Summary Report for Conduct of Kozloduy NPP Stress Tests

“Kozloduy NPP” PLC
SUMMARY REPORT
for Conduct of Kozloduy NPP
Stress Tests
Level 1 for SFP fuel damage frequency that most precisely reflect the actual state of the units are
provided in Table 1.7-1 below (events per reactor year).
Table 1.7-1 Summary results of the fuel damage frequency in SFP
Initiating event category
SFP fuel damage frequency [ per year]
Internal initiating events
2,0E-05
Seismic impact
1,9Е-06
Fire impact
2,3Е-06
Total SFP fuel damage frequency
2,4E-05
The main contributors to the risk of damage to the fuel in the SFP, related to the internal
initiating events are the initiators "Leakage from SFP", 48% of total risk, and "Loss of off-site
power" and "Loss of non-service water", with 22%.
On the basis of PSA Risk Monitor was developed, which is used in the daily operation of
units 3 and 4 of Kozloduy NPP. Furthermore, other PSA applications were developed, such as
probabilistic assessment of operational events.
It should be noted that the above estimates of the current state of the units are too
conservative, given the constantly decreasing heat in the assemblies and additional technical and
organizational measures for the prevention and mitigation of potential emergency situations. These
measures were developed after bringing the unit in state "SNF storage in SFP" and are not reported
in PSA [34], [35].
1.7.2 Scope and main results of Units 5 and 6 PSA
The first probabilistic safety assessment for VVER-1000 reactors ever was performed for
units 5 and 6 of Kozloduy NPP. Since multiple tests have been extended in the scope, methodology
and structure. The aim was to reflect both the current state of the plant, after numerous upgrades,
and the development of analyses methods. The results of PSA Level 1 in the table below represent
the state at the time:
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