Software of Safety Important I&C Systems - Nuclear Power Plant Instrumentation and Control Systems for Safety and Security - page 147

etc. Software of different forms and types has
specific properties. Moreover functions that are
performed by software impose constraints on both
software as a product and software lifecycle as a
processes. For example, use of operating systems
and application software has a very limited scope
in safety important systems.
In the context of safety important I&C sys-
tems, increase in portion of software-produced
or software-supported functions requires more
attention to software. In this chapter software (SW)
for nuclear power plant's (NPP) instrumentation
and control (I&C) systems is concerned. That
means that references to specific regulations for
nuclear power engineering are given, particular
terminology and classifications are used.

systems have already been in use for several
decades, allows one to determine that in the past
40 years each fifth accident is related to failure
of a digital control system. Six of seven failures
of these systems were caused by the occurrence
of software defects. One such defect of computer
software of the Ariane-5 navigational system in
1997 led to an accident which cost nearly one half
billion dollars (Adziev, 1998). In nuclear power
generation programmable I&C systems have had
a shorter history, however, here also there have
been accidents due to software defects.
The reliability of software, as for the I&C
system as a whole, depends on the design qual-
ity at stages that directly precede development
of the software:

• Development of requirements for I&C
system.
• Mathematical models.
•

BACKGROUND

The increase of the number of nuclear power
plant I&C software executed functions causes
an increase of the “weight” of software device
defects and its possible sources of failures. Based
on different estimates such defects cause up to
70% of the failures of computer systems of criti-
cal application complexes, of the total number
of those attributed to nuclear power plant I&C
systems (Everett, 1998) (Lyu, 1996). Given this,
the present trend is having an increasing dynamic
role over time.
In the 1960s software defects caused up to
15% of the failures, and in the 1970s it was 15-
30%, and by the year 2000 they were the cause
of up to 70% of computer system failures. This
trend shows up even more in space rocket tech-
nology (Aizenberg, 2002). Analysis of the cause
of accidents and catastrophes of space rocket
systems, where on board and ground computer

Software created functioning algorithms.

Errors committed at these stages become
sources of complex defects in software. In this
sense, software, on the one hand, accumulates
the deficiencies of the preceding stages, and on
the other hand, is the “field,” in which they can
show up and be eliminated. However, the efforts
that must be made to do this, increase by an order
of magnitude.
Consequently, software is becoming an even
more important factor determining the safety of
nuclear power plant I&C system. This explains
the fact that software of nuclear power plant I&C
system, in accordance with national and interna-
tional normative documents, is a separate and
very important object of safety standardization.