Physico–statistical approach taking defects into account and using binomial distribution - Probabilistic Safety Assessment for Optimum Nuclear Power Plant Life Management (PLiM) - page 100

- the level of reliability of a reactor depends on the reliability of
equipment and pressure pipelines of the primary and secondary circuits;
- the reliability criterion for equipment and piping is based on the
requirements of normative and technical documentation, domestic and
international operating experience of similar plants based on the safe
operation of the reactor core and nuclear power plants in general;
- satisfying the reliability criterion of test equipment and pipelines if
the determining factor is the propagation of defects in metal parts. This
is achieved by issuing recommendations for the characteristics of metal
inspection and, if necessary, for their design and working conditions.
Analysis of reliability of equipment and pipelines of VVER reactors based
on probabilistic methods of fracture mechanics consists of four main steps:
- analysis of input data;
- calculation of the probability of destruction;
- definition of reliability;
- preparation of recommendations for achieving the necessary reliability.
To determine the reliability of equipment or piping it is necessary
to construct the physical and mechanical model of destruction of their
elements. This model, which includes all the stages of fracture (nucleation,
fatigue growth, the critical growth of defects under corrosion and mechanical
effects), is called the unified model of fracture 61 . The idea of this model is
as follows. Before operation, an element of a composite piece of equipment
may contain defects that were missed during testing of metal and also
nuclei of future defects may be found. In service, parts of equipment or
piping are subjected to thermal, mechanical and corrosive influences. This
may be accompanied by the nucleation of new defects and propagation of
existing defects from the start of operation. The ratio of the rates of these
processes may differ depending on the service conditions of equipment
or piping. Two cases are of particular interest here. In the first case the
nucleation of new defects is much slower than the process of propagation
of the already existing defects. In this case, in determining, for exmple,
SG reliability indices, the nucleation of new defects can be neglected. This
case requires mandatory testing and validation, as commonly used in design
analysis. In the second case, the rate of nucleation of new defects exceeds
the rate of propagation of existing defects and this leads to merging of
the nucleated and existing defects. Nucleation of new defects in this case
is of fundamental importance for determining the reliability of equipment
or pipeline. Thus, the damage cumulation in a section of equipment or
piping can lead to an increase in both the number and size of the existing
defects. In addition, the operating rules and regulations define the extent
and frequency of testing of metal which in turn lead to the identification of
defects during the plant preventive maintenance. This reduces the number
of dangerous defects in components of equipment or pipeline.
The physical-mechanical models of faults due destruction of parts of
equipment or pipeline have been developed using the terms defined below.

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