Software of Safety Important I&C Systems - Nuclear Power Plant Instrumentation and Control Systems for Safety and Security - page 151

expressed by Safety Class. Moreover, safety class
defined for I&C system imposes requirements for
software of all components, platforms and even
related automation devices.

d. Requirements for stability of function
execution against external influences.
e. Requirements for lack of influence on
other systems.
f. Requirements for procedures and
processes that support meeting require-
ments for functions, quality, reliability
and stability.

The Criteria of Selection and
Structuring of Requirements

Selection and any activities aimed at meeting
requirements are impossible without establishing
a clear classification features, determining factors
and selection criteria. The main factors and criteria
are considered below.

This set should be designed for the full set of
software requirements and should be correspond-
ingly supplemented and specified. In particular,
the subsets of requirements for processes of soft-
ware development and verification, which play
a priority role from the standpoint of assurance
of reliability and safety, should be expanded and
worked out in maximum degree.

1. General criteria for selection of requirements
or, in other words, “requirements for require-
ments.” Among such criteria for nuclear
power plant I&C software the most important
are the criteria of necessity, completeness,
adequacy, correctness, verifiability, and
openness. These criteria are related to the
criteria that were developed and are used for
evaluating the execution of requirements for
software during expert analyses (Vilkomir,
1999), (Vilkomir, 2000). For example, in ac-
cordance with the criterion of completeness
during generation of many requirements for
software elements must be separated and
taken into account that reflect “covering”
by requirements of these components such
as: completeness of conformity to specifi-
cations; completeness of consideration of
software lifecycle stages; completeness of
the diagnostics, and so forth.
2. Classification and content of I&C system
requirements as a whole. The full set of
these requirements includes:
a. Requirements for the composition of
the functions.
b. Requirements for quality of the execu-
tion of these functions.
c. Requirements for reliability of function
execution.

3. Particular features of software as an object
of safety standardization. The following set
of the software features have a direct effect
on the selection of classification features
and generation of subsets of requirements:
a. Software is both a component of the
system for which regulatory require-
ments have been established and a
means that assures fulfillment of
the regulatory requirements for I&C
system. Consideration of this feature
is most important in defining require-
ments for monitoring and diagnosis,
reliability and stability. In doing so
different external disturbing influences
for software should be examined.
b. Software is a possible source of com-
mon cause failure. Nature of software
makes it necessary to have require-
ments for protection from common
cause failures due to improvement of
software development and verifica-
tion processes and use of the diversity
principle, which in turn determines
the necessity of classification features