Terminology, concepts and definitions - Probabilistic Safety Assessment for Optimum Nuclear Power Plant Life Management (PLiM) - page 40

collected by GRS (Germany), are given in Figs. 1.24-1.26 9 .
It should be noted that the justification of the strength and service life is
conducted in deterministic formulation using a safety factor. For example,
a 10-fold margin of time (number of cycles of repetition of various modes
of operation) to the appearance of fatigue cracks is used, i.e. crack should
form no sooner than after 300 years of operation at the assigned operating
time of 30 years.
In fact, as mentioned above, cracks and failure occur even in the design
life. This is due to the probabilistic nature of strength . The probabilistic
nature of strength and lifeability of structural elements served as one of the
reasons for creating Farmer charts (Fig. 1.23), reflecting the relationship
between the probability of occurrence of accidents and their radiological
consequences.
The section of the science of strength which studies the probabilistic
laws of resistance of structural elements to destruction can be called
strength reliability or the statistical theory of structural strength. To describe
the probabilistic laws of strength reliability we can use conceptual and
mathematical tools of mathematical reliability theory, formulated within
the framework of the so-called system reliability theory for electronic and
radio systems. This approach can be effective for studying the strength of
mass production of items, such as automobiles and in the manufacture of
agricultural machinery. The formal mathematical approach cannot be used in
nuclear power engineering to understand the probabilistic laws of strength
because of the lack of sufficient statistical data on the damage of similar
structural elements. There are almost no data on large-scale destruction.

1.5 Ageing of equipment and pipelines.

Ageing considered and not considered in design

Ageing is a term that has long been used in metals science where it refers
to changes in metal under the influence of strain, temperature, other effects,
or without them. Typically, ageing changes the mechanical properties of the
metal.
In nuclear industry the term ageing has a broader meaning. Here ageing
includes any changes to the metal or structure that occur during operation. In
this case, the term 'ageing' is often replaced the term 'degradation'.
According to the documents 3,4,6-8,etc. all ageing processes must be
considered in the design. Equipment and piping should operate without
damages and destructions during the whole lifetim. As a rule, all ageing
degradations considered in the design processes and their limits are specified
in designn strength calculations.
In fact, during the operation some elements could be damaged to the
extent of design limits, and even destruction of individual elements and
piping may take place. Typical ageing mechanisms excluded by the design
process, but appeared during the operation of NPP are:

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