Overview of ageing and degradation issues in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 344

1.2 Degradation mechanisms and materials

ageing issues in nuclear steam supply
systems (NSSS)

As shown in Tables 1.2 and 1.3, we note that a large number of materials are
used for various components in nuclear power systems. It is important to
state here that relatively large structures can only be fabricated using welded
joints and the designers need to account for the varied properties of the dif-
ferent materials and their welds; often welds are known to be more sensi-
tive to radiation and corrosive environments. As pointed out by Roberts, 3 i n
many cases, nuclear grade materials are fabricated to more stringent speci-
fi cations than those for other technologies and are subjected to inspection
and surveillance following in-reactor exposures. Stress states experienced
by different components vary depending on locations, for example biax-
ial stresses in thin-walled cladding tubes and more complex ones in pipes,
elbows and their welds; a time dependent constant load leads to creep failure
in out-of-core structural components while in-core materials experience irra-
diation enhanced damage thereby further shortening their life. All structures,
especially massive ones such as reactor pressure vessels (RPVs), invariably
contain fl aws and cracks that need to be taken into consideration through
fracture mechanics and structural integrity analyses. It is therefore necessary
to develop appropriate constitutive laws and models taking into account the
individual or combined effect of: (i) instantaneous elastic and plastic defor-
mation, (ii) time dependent recoverable deformation (anelastic strain), (iii)
time dependent plastic deformation (creep), (iv) strain accumulation due to
cyclic loading (fatigue), (v) corrosive environment effects, (vi) compositional
effects such as dynamic strain ageing leading to premature failures and fi nally
(vi) radiation damage and effects. The common ageing-related degradation
mechanisms are described in the subsections which follow while more details
are given in various chapters of the topic - Part I covers major phenomena
and Part II pertains to specifi c structural components and varied NSSSs.

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1.2.1 Radiation damage

High energy neutron exposure results in accumulation of many defects
like point defects (vacancies and interstitials) and dislocations, and causes
redistribution in the chemistry (phase change or radiation-induced segre-
gation (RIS)) in the materials. These modifi cations lead to deterioration in
mechanical and corrosion properties of the exposed materials. The micro-
scopic defects produced in materials due to irradiation are referred to as
radiation damage . The crystal defects thus produced modify the macro-
scopic properties (physical, thermal and mechanical) of materials which are
referred to as radiation effects .