Properties of Safety Important I&C Systems and their Components - Nuclear Power Plant Instrumentation and Control Systems for Safety and Security - page 74

restrictions for technological variable and other
important parameters, at which operation of NPP
is allowed. According to NP 2008,a deviation
from specified in the design operational limits
that did not cause emergency is considered as an
operational event).
To except execution of unnecessary preventive
actions, a margin between upper limit of work-
ing values of each of controlled parameters and a
value of a set-point, specified for it in the normal
operation of the I&C system, is provided.
Failures of components, which can cause un-
availability of the normal operation I&C system
for performance of required safety important
functions with a specified reliability and quality,
are automatically detected and displayed in such
a form that simplifies taking measures for oper-
ability recovery by personnel.
Possibility to continue power unit operation in
case of components' failures of safety important
normal operation I&C systems, allowed design
limits and restrictions and time, during which
operation in such conditions is allowed, is speci-
fied in Technical Specification on Safe Operation.
Post-accident monitoring systems provide
informational support of personnel and safety
experts during control of accidents, elimination
of their effects and return of reactor facility into
controlled state, and also during the process of
further analysis of occurrence reasons and behav-
ior of design and beyond design basis (including
severe ones) accidents.
For this purpose post-accident monitoring
systems perform monitoring functions, archiving,
display and register of data of occurrence of initi-
ating events, emergencies and accidents, actions
of a protection system and operational personnel,
intended for safety assurance, and also about
controlled thermo-hydraulic, neutron physical
and other parameters, defining a state of power
unit and protection systems, integrity of physical
barriers (fuel matrix, fuel claddings, boundaries

of the primary coolant system and containment),
radiation environment in rooms and on the NPP
territory, in sanitary protection zone and control
area.
Information obtained from I&C system of post-
accident monitoring is supposed to be used for:

• Identiication of a postulated initiating
event that was a cause of failure of a speci-
ied operating limit(s); recreation of a se-
quence of further events, including pro-
tective actions of safety systems and / or
operational personnel; to be convinced that
during the execution of these actions limits
of safety operation were not exceeded and
physical barriers on a way of spread of ra-
dioactive substances and ionizing radiation
were undamaged.
• Identiication of the cause, that provoked
an emergency situation, and recreation of
the sequence of further events, including
actions of operational personnel to return
unit in a normal operation; assessment of
possible efects of exceeding normal oper-
ation limits; to be convinced that release of
radioactive substances and ionizing radia-
tion did not exceed the limits speciied by
the design.
• Identiication of a cause and recreation of
the sequence of events in development of
the emergency into an accident; analysis
of release of radioactive substances and /
or ionizing radiation over speciied limits;
execution of required actions for accident
control; detection of necessity to take of-
site emergency actions.
• Assessment of damages to structures, sys-
tems and components, including physical
barriers, caused by design basis or beyond
design basis accident, and use of these data
during mitigation of its efects.