Environmental Engineering Reference
In-Depth Information
Table 4.2
Chemical compositions of various stainless steels and Ni base alloys
Material
Fe
(wt%)
Ni
(wt%)
Cr
(wt%)
Mn
(wt%)
Si
(wt%)
Mo
(wt%)
Ti
(wt%)
Nb
(wt%)
Al
(wt%)
AISI 304
Bal.
10
19
ื2
ื0.75
DIN 1.4541
Bal.
11
18
ื2
ื0.75
0.4
Inconel X-750
7
Bal.
15
ื1
ื1
2.6
1
0.7
Inconel 718
17
Bal.
19
0.5
0.75
3
0.7
5
0.6
Inconel 625
2.5
Bal.
22
0.3
0.1
8.8
0.3
3.9*
0.2
*(Nb + Ta) = 3.9 wt%.
Source: A.N.T. International (2011) and Cox
et al
. (2006).
need to develop more corrosion-resistant alloys. However for BWRs, whilst
the material originally selected, namely Zircaloy-2 (Zry-2), appeared to have
adequate corrosion performance, recent burnup experience has shown that
alloys with better corrosion and hydriding resistance are needed. Improved
versions of Zry-2 and advanced alloys are being developed. Initially pure
Zr-sponge liner was used as a PCI remedy for BWR applications. It was later
found that the pure Zr sponge material results in a tendency for secondary
degradation of failed fuel and therefore all fuel vendors added some alloying
elements to increase the resistance towards secondary degradation of failed
rods. The most potent alloying element to obtain this increased resistance is
Fe by improving corrosion resistance of the liner material. However, Fe also
has a tendency to decrease PCI performance. In RBMK and VVER reactors,
E110 has always been used as fuel cladding material while E125 is being used
for some of the structural components in the fuel assemblies.
4.3
Effects of irradiation on zirconium alloys
We proceed with sections describing fundamental metallurgical properties
and phenonema which ultimately affect core component behavior.
4.3.1 Basic irradiation damage
In structural materials like Zircaloy, the overwhelming majority of defects
are caused by neutrons, and the most important type of defect is the dislo-
cation loop. Two types of loops predominate: <a> and <c> loops. The <a>
loop lies on a prism plane and has a Burgers vector in the <a> direction of
the HCP lattice. Table 4.4 lists some important characteristics. Both vacancy
and interstitial loops exist, but more than half have vacancy character. They
are very small (100 nm 'black spots') and are diffi cult to analyze even with
the transmission electron microscopy (TEM) (see Fig. 4.7 ).
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