Environmental Engineering Reference
In-Depth Information
SOFTWARE OF NUCLEAR POWER
PLANT I&C AS AN OBJECT
OF SAFETY REQUIREMENT
ESTABLISHMENT
program copies increases the likelihood of
failures and faults, caused by their hidden
defects.
3. At different stages of the software lifecycle
(primarily design, coding, integration and
testing) different tools are widely used.
These tools are also software products,
which are intended to reduce the number
of defects and increase the reliability of
I&C software. However, defects can also be
introduced into the I&C software through
the software tools. It is the common ap-
proach when control systems are based on
programmable logic controllers (PLC) for
which specialized computer-aided design
(CAD) tools are used, and in view of the
complexity of such CAD tools both intrinsic
defects of a tool and improper use of a tool
can be the source of I&C software defects.
Therefore, requirements for software must
include requirements for software tools used
in development and verification.
4. Because documentation is an integral part of
software, the requirements for I&C software
also include requirements for documentation
that is used at all stages of the lifecycle.
5. Software must be examined not only as an
independent object of safety standardization,
but as a necessary means that will ensure con-
formity of the I&C system to requirements
established for it with regard to redundancy,
maintainability, technical diagnostics and so
forth.
6. Software requirements are not permanent.
The experience with creation and use of
I&C system as well as improvement of the
information technologies lead to the neces-
sity to improve the requirements. Therefore,
requirements must reflect basic and most
stable situations considering this experi-
ence and prospects of software development
technologies.
Aspects of Software in Establishing
Safety Requirements
Software has a number of important features
that should be taken into account in establishing
requirements for it. The main of these features
are listed below.
1. On the one hand, software is a component of
I&C system and shall comply with general
requirements for the system, and on the other
hand it is an independent and specific object
for establishment of requirements, which is
confirmed by a large number of international
and national standards and methodological
normative documents completely devoted
to software.
2. Defects that are committed during the devel-
opment and are not revealed during software
verification, can be actuated under certain
conditions in the I&C system operating
process and lead to their failure. This failure
cannot be compensated even if redundant
channels are available. If that channels use
identical software versions, software defects
are in all channels and reveal themselves
simultaneously leading to the same kind
of distortion of information at the outputs.
Therefore, software defects are potential and
quite likely source of common cause failure.
For this reason, on the one hand software
requirements include both requirements for
its characteristics (structure, functions and
properties) and software lifecycle processes;
on the other hand there is a requirement for
whole I&C system related to adherence of
diversity principle, that is addressed primar-
ily to software, because the use of several
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