Summary Report for Conduct of Kozloduy NPP Stress Tests

“Kozloduy NPP” PLC
SUMMARY REPORT
for Conduct of Kozloduy NPP
Stress Tests
The SG alternative makeup system to ensure primary circuit heat removal only during time a
little over 24 hours [90]. This time is fully sufficient for cooldown of one unit to cold state. In spite
of this, it should be taken into account that the use of SG alternative makeup system depends on
capability of MDG to move to the point of connection.
For other units (to which MDG is not connected) the heat removal function to the secondary
circuit can be used after reaching of the water level to Kozloduy NPP site. In this case, to prevent
from heating of the reactor installation and to recover the reactor core cooling, the operating
personnel shall use instructions for effective recovery from SBEOPs package (see instructions
ВФЗ-2.1 and ВФЗ-3.1.), [101] and [102].
In the most general case, the actions are limited to cooldown of the reactor installation to
150°С and lowering the primary circuit pressure to parameters at which it is possible to connect
emergency normal cooldown, [101] and [102], i.e. to performance of feed-and-bleed procedure.
Obviously, when discussing scenario of combination of beyond design basis earthquake
with beyond design basis flooding of major importance is the time for the water level reaching to
Kozloduy NPP site and the time for flooding elevation - 4,20, i.e. the time from the earthquake to
achieving the emergency feedwater pumps failure.
According to the analyse [89] it can be demonstrated that about 20 hours are available from
the destruction of water supply systems “Iron Gates” 1 and 2 to the reaching of the water level to
the kilometre of Kozloduy NPP site. Obviously, it can be anticipated that the loss of emergency
feedwater pumps will be occur at the end of the reactor cooldown or after its completion (depending
on the cooldown speed, selected by the operating personnel). Therefore, based on the analyses [34]
it can be accepted that the water inventories in the SGs are sufficient for successful completion of
cooldown and performance of actions in ВФЗ-2.1 [101] and ВФЗ-3.1 [102].
Ensuring stable over time cooling of the reactor core for a period of 72 hours without
external supply can be achieved when the unit is cooled to cold state and core cooling is provided
through emergency and normal cooldown system and the heat from the emergency core cooling
system heat-exchanger is removed from the essential components service water supply system to
the ultimate heat sink – water in spray pools.
The water in the spray pools and operational inventories of diesel fuel of the site are
sufficient to ensure safe condition of the reactor in cold state for 7 days without additional supplies.
The core cooling function by primary circuit will be affected by the combination of the
external events, as follows:
· unit 5: performance of the function for the line of scheduled and maintenance
cooldown will be limited to two trains of Unit 5 emergency and scheduled cooldown
system. Due to flooding of ShPS, the inventories in the spray ponds ensure 60 hours’
performance of the function at stage by stage switching of the available trains;
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