Diversity and Multi-Version Systems - Nuclear Power Plant Instrumentation and Control Systems for Safety and Security - page 221

intangible values and human experience, improv-
ing the available resources in the decision making
process in this complicated area.
Usually, in a quantitative setting, the informa-
tion is expressed by means of numerical values.
However, when we work in a qualitative setting,
that is, with a vague or imprecise knowledge, the
information cannot be estimated with an exact
numerical value. In that case, a more realistic
approach may be to use linguistic assessments
instead of numerical values, that is, to suppose
that the variables, which participate in the problem
area, are assessed by means of linguistic terms
(Zadeh L. 1999, Mendel, 2002).This approach is
appropriate for a lot of problems, since it allows a
representation of the information in a more direct
and adequate form if we are unable to express it
with precision.
A linguistic variable differs from a numerical
one in that its values are not numbers, but words or
sentences in a natural or artificial language. Since
words, in general, are less precise than numbers,
the concept of a linguistic variable serves the
purpose of providing a means of approximated
characterization of phenomena, which are too
complex or too ill-defined to be amenable to their
description in conventional quantitative terms.
In fact, considering the approach suggested
in (NUREG/CR-7007, 2009; NUREG/CR-6003,
1994), it is often difficult to determine the precise
values of diversity attributes' weights and rank of
all alternatives on diversity criteria. We need to
evaluate all appropriate experience of applications
of different diversity approaches in all industrial
area, take into account all relevant statistics of
I&C failures caused by CCFs etc. A part of this
information is often represented as linguistic
information, being the expert's subjective opin-
ions. The transformation and formalization of this
linguistic information into precise form without
application of special methods is characterized
by loss of important information. This is another
aspect, which increases the difficulties of the I&C
diversity assessment.

At the initial stage of selection of secondary
(primary) RTS it is more convenient approach for
the experts to compare the possible alternatives of
primary (secondary) RTS and express their pref-
erences using the natural language expressions.
The experts have to deal with portion of a
qualitative information stipulated by several types
of the following uncertainties:

• Uncertainties caused by lack of a suicient
and objective information on RTSs, which
could be considered as an alternative for
given RTS. The lack of required informa-
tion is stipulated by policies of some I&C
company-manufacturer to conceal the part
of information related to its possible short-
ages and defects. In addition, a part of
information on RTS features is coniden-
tial and not available for objective expert
assessment.
• Strategic Uncertainties caused by depen-
dencies on activities of other subjects in-
volved (directly or indirectly) in the process
of selection of alternative RTS (partners,
suppliers etc.)
• Uncertainties caused by application of an
imprecise information (diferent system
parameters) expressed in natural language
(for example the linguistic nature of some
diversity attributes).

On the one hand, it is possible to neglect all
these uncertainties and use deterministic ap-
proaches for selection of the most diverse I&C
system for a given one. But on the other hand,
some of important information might be lost.
We suggest using fuzzy metrics, derived from
application of Computing, with words (CW)
methodology to form the initial subset of possible
alternatives and determine the most diverse I&C
system under uncertainties.
Diversity strategies description: According
to (NUREG/CR-7007, 2009; NUREG/CR-6003,
1994) the rational choice of a pair of primary