Summary Report for Conduct of Kozloduy NPP Stress Tests

“Kozloduy NPP” PLC
SUMMARY REPORT
for Conduct of Kozloduy NPP
Stress Tests
of water intake from the pumps of service water system for cooling of safety system consumers of
units 3 and 4.
In addition, analysis was conducted for beyond design basis accident with complete loss of
water in the SFP due to beyond design basis earthquake, at which ground stability is affected and
significant cracks open in the reinforced concrete of the SFP.
Drainage of fuel storage compartments at existing condition of SFS and simultaneous failure
to all systems and unavailability of natural ventilation will bring to significant increase in
temperature of fuel elements (to 600°С) and of the civil structure (up to 340°С), which is not
permissible from the point of view of safety margins for safe SNF storage but will not result in fuel
melting [113].
Analysis of events with complete loss of power supply cover also events with complete loss
of cooling of the SFP (loss of ultimate heat sink).
5.4.4 Measures to improve robustness of SFS to loss of ultimate heat sink
In relation with SFS, no measures are proposed to enhance robustness to loss of ultimate
heat sink.
5.5 CONCLUSION
A reassessment of the safety margins of the nuclear installations at the Kozloduy site is
performed for the events with consequent loss of safety functions that would lead to a severe
accident. The scope includes the nuclear reactors and the spent fuel pools of units 5 and 6, the spent
fuel pools of units 3 and 4 and the spent fuel storage facility. The reassessment is based on safety
analysis performed with deterministic approach.
The results of the analysis of postulated initiating events with loss of power and loss of
ultimate heat sink show in general strong robustness of the provisions and adequate time period for
implementation of additional recovery actions. Specifically, the results imply that:
· Due to the low value of residual heat loss of electrical power and/or ultimate heat
sink in SFP does not lead to severe damage to the fuel, including total loss of water in the
storage pool;
· The low value of residual heat in SFP-3 and SFP-4 provides sufficient time for
recovery of failed safety functions (6.8 days for SFP-3 and 9.2 days for SFP-4);
· Most vulnerable are Units 5 and 6 and their fuel storage pools:
During power operations and in cold state but with pressurized primary circuit the reactors
are the most robust against loss of power and ultimate heat sink because:
· The residual heat from the core can be removed to the secondary circuit. The
ultimate heat sink is air (steam discharge from safety or relief valves of SG) ;
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