Formal–statistical methods - Probabilistic Safety Assessment for Optimum Nuclear Power Plant Life Management (PLiM) - page 50

2

Formal-statistical methods

2.1 The simplest model

In Ref. 17-26 it was attempted to assess the reliability of individual items
of equipment and pipelines through the analysis of their operation. This
approach can be called simple, because it is based on a simple analysis and
statistical processing of results of operation.
In Ref. 17 it was pointed out that nuclear power plants as large technical
systems use a large number of different materials which operate under
different conditions. Many factors could cause degradation of the functional
ability of their elements. According to operating experience, failure of
mechanical elements has been due to degradation processes, such as general
and local corrosion, erosion, radiation and thermal effects, which caused
embrittlement, fatigue and wear of materials 17,18 .
Examples of some of the failures associated with ageing are as follows:
• Rupture of carbon steel feed water pipelines caused by erosion-
corrosion wear;
• Reduction of the wall thickness of pressure vessel walls (loss of metal
1-9 mm) caused by erosion-corrosion wear;
• Fracture of thermocouple pressure pipes caused by blistering of the
hydride;
• Degradation of the elements of engine valves due to cavitation-induced
erosion;
• Corrosion of the exhaust valve of high-pressure reactor cooling
systems, caused by boric acid;
• Failure of steam generator tubes due to intercrystalline stress corrosion,
pitting;
• Degradation of plant cells due to erosion of the plates to the
communication bus;
• Failures of stop valves;
• Failures of safety injection pumps due to erosion and vibration;
• Damage to pipes due to vibration.
The authors draw attention to the fact that the above failures, associated

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