NPP: Power Grid Mutual Safety Assessment - Nuclear Power Plant Instrumentation and Control Systems for Safety and Security - page 410

Figure 8. The fragment of the operational hierarchy of power grid criticality matrices for Zaporozhe NPP

lated considering the equipment parameter change.
The criticalities for M crt

The total influence value for NPP subsystems
considers not only the physical influence change,
but also the organizational (through NPP unit load
procedure) influence between NPP units.
The fragment of the operational hierarchy of
PG criticality matrices after Zaporozhe NPP unit
2 shut down is shown on Figure 9.
As a result of the power unit shut down, the
criticality of the power unit 3 was increased. The
new criticality values were calculated considering
the influence matrix constructed for the NPP
subsystem. The new criticality values allow
evaluating the new power grid safety value.
To understand the nature of influence between
NPP and power grid we introduce the approach
for formalization based on application the influ-
ence matrix. The influence formalization might
be very useful NPP safety assessment. The influ-
ence might be useful for the risk analysis based on
FMECA as the additional information to compare
the possible failures criticalities. The conditional
criticality complements the traditional criticality
assessment and considers the mutual failures
criticality changes. Using the different metrics
we could evaluate the strength of influence. The

S 0 are calculated as a sum
of criticalities of the lower level hierarchy. The
study for creating and implementing criticalities
evaluation software is conducted by the research-
ers of the Centre of Safety Infrastructure-Orient-
ed Research and Analysis, Ukraine. The system
criticality change on each level of operational
hierarchy determines the power grid safety trend.
The following stage of the power grid safety as-
sessment is constructing the operational hierarchy
of criticality matrices considering the occurrence
of non-critical failures.
The shut down of the Zaporozhe NPP power
unit 2 (2011) caused by the power grid disturbances
was studied with constructing of the operational
hierarchy of power grid criticality matrices. Any
change of PG system operational mode could be
considered a potential conditional event or risk
factor as it was in Sayano-Shushenskaya HPP
accident.
The incident involved only the second level
hierarchy. The new operational hierarchy of
criticality matrices considering the total influ-
ence matrix was constructed for NPP subsystem.