Terminology, concepts and definitions - Probabilistic Safety Assessment for Optimum Nuclear Power Plant Life Management (PLiM) - page 49

assessment (probability of failure) determines the optimum frequency of
non-destructive testing in nuclear power plants, and is used to prepare in a
timely manner the schedule of repair and supply of spare parts, and decide
on the appropriate test of strength by hydraulic tests.

1.7 Formal-statistical and physico-statistical

approaches to predicting the reliability of
technical systems

Reliability issues were first raised in connection with the statistical
interpretation of the strength margin (safety factor) in the 30's of
last century. However, in its present form the reliability theory
developed in connection with the rapid development of electronics and
computing. Electronic systems are composed of many elements and their
reliability was known from the bench tests. The main task of the theory of
reliability was to determine the reliability of the system using the known
characteristics of the reliability of its elements. The mechanism of failure
of individual elements are not considered. Such an approach in reliability
theory is called the system reliability theory . It can also be called the formal-
mathematical or formal-statistical or (due to the fact that the mechanisms
of damage and failure are not considered in this theory) phenomenological.
Mechanical elements such as plant equipment and piping, are usually
small-scale products. Their failure in service is a rare event and, therefore,
in most cases it is not possible to obtain their characteristics of reliability
from operating experience. Bench tests of the reliability with adequate
simulation of operating conditions cannot usually be carried out. Therefore,
to obtain estimates of the reliability characteristics of mechanical equipment
of NPP, it is necessary to study the processes of damage, ageing and
degradation. The results of these studies as well as construction of a physical
model and introduction to this model of the statistical characteristics of these
variables which have a significant impact on the reliability characteristics
of the element, provide reliable estimates of reliability. This approachis
called the structural approach 17,etc. , in contrast to the phenomenological
approach. The structural approach requires an understanding of the processes
of damage (ageing, degradation) occurring in service and construction
of the corresponding physical models. These models can also be called
physical-statistical .

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