Overview of ageing and degradation issues in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 359

1.3 Radiation effects

As described in Section 1.2.1, exposure of materials and structures to high
energy neutrons leads to the creation of microscopic defects such as vacan-
cies, interstitials, Frenkel defects, dislocations and faulted loops, as well as
voids and cavities. Figure 1.12a depicts voids and precipitates in irradiated
stainless steel 15 while large Frank loops are shown in Fig. 1.12b. 16 Similar
faulted Frank loops are noted in irradiated aluminium and copper as well
as iron (Fig. 1.12c). 17
Materials undergo many changes on exposure to neutron radiation: defect
concentration increases, neutron transmutation occurs, chemical reactivity
changes (generally gets enhanced), diffusion of the elements increases and
new phases (both equilibrium and non-equilibrium) form. The extent of
change in properties is, in general, proportional to radiation fl ux, particle
energy and irradiation time, while it decreases with an increase in irradia-
tion temperature. The creation of voids, cavities and depleted zones leads
to decreased density of the material with a corresponding increase in vol-
ume known as radiation swelling. Increased defect concentration leads to
increased electrical resistivity and decreased thermal conductivity while
magnetic susceptibility decreases. The threshold neutron fl uence or dpa that
leads to extensive degradation in a material depends on the crystal structure
and nature of atomic bonding - semiconductors and polymers degrade at
much lower neutron fl uences compared to ceramics and metals. The reader
is referred to various monographs on nuclear materials and radiation effects
for more details. 18 These defects result in hardening and embrittlement of the
material with an increase in strength and accompanying decrease in ductility
commonly referred to as radiation hardening and radiation embrittlement ;
strain hardening in the material decreases accompanied by a decreased uni-
form elongation and an increase in DBTT (or RT NDT ), which decreases the
fracture toughness. The increased defect density enhances the diffusivity in
the material which in turn increases the creep rates and reduces the rupture
time. These various phenomena will be discussed in detail in the following
sections.

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1.3.1 Mechanical behaviour

It was mentioned in earlier sections that BCC materials such as iron and
some steels show a distinct yield point (Fig. 1.13) where as FCC and HCP
materials show a continuous transition from elastic to plastic range (Fig. 1.3).
The distinct yield point is due to the locking of the dislocation sources by
interstitial impurities such as C and N in low alloy steels that increases the
stress resulting in a sudden increase in free or mobile dislocation density. The
velocity of these dislocations decreases in order to maintain the imposed