Probabilistic analysis of safety: Increasing the reliability and safety of nuclear power plant components - Probabilistic Safety Assessment for Optimum Nuclear Power Plant Life Management (PLiM) - page 214

residual defectiveness in comparison with the regulatory curve of the
residual defectiveness as in the first variant (Fig. 6.9-6.11).

6.4 Improving the safety of main circulation
pipelines of nuclear power plant with first-
generation VVER-440 reactors

In the preparation of a project for reconstructing the energy units of the
first-generation with the incomplete safety systems, it was necessary to solve
the problem of ensuring the reliability of main circulation pipelines (MCP)
using the criterion of fracture resistance (beginning of the maximum design-
basis failure). This problem was solved in the framework of the concept
'leak before break' (LBB), developed in the USA, or the technology of
preventing fracture, developed in Germany. Analysis of Western approaches
shows that the application of these approaches to the nuclear power plant
does not ensure the safety on the basis of the fracture criterion of MCP
because it does not take into account the possibility of fracture of the
pipeline without a leak 9 .
Therefore, investigations were carried out to develop a Russian
technology of ensuring the safety of the main circulation pipelines on
the basis of the criterion of fracture resistance using the System concept
of strength and the concept of the leak before break, based on the
determination of the probability of failure of the pipeline. This technology
has been subjected to international analysis in the framework of the project
TACIS 9 and, subsequently, the safety systems LBB were applied for the
units 3, 4 the Novovoronezh' NPP and the units 1, 2 of the Kola NPP. The
technology is described in detail in Ref. 9, and some of the main results
and conclusions, obtained using the method described in section 6.3, are
given below.
Figure 6.12 shows the characteristics of service safe operation of
the main circulation lines on the basis of the fracture criterion in the
'probability of break - operating time in the generalised heating-cooling
cycles' coordinates. The curves 1, 2 characterise the safety level prior to the
application of the 'leak before fracture' system technology; the curves 3,
4 indicate the actual level of safety in the units 3, 4 of the Novovoronezh'
NPP and units 1, 2 of the Kola NPP.
Reliability and safety were improved as a result of specially organised
non-destructive inspection and hydraulic tests (HT) (the effect of hydraulic
tests on the probability of failure of the vessels and pressure pipelines was
discussed in sections 8.1 and 8.2).
When using the 'leak before break' system technology, and the
probabilistic evaluation of safety and reliability in the four units of the
nuclear power plants in operation, the high level of safety and reliability
was reached quite rapidly and the beginning of the maximum design-basis

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