Physico–statistical approach taking defects into account and using binomial distribution - Probabilistic Safety Assessment for Optimum Nuclear Power Plant Life Management (PLiM) - page 117

determining the reliability of the elements of the existing energy unit, with
indicators of reliability (reliability level) in previous instances (or equivalent).
It is assumed that the reliability criterion is satisfied if reliable operation
of the elements ensures:
- safe operation of the core and nuclear power plant in general;
- transfer of the element to the safe state when a failure occurs (the
principle of safe failure);
- the required value of the readiness coefficient;
- optimum cost of repairs related to failure.
Based on the first principle, acceptable level of reliability is determined
on the basis of the PSA data the corresponding generalised IAEA data.
At the present time the databases in Russia and the European Community
contain information regarding the reliability.
For nuclear power plants these reliability indices ensure safety of
the reactor core, i.e. in accordance with domestic and foreign safety
requirements the probability of core damage is less than 10 -5 per reactor
per year. These reliability indices, which characterise the reliability of
components and systems, comply with such a (acceptable) level of reliability
at which the safe operation of nuclear installations and NPP is ensured as
a whole.
The second principle of design and regulatory documentation is used
to determine the permissible level of reliability in terms of transfer of an
item into a safe state when a failure occurs in it.
The third principle is based on the availability factor of nuclear power
plant and the reactor, the availability factor of the elements should satisfy
the required availability factor for nuclear power plant.
The fourth principle relates the allowable reliability indices and the
required factor of economic efficiency of operation of the element, reactor
installation the nuclear power plant as a whole.
If the resulting level of reliability (and/or failure rate and the limiting
state) of equipment does not meet the reliability criterion, then based on
the reliability criterion, it can be improved (and/or reduced to the failure
rate and limiting state), for example, due to: changes of periodicity of
metal inspection, design requirements for flaw inspection with regard to the
allowable defect sizes with the corresponding allowable number of defects
or more accurate determination of its service life.

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