International Standard Bases and Safety Classification - Nuclear Power Plant Instrumentation and Control Systems for Safety and Security - page 37

It should be taken into account that a degree of
“rigidity” of regulatory requirements depends con-
siderably on a type of class, to which an object of
standardization and safety assessment is referred.
Standards IAEA NS-R-1 (IAEA, 2000,a) and
IAEA SSR-2/1 (IAEA,2012), covering not only
the I&C, but also various NPP systems, determines
that all items (i.e. systems, structures and com-
ponents) important to safety, shall be identified
and shall be classified on the basis of their func-
tions and their safety significance. The method
for classifying the safety significance shall be
based primarily on deterministic methodologies
complemented where appropriate by probabilistic
methods, with account taken of factors such as:

IAEA NS-G-1.3 (IAEA, 2002) expands the
principle of classification to all I&C functions,
systems, and components to fit into one of two
safety categories: important to safety or not im-
portant to safety (Figure2). Functions, systems,
and components important to safety are those
whose malfunction or failure could lead to radia-
tion exposure of the site personnel or members
of the public.
Functions, systems, and components important
to safety are further categorized as either safety
or safety related .
Safety functions, systems, and components
are those provided to ensure the safe shutdown
of the reactor, the residual heat removal from the
core, control of reactivity, control of planned ra-
dioactive release, limitation of the consequences
of anticipated operational occurrences or design
basis accidents.
I&C safety systems initiate actuation and con-
trol actions of technological systems, performing
emergency reactor trip, emergency core cooling,
emergency containment isolation and others.

• The safety function to be performed.

• The consequences of failure to perform the

safety function.

• The frequency with which the item will be

called upon to perform a safety function.

• The time following a postulated initiating

event at which, or the period for which, it
will be called upon to operate.

Figure 2. IAEA safety classification of I&C systems