Terminology, concepts and definitions - Probabilistic Safety Assessment for Optimum Nuclear Power Plant Life Management (PLiM) - page 35

All elements of NPP

Elements important
for service

Elements important
for safety

Active elements
important for safety

Passive elements
important for safety

1.20 Ranking of equipment, pipelines and nuclear plant structures.

elements that are important to safety; elements which degrade with age in
service and which can limit the useful life of NPP; the elements that are
important to safety and could limit the useful life.
Ranking the elements and systems of NPP with VVER-440 is shown
in Fig. 1.20. All the structural elements listed in Fig. 1.20, can be divided
into four groups:
- elements of the core;
- pressure vessels and piping;
- mechanisms and internals;
- building structures.
Vessels and pipelines play the important role in the safety and lifetime
assessment due to:
- the greatest impact on nuclear safety of nuclear power plants;
- they are the most numerous group of structures and components on
the power unit:
- they are in many cases high-energy, representing a major threat in
terms of not only nuclear, radiation, but also industrial safety;
- their replacement (or repair) is connected, usually with high material
costs, and some structural elements, such as reactor vessels, are virtually
impossible to replace.
In the design stage, materials, vessels and pipelines are selected so
that their strength is ensured throughout the entire life cycle. In this case,
nucleation of any cracks and also the transition of the section of the wall of
the vessel or pipeline to the plastic state 3,4,etc are not allowed. The margin of
yield strength for the membrane (i.e. the average over the wall cross section)
stress is n 0.2 = 1.5. This means that the level of allowable membrane stress
[σ] is lower than the yield stress and considerably lower than the tensile
strength of the material from which the structure is produced.

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