Terminology, concepts and definitions - Probabilistic Safety Assessment for Optimum Nuclear Power Plant Life Management (PLiM) - page 39

and others, subjected to mechanical loading determine their technical safety
(it is enough to remember the accident with fatalities at the Mihama NPP
in Japan in 2004).
Radiation and nuclear safety of the nuclear power plants are based on
the concept of deeply layered protection in the path of propagation of
ionising radiation and radioactive substances 3-6, etc . Physical barriers to the
spread of ionising radiation and radioactive substances are called safety
fence. The structural elements of the safety barriers belong in the group of
passive safety elements of NPP (Fig. 1.22) and their strength is obviously
associated with radiation and nuclear safety.
The processes leading to damage which are taken into account in
substantiating the strength at the design stage of structures, pipelines and
equipment of NPP, include:
- corrosion;
- radiation damage;
- plastic deformation;
- failure under static loading;
- fatigue;
- creep;
- wear, abrasion, erosion.
In accordance with these processes, strength and service life are
determined by the following criteria 4 :
1. resistance to plastic deformation over the ensire section, including
the appearance of residual deformation which makes further operation
impossible;
2. fracture resistance under static loading (by the viscous and brittle
mechanisms);
3. fatigue strength;
4. creep resistance;
5. resistance to buckling.
The strength and service life of a structure with corrosion and radiation
damage taken into account are ensured on the one hand by the choice of
material and, on the other side, by the safety factor and allowances for the
wall thickness of the construction.
Equipment, pipelines and nuclear power plant construction elements must
not fail during operation. Such a requirement is contained in the Norms
of trength of nuclear power plants 4 , and other regulatory documents 2-4,5-8
etc., which also identify technologies and tools that ensure the above norm.
At the same time, nuclear power plants all over the world show every
year damage in structural elements, including cracks, irrespective of their
types, design features and operating conditions. Some of these cracks lead
to the formation of leaks or even the complete destruction of the structural
element, including fatalities, as the already mentioned Mihama nuclear
power plant in Japan in 2004. Some summary data on the destruction
of pipelines at nuclear power plants produced in the western countries,

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