Summary Report for Conduct of Kozloduy NPP Stress Tests

“Kozloduy NPP” PLC
SUMMARY REPORT
for Conduct of Kozloduy NPP
Stress Tests
5.2.2 Measures to improve robustness at loss of power supply
These systems for power supply to the SFP are the same which ensure power supply to units
5 and 6, with exception of the additional DG and mobile DG. If the proposal is implemented related
to units 5 and 6, reliability of the power supply to the SFP cooldown system will enhance.
5.2.3 Loss of ultimate heat sink
At failure to all the trains of SFP cooling system, as well as at isolation of the containment,
decay heat removal from the stored fuel is provided through pool water steaming. To prevent
impermissible level reduction in fuel compartments and uncoverage of the fuel emergency scheme
is used for filling through one of containment spray system pumps with water from ECCS tanksump.
The pump motor is cooled by service water system for cooling of safety system consumers.
Response of the structure at loss of ultimate heat sink is the same, as at loss of power
supply, but heat removal stops significantly later – after exhaust of emergency water volume from
the cold channel and termination of spray pools make-up. Thus durations, analyzed for the case of
complete loss of alternate current power supply, specified above, prolong on 197 hours (8.2 days).
A possibility exists to supply service water with the diesel-pumps from the inlet chamber of
CPS-3 (CPS-4) to the first train of service water system for cooling of safety system consumers.
Scheme is proposed, according to which cooling can be ensured for the motors of the pumps of the
first train of the emergency core cooling system. Thus, due to water reserve in the tank-sump,
steaming from SFP may be replenished for during 32 h 15 min at the most severe conditions (the
fuel is relocated from the reactor to the SFP).
If boric solution reserve is used in the tanks of boron control system, time period during
which loss due to steaming can be compensated for, prolongs on at least 19 hours, at this operation
of the pumps, used for filling, is independent from operation of service water system.
5.2.4 Measures to improve robustness to loss of ultimate heat sink
The main mean to improve robustness of SFP-5 and SFP-6 is to ensure make-up of the spray
pools (proposal introduced above in par. 5.1.4.
5.3 SFP OF UNITS 3 AND 4
5.3.1 Loss of power supply
5.3.1.1 Loss of off-site power supply
5.3.1.1.1 Design schemes at loss of off-site power supply, sources, capacity and availability for
operation of emergency power supply
Main and standby power supply
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