Summary Report for Conduct of Kozloduy NPP Stress Tests

“Kozloduy NPP” PLC
SUMMARY REPORT
for Conduct of Kozloduy NPP
Stress Tests
elevation of flooding, and the pipes are placed in watertight protective channels. Accordingly, there
is no risk of flooding safety related SSCs through them. The domestic and rain waters are
discharged directly into drain channel-1 without treatment and the possibility of reaching the water
to the buildings is precisely through them.
For the purposes of safety margins reassessment a review was performed on two buildings
of additional system for steam generator emergency makeup. The review is aimed at determining
the risk of flooding in sewage network. Level 0,00 of additional system for steam generator
emergency makeup is established as 35.60. During the review it was confirmed that two buildings
of additional system for steam generator emergency makeup have siphon at elevation -4,50 = 31,10
m which is discharged into shaft 1591 of Unit 3 and shaft 1766 of unit 4.
The availability of these siphons determined the ability of returning of rainwater sewage and
as a result, two buildings of additional system for steam generator emergency makeup can be
flooded if the water elevation exceeds 31.10. The difference between the critical height of the
flooding and MWL is -1,83 m, i.e. these buildings are likely to be flooded first through rainwater
sewage. Obviously, the buildings of additional system for steam generator emergency makeup have
margin of external flooding at MWL.
In accordance with [89], only the pumps of additional system for steam generator
emergency makeup will be affected by flooding of the building because they are located at
elevation -3.00 m, which determines that it will be flooded with water level above 32.50 m. The
loss of these pumps (1 ÷ 4 CEFWS PUMPS) is relevant only if spent fuel is in the reactor as they
are part of an alternative logic of the primary circuit cooling through the secondary circuit at loss of
the design cooling system. ASSGEM pump system (1 ÷ 4 CEFWS PUMPS) is not related to
cooling of spent nuclear fuel when it is in the SFP.
Emergency diesel generators of additional system for steam generator emergency makeup
remain available with their section for power supply of different systems, including pumps for spent
fuel storage pool cooldown.
Obviously, it follows from the flooding of building additional system for steam generator
emergency makeup that there will be loss of the only alternative for cooling of the spent fuel by
water-water cooling of the reactor by the steam generators for cases where the fuel is placed in the
reactor under conditions of [46], [65], [66] (use of emergency cooling logic at failure of the
standard logic - using ПР with ultimate heat sink for essential components service water supply).
Since MWL does not affect the operation of standard logics for safety functions assurance, it
can be assumed that the effect of flooding of the building of additional system for steam generator
emergency makeup is expressed only in reduction of the number of systems capable of performing
the same safety function.
3.4.1.3.2 Units 5 and 6 of Kozloduy NPP
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