Environmental Engineering Reference
In-Depth Information
The high energy neutron knocks out a stationary atom from its equilib-
rium position and transfers some kinetic energy (KE) to it which in turn
displaces more atoms to cause a cascade effect, resulting in a number of
interstitials leading to Frenkel defects.
5
This process continues until the
energy of all primary and secondary knock-on atoms is insuffi cient (<25 eV)
to displace those from the lattice sites. The atomic displacements per atom
(dpa), defi ned as the number of times each atom is displaced from the lat-
tice site, is estimated using various models, among which the Kinchin-Pease
model is often used:
Λ
E
E
4
A
dpa
,
[1.1 ]
n
Φ
with
with
with
Λ
σ
=
=
l
el
2
,
A
4
(
)
1
+
d
is neutron fl ux (/m
2
s),
t
is irradiation time,
E
̅
where
n
is
the mean neutron energy,
E
d
is the atomic displacement energy,
A
is atomic
mass (in amu) and
Φ
=
φ
t
is the fl uence,
φ
el
is the elastic neutron scattering cross section. This dpa
is a better damage evaluation unit than the commonly used fl uence (
σ
) since
it takes into account the spectral distribution of neutron energy in a given
reactor.
5
This process continues until the KE falls below that needed to cause
further displacements. The interstitials thus formed segregate as small disc
shaped clusters.
6
They can either dissolve by vacancy emission or coalesce to
large nano-voids. Clearly, the creation of excess point defects not only changes
the physical dimensions due to a reduction in density but also enhances the
diffusion kinetics and the phenomena controlled by atomic diffusion such as
creep, while the production of line, areal and volumetric defects result in radi-
ation hardening and embrittlement. In RPV steels, the excess vacancies pro-
duced by irradiation favour long range diffusion of copper atoms to form a
copper-rich-phase, which has a BCC structure and is coherent with the steel
matrix. This phase gets enriched in iron, nickel and manganese, and increases
in volume.
7
These phenomena pertaining to RPV steels are further discussed
in detail in a later chapter of the topic. Before discussing the radiation effects a
brief description of various materials properties and phenomena is presented.
It should be noted that materials issues specifi c to various reactor compo-
nents as well as detailed descriptions on various phenomena are included in
specifi c chapters later in the topic: Part I covers various fundamental materi-
als phenomena and Part II covers reactors and components.
Φ
1.2.2 Plastic deformation
Structural materials experiencing complex stresses due to varied external
forces may suffer elastic, anelastic and plastic deformations. Elastic strain
is an instantaneous and completely recoverable deformation, the extent of
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