthquake with beyond
design basis flooding is not considered as the effect of the external events has been defined under
[44].
2.10.5.3 Measures to enhance DSFS robustness
With regard to DSFS measures to increase robustness are not proposed as its margin is fully
dependent on the capacity of the civil structure.
2.11 CONCLUSION FOR EVALUATION OF THE INFLUENCE OF SEISMIC IMPACTS
ON THE SITE OF KOZLODUY NPP
According to the review of the seismic characteristics of Kozloduy NPP site reevaluated in
1992 (Project BUL 9/012 "Site and Seismic Safety of Kozloduy and Belene NPPs") and additional
studies of 1995 (Studies for local earthquakes and probabilistic determination of seismic effects for
seismic PSA) it can be concluded that the re-evaluated seismic bases at the site comply with current
regulations.
The safety related equipment involved in the emergency scenarios for seismic resistance,
defined as parameters of functions describing its conventional probability of failure (fragility
curves) is are analized. Limits of seismic accelerations that any nuclear facility can withstand
without severe damage to the fuel and disposal of radioactive substances into the environment are
defined.
It follows from the above findings that the analysis of beyond design basis seismic impact is
sufficiently conservative and gives confidence that in seismic terms the SSCs of Kozloduy NPP are
able to ensure the safety of the plant for real maximum seismic impacts probable for the site.
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