International Standard Bases and Safety Classification - Nuclear Power Plant Instrumentation and Control Systems for Safety and Security - page 46

• Key instrumentation and displays to per-
mit pre-planned operator actions that are
deined in the NPP operating instructions,
and that are required to ensure NPP safety
in the short term.

room staf to their failures, when no alter-
native means (e.g. periodic tests) are pro-
vided to verify their availability.

The I&C functions assigned to category B are
necessary for:

An I&C function shall be assigned to category
B if it meets any of the following criteria and is
not otherwise assigned to category A:

• Used fuel pool cooling system.
• Main cooling system isolation.
• Post-accident monitoring system.
• Automatic control of the NPP primary and
secondary circuit conditions, keeping vari-
ables in the limits assumed in the safety
analysis, and prevention of events from es-
calating to accidents.
• Monitoring/controlling the handling of
fuel where failure could cause radiation re-
lease or fuel degradation outside the limits
and conditions of normal operation.

• Functions required after the non-hazardous

stable state of a DBE has been reached,
to prevent it from leading to unaccept-
able consequences, or to mitigate the
consequences.

• Functions required to provide information

or control capabilities that allow speciied
manual actions necessary after the non-
hazardous stable state has been reached
to prevent a DBE from leading to unac-
ceptable consequences, or mitigate the
consequences.

Typical I&C systems are as follows:

• Functions, the failure of which during nor-

mal operation, would require the operation
of a category A function to prevent an ac-
cident which study is required.

• NPP automatic control system or preventa-
tive protection system.

• Part of the decay heat transport to ultimate

heat sink not necessary in the short term;

• Functions to reduce considerably the fre-

quency of a DBE as claimed in the safety
analysis.

• Instrumentation needed to apply operating

procedures for DBE.

• Safety circuits and interlocks of fuel han-

dling systems used when the reactor is shut
down.

• Plant process control functions operating

so that main process variables are main-
tained within the limits assumed in the
safety analysis, when these control func-
tions are the only means of control of these
variables.

An I&C function shall be assigned to category
C if it meets any of the following criteria and is not
otherwise assigned to category A or category B:

• Functions used to prevent or mitigate a ra-

dioactive release or fuel degradation out-
side of the limits and conditions of normal
operation as deined in the safety analysis.

• Plant process control functions so that the

main process variables are maintained
within the limits assumed in the safety
analysis and whose failure would not
lead directly to operation of category A
functions.

• Functions that provide continuous or in-

termittent tests or monitoring of functions
in category A to indicate their continued
availability for operation and alert control