Summary Report for Conduct of Kozloduy NPP Stress Tests

“Kozloduy NPP” PLC
SUMMARY REPORT
for Conduct of Kozloduy NPP
Stress Tests
available to allow for adequately arranging and effectively implementing all the required measures
for recovery of access to switchgear 0,4 kV. For more detail for the time available see item 5.3.1.2
of the report.
In the early phase of accident process, ASSGEM can be used for performance of any one of
its functions. After the water level reaches Kozloduy NPP site, ASSGEM can be used only as an
alternative source of supplying pool cooling pumps due to flooding of the basement of ASSGEM.
The power supply from ASSGEM is guarantees for 72 hours as each EDG has at least 72 hours of
fuel inventory [45].
As the ultimate heat sink from the heat exchanger for SFP cooling, at simultaneous loss of
off-site power and emergency power supply from the diesel-generators in DGS-2, only the cooling
water supplied from the diesel pumps to the fire pump station 2 will remain.
Obviously, the SFP decay heat removal function can be ensured at selection of a scenario for
combination of the external events.
At this scenario the only significant consequence is associated with the seismic impact itself,
i.e. with possible falling of covering elements in the SFP. As stated in [41], at mechanical damage
of the fuel, release of fission products from the gas gap of the fuel elements can be anticipated. This
activity will be released into the environment, as the Central hall does not have the status of a
confined compartment.
It can be concluded that the degree of redundancy of SFP fuel cooling function was reduced
to a greater extent than at independent effect of each external event included in the scenarios.
In the event that before the earthquake and flooding, the fuel is placed in the reactor
according to the conditions of [46], [65] and [66], then due to the loss of the ПР system and loss of
ASSGEM pumps, heat removal can be provided only by fire trucks and SG flow rate can be ensured
by opening of SG SVs. [154],[155].
Regardless of low energy release of the fuel, the effectiveness of application of such cooling
logic should be analytically reviewed.
At the assessment of applicability of the cooling logic with fire trucks the following
complicating circumstances shall be considered:
· the fire trucks owned by the Kozloduy NPP are situated opposite the entrance to
units 5 and 6. This means that their access to points of connection will not be hampered by
the water level. Building structures and facilities of the fire service are designed according
to the national seismic standards. This means that they are guaranteed for seismic impacts
with Кс=0.10, assuming non-linear deformities and partial damages. This means that at an
earthquake within the range of 0,26 < PGA ≤ 0,36g, the buildings under fire will be severely
damaged or destroyed, thus, hindering release of the vehicle from the garages. Therefore,
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